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Element Selective Probe of the Ultra-Fast Magnetic Response to an Element Selective Excitation in Fe-Ni Compounds Using a Two-Color FEL Source

Element Selective Probe of the Ultra-Fast Magnetic Response to an Element Selective Excitation in... hv photonics Article Element Selective Probe of the Ultra-Fast Magnetic Response to an Element Selective Excitation in Fe-Ni Compounds Using a Two-Color FEL Source 1 , 2 , † 1 , 3 4 5 6 Eugenio Ferrari , Carlo Spezzani , Franck Fortuna , Renaud Delaunay , Franck Vidal , 1 1 1 1 1 Ivaylo Nikolov , Paolo Cinquegrana , Bruno Diviacco , David Gauthier , Giuseppe Penco , 1 1 1 7 Primož Rebernik Ribic ˇ , Eléonore Roussel , Mauro Trovò , Jean-Baptiste Moussy , 8 6 , 9 1 , 10 1 , 11 Tommaso Pincelli , Lounès Lounis , Cristian Svetina , Marco Zangrando , 1 1 1 1 Nicola Mahne , Lorenzo Raimondi , Michele Manfredda , Emanuele Pedersoli , 1 1 1 , 12 1 Flavio Capotondi , Alexander Demidovich , Luca Giannessi , Maya Kiskinova , 1 , 13 1 1 , 6 , 14 , Giovanni De Ninno , Miltcho Boyanov Danailov , Enrico Allaria * and Maurizio Sacchi * ELETTRA—Sincrotrone Trieste, Area Science Park, 34149 Trieste, Italy; eugenio.ferrari@psi.ch (E.F.); carlo.spezzani@elettra.eu (C.S.); ivaylo.nikolov@elettra.eu (I.N.); paolo.cinquegrana@elettra.eu (P.C.); bruno.diviacco@elettra.eu (B.D.); david.gauthier@elettra.eu (D.G.); giuseppe.penco@elettra.eu (G.P.); primoz.rebernik@elettra.eu (P.R.R.); eleonore.roussel@elettra.eu (E.R.); mauro.trovo@elettra.eu (M.T.); Cristian.Svetina@elettra.eu (C.S.); Marco.Zangrando@elettra.eu (M.Z.); Nicola.Mahne@elettra.eu (N.M.); Lorenzo.Raimondi@elettra.eu (L.R.); michele.manfredda@elettra.eu (M.M.); emanuele.pedersoli@elettra.eu (E.P.); flavio.capotondi@elettra.eu (F.C.); alexander.demidovich@elettra.eu (A.D.); lucagiannessi@gmail.com (L.G.); maya.kiskinova@elettra.eu (M.K.); giovanni.deninno@elettra.eu (G.D.N.); miltcho.danailov@elettra.eu (M.B.D.) Dipartimento di Fisica, Università degli Studi di Trieste, 34127 Trieste, Italy Laboratoire de Physique des Solides, Université Paris-Sud, CNRS-UMR 8502, Bât. 510, 91405 Orsay, France Centre de Sciences Nucléaires et de Sciences de la Matière, Université Paris-Sud, CNRS UMR 8609, Bât. 104-108, 91405 Orsay, France; fortuna@csnsm.in2p3.fr Laboratoire de Chimie Physique Matière et Rayonnement, Sorbonne Universités, UPMC Univ Paris 06, CNRS UMR 7614, 75005 Paris, France; renaud.delaunay@upmc.fr Institut des NanoSciences de Paris, Sorbonne Universités, UPMC Univ Paris 06, CNRS UMR 7588, 75005 Paris, France; franck.vidal@insp.jussieu.fr (F.V.); lounis@insp.jussieu.fr (L.L.) Service de Physique de l’Etat Condensé, DSM/IRAMIS/SPEC, CNRS UMR 3680, CEA Saclay, 91191 Gif-sur-Yvette, France; jean-baptiste.moussy@cea.fr Dipartimento di Fisica, Università degli Studi di Milano, 20133 Milano, Italy; tommaso.pincelli@gmail.com Ecole Normale Supérieure, PSL Research University, 75231 Paris, France Graduate School of Nanotechnology, Università degli Studi di Trieste, 34127 Trieste, Italy Istituto Officina dei Materiali, Consiglio Nazionale delle Ricerche, 34149 Trieste, Italy ENEA, Centro Ricerche Frascati, Via E. Fermi 45, 00044 Frascati, Italy Laboratory of Quantum Optics, University of Nova Gorica, 5001 Nova Gorica, Slovenia Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, B.P. 48, 91192 Gif-sur-Yvette, France * Correspondence: enrico.allaria@elettra.eu (E.A.); maurizio.sacchi@synchrotron-soleil.fr (M.S.) † Current Address: Particle Accelerator Physics Laboratory, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland. Received: 18 December 2016; Accepted: 20 January 2017; Published: 26 January 2017 Abstract: The potential of the two-color mode implemented at the FERMI free-electron laser (FEL) source for pumping and probing selectively different atomic species has been demonstrated by time-resolved scattering experiments with permalloy (FeNi alloy) and NiFe O samples. 2 4 We monitored the ultra-fast demagnetization of Ni induced by the pump FEL pulse, by tuning the linearly-polarized FEL probe pulse to the Ni-3p resonance and measuring the scattered intensity in the transverse magneto-optical Kerr effect geometry. The measurements were performed by varying the intensity of the FEL pump pulse, tuning its wavelength to and off of the Fe-3p resonance, and by spanning the FEL probe pulse delays across the 300–900 fs range. The obtained results have Photonics 2017, 4, 6; doi:10.3390/photonics4010006 www.mdpi.com/journal/photonics Photonics 2017, 4, 6 2 of 10 evidenced that for the case of NiFe O , there is a sensible difference in the magnetic response at the 2 4 Ni site when the pump pulse causes electronic excitations at the Fe site. Keywords: free electron laser; two-color source; ultra-fast dynamics 1. Introduction The most common approach to ultra-fast demagnetization relies on optical-laser based time-resolved magneto-optical Kerr effect (MOKE) studies [1,2]. Modern pulsed X-ray sources, notably free-electron lasers (FEL) and high-harmonic generation sources, have introduced the use of X-rays as a complement to optical lasers. The most common set-up makes use of an optical laser as the pump and of an X-ray beam as the probe, providing element selectivity by fine-tuning the X-ray wavelength to a core resonance [3–6]. We developed a new scheme where, in a given multi-element magnetic sample, we pump one selected element and probe another element, also selectively, using two FEL pulses of different wavelengths [7]. The interest of this kind of experiment resides in the possibility of associating the pump energy with a specific electronic excitation of one component of a complex system. The combination of an element selective excitation with an element selective probe offers new paths for unraveling the fundamental mechanisms that drive magnetization loss by separately addressing the weight of possible contributions in this complex problem. Important classes of magnetic materials, in this respect, are ferrites [8,9] and transition metal (TM) rare-earth (RE) compounds [10–13]. Beyond their applicative interest, the (O-mediated) TM-TM coupling in magnetic oxides and the (5d-mediated) 3d-4f coupling in TM-RE compounds make these materials ideal candidates for resonant-pump resonant-probe magnetization dynamics studies. The highly localized 3d and 4f orbitals and the mediated coupling make it plausible that associating the pump energy with a specific electronic excitation can influence the magnetization dynamics profoundly, compared to using a non-resonant pump. 2. Materials and Methods 2.1. Source Design The conceptual and experimental development of two-color schemes prompted major research efforts at all FEL facilities worldwide [14–18], with the ambition of optimizing wavelength and timing control. In general, two-color sources are constrained by limited wavelength tunability. Recently, a new configuration of the FERMI FEL-1 seeded source [19] was implemented that delivers two time-delayed FEL pulses with different wavelengths, each independently tunable over a broad spectral range. The principle of the two-color double-resonant source was set out in reference [7]. Here we summarize its most relevant features. The twin-seed layout starts from a common ~780 nm laser source that produces two ultraviolet (UV) pulses along two distinct optical paths. One comprises a third-harmonic generator and a delay-line for adjusting the delay Dt between the two pulses. The other features an optical parametric amplifier (OPA) for adjusting the pulse wavelength over the 230–260 nm range. The former, which has a fixed 261.5 nm wavelength and a FWHM pulse duration t of ~120 fs, serves as a UV seed for seed producing the Ni-3p resonant FEL pulse. The latter, with t ~150 fs, is used for seeding the Fe-3p seed resonant FEL emission by setting its wavelength to 255 nm. The two UV seeds are recombined and kept along the same optical path by using a feed-back system based on multiple motorized piezoelectric tip-tilt devices. Figure 1 sketches the principle of the two-color resonant FEL mode used in our experiment. The two UV laser seeds are aligned with the electron beam trajectory within the FEL modulator section, where they interact with the electron bunch, stretched in time to ~1 ps length. The wavelength Photonics 2017, 4, 6 3 of 10 separation between the two seeds is smaller than the bandwidth accepted by the modulator (~3%), but larger than that accepted by the radiator (~0.7%). The FERMI FEL-1 radiator section is composed of six undulator modules. For most of the measurements, five modules (Rad_2 in Figure 1a) are tuned to the 11th harmonic of the 255 nm seed laser, generating a FEL pump pulse at 23.2 nm (53.5 eV), i.e., Photonics 2017, 4, 6 3 of 10 at the Fe-3p resonance (Figure 1a). By controlling the seed laser intensity via cross-polarizers, the of six undulator modules. For most of the measurements, five modules (Rad_2 in Figure 1a) are energy per FEL pulse spans the 0 to 10 J range, corresponding to a pump fluence (F) of 0–40 mJcm tuned to the 11th harmonic of the 255 nm seed laser, generating a FEL pump pulse at 23.2 nm (53.5 eV), at the sample position. Off-resonance pumping (Figure 1b) is obtained by tuning Rad_2 to the i.e., at the Fe-3p resonance (Figure 1a). By controlling the seed laser intensity via cross-polarizers, the −2 10th harmonic, corresponding to 25.5 nm (48.6 eV). The remaining radiator module (Rad_1) is tuned energy per FEL pulse spans the 0 to 10 µ J range, corresponding to a pump fluence (F) of 0–40 mJ· cm at the sample position. Off-resonance pumping (Figure 1b) is obtained by tuning Rad_2 to the 10th to the 14th harmonic of the 261.5 nm seed pulse, resulting in a FEL emission at 18.6 nm (67 eV, Ni-3p harmonic, corresponding to 25.5 nm (48.6 eV). The remaining radiator module (Rad_1) is tuned to resonance), with a maximum energy of ~0.8 J per pulse. We also tested a different source scheme the 14th harmonic of the 261.5 nm seed pulse, resulting in a FEL emission at 18.6 nm (67 eV, Ni-3p with inverted pump and probe wavelengths and an equal distribution of the radiator modules on resonance), with a maximum energy of ~0.8 µ J per pulse. We also tested a different source scheme the Rad_1 and Rad_2 sub-sections (Figure 1c). The delay Dt between the two UV seeds, measured with inverted pump and probe wavelengths and an equal distribution of the radiator modules on by a cross-corr the Rad_1 elator and ,Rcan ad_2 be sub adjusted -sections (F over igure the 1c). Th 300–900 e delay Δ fs t bet range weenwith the twnegligible o UV seeds, (<5 measu fs) red jitter by [20,21]. a cross-correlator, can be adjusted over the 300–900 fs range with negligible (<5 fs) jitter [20,21]. The The upper limit is imposed by the electron bunch length of ~1 ps. Cross-talk and instabilities are upper limit is imposed by the electron bunch length of ~1 ps. Cross-talk and instabilities are observed in the double FEL emission below 300 fs separation, hampering the collection of reliable data observed in the double FEL emission below 300 fs separation, hampering the collection of reliable at shorter delays. The duration t of each FEL pulse can be conservatively estimated from t by FEL seed data at shorter delays. The duration tFEL of each FEL pulse can be conservatively estimated from tseed 0.3 the scaling law t = t  h ,−0h .3 being the radiator harmonic. Therefore the FEL pulses in our FEL by the scaling law seed tFEL = tseed × h , h being the radiator harmonic. Therefore the FEL pulses in our experiment expe ar rim e e expected nt are expeto ctehave d to ha a ve 50 a 5 to 0 t70 o 70 fs fsduration, duration, fo for r an an ovoverall erall timetime resolu resolution tion better th better an 100 than fs. 100 fs. Figure 1. Twin-seed split-radiator source scheme. (a) Ni-3p resonant probe, Fe-3p resonant pump. Figure 1. Twin-seed split-radiator source scheme. (a) Ni-3p resonant probe, Fe-3p resonant pump. The two UV seed pulses, controlled in intensity and separated by a well-defined time delay Δt, The two UV seed pulses, controlled in intensity and separated by a well-defined time delay Dt, interact interact with the electron bunch within the modulator (Mod) section. After going through the with the electron bunch within the modulator (Mod) section. After going through the dispersive section dispersive section (DS), the electron excitation induced by the probe seed pulse (λseed_1 = 261.5 nm) is (DS), theaelectr mplifieon d bexcitation y the first oinduced f the six ra by diathe tor m pr od obe ulesseed (Rad_pulse 1) tune(dl to harm =on 261.5 ic 14 nm) (h_14)is , g amplifi enerating ed by the seed_1 18.6 nm free-electron laser (FEL) radiation (67 eV, Ni-3p resonance). This excitation is not amplified first of the six radiator modules (Rad_1) tuned to harmonic 14 (h_14), generating 18.6 nm free-electron by the five other radiator modules (Rad_2). Conversely, the UV pump seed (λseed_2 = 255 nm) induces laser (FEL) radiation (67 eV, Ni-3p resonance). This excitation is not amplified by the five other radiator an excitation of the electron bunch that is amplified by tuning Rad_2 to the 11th harmonic, modules (Rad_2). Conversely, the UV pump seed (l = 255 nm) induces an excitation of the seed_2 generating a 23.2 nm FEL pulse (53.5 eV, Fe-3p resonance), but this excitation is not amplified by electron bunch that is amplified by tuning Rad_2 to the 11th harmonic, generating a 23.2 nm FEL Rad_1. The source delivers two FEL pulses tuned in energy to the Fe-3p and Ni-3p resonances, with pulse (53.5 eV, Fe-3p resonance), but this excitation is not amplified by Rad_1. The source delivers two controlled intensities and time separation; (b) Ni-3p resonant probe, non-resonant pump. The same FEL pulses seed tuned ing schem in e ener as in gy (a) to appl the ieFe-3p s, but the and Rad_2 Ni-3p radia resonances, tor sub-sectiowith n is tucontr ned to olled h_10 of intensities the 255 nm and time pump seed, generating 25.5 nm FEL radiation, whose photon energy is insufficient to photo-excite separation; (b) Ni-3p resonant probe, non-resonant pump. The same seeding scheme as in (a) applies, the core electrons of either Fe or Ni; (c) Fe-3p resonant probe, Ni-3p resonant pump. The but the Rad_2 radiator sub-section is tuned to h_10 of the 255 nm pump seed, generating 25.5 nm FEL wavelengths of the two UV seeds are inverted (λseed_1 = 255 nm, λseed_2 = 261.5 nm) and the radiator is radiation, whose photon energy is insufficient to photo-excite the core electrons of either Fe or Ni; split in two sub-sections of three modules each. Rad_1 provides the 23.3 nm FEL probe pulse tuned (c) Fe-3p resonant probe, Ni-3p resonant pump. The wavelengths of the two UV seeds are inverted to the Fe-3p resonance, while Rad_2 generates the Ni-3p resonant 18.6 nm pump pulse. (l = 255 nm, l = 261.5 nm) and the radiator is split in two sub-sections of three modules seed_1 seed_2 each. Rad_1 provides the 23.3 nm FEL probe pulse tuned to the Fe-3p resonance, while Rad_2 generates the Ni-3p resonant 18.6 nm pump pulse. Photonics 2017, 4, 6 4 of 10 Photonics 2017, 4, 6 4 of 10 2.2. Experimental Details 2.2. Experimental Details The measurements were performed at the DiProI beamline [22,23] of the FERMI FEL source, The measurements were performed at the DiProI beamline [22,23] of the FERMI FEL source, using the IRMA (Instrument pour la Réfléctivité MAgnétique) scattering chamber [24]. Figure 2 using the IRMA (Instrument pour la Réfléctivité MAgnétique) scattering chamber [24]. Figure 2 shows shows a sketch of the experimental set-up and of the data collection method. After aligning the a sketch of the experimental set-up and of the data collection method. After aligning the sample sample using the two-axis goniometer, the pump-probe results were monitored using a using the two-axis goniometer, the pump-probe results were monitored using a charge-coupled device charge-coupled device (CCD) detector. The grating-like structure of the samples separated the pump (CCD) detector. The grating-like structure of the samples separated the pump and probe contributions and probe contributions spatially, allowing us to discriminate the diffracted intensities at the two spatially, allowing us to discriminate the diffracted intensities at the two wavelengths. The diffraction wavelengths. The diffraction peaks corresponding to the pump and probe FEL wavelengths were peaks corresponding to the pump and probe FEL wavelengths were collected simultaneously on collected simultaneously on the CCD. Blanking out one of the FEL beams allowed us to check for the CCD. Blanking out one of the FEL beams allowed us to check for cross-talking between them. cross-talking between them. We confirmed its absence for delays exceeding 300 fs, while interference We confirmed its absence for delays exceeding 300 fs, while interference between the two sources was between the two sources was observed at shorter delays. In order to maximize the magnetic signal, observed at shorter delays. In order to maximize the magnetic signal, we worked in transverse-MOKE we worked in transverse-MOKE geometry [25,26], setting the incidence angle to 46.5°, i.e., close to geometry [25,26], setting the incidence angle to 46.5 , i.e., close to the Brewster angle. We used linear the Brewster angle. We used linear vertical polarization of the FEL pulses, taking advantage of the vertical polarization of the FEL pulses, taking advantage of the Apple-II undulators in the FERMI Apple-II undulators in the FERMI radiator [27]. radiator [27]. Figure 2. Schematics of the scattering measurement setup. The grating samples are mounted Figure 2. Schematics of the scattering measurement setup. The grating samples are mounted in a in a reflectometer featuring a vertical scattering plane. A magnetic field (up to 1.5 kOe pulsed, reflectometer featuring a vertical scattering plane. A magnetic field (up to 1.5 kOe pulsed, 500 Oe 500 Oe permanent) can be applied normal to the scattering plane and parallel to the sample permanent) can be applied normal to the scattering plane and parallel to the sample surface by a surface by a horseshoe electromagnet. The FEL pulses with linear vertical polarization impinge horseshoe electromagnet. The FEL pulses with linear vertical polarization impinge on the sample on the sample grating at 46.5 and are diffracted at different angles according to their wavelengths. grating at 46.5° and are diffracted at different angles according to their wavelengths. Diffracted Diffracted intensities from the pump and probe FEL pulses are collected simultaneously by using a intensities from the pump and probe FEL pulses are collected simultaneously by using a two-dimensional charge-coupled device detector. two-dimensional charge-coupled device detector. We measured two samples. The first is a 20 nm permalloy (Py) film, deposited on a 605 nm period We measured two samples. The first is a 20 nm permalloy (Py) film, deposited on a 605 nm Si grating and protected by a 3 nm Al capping layer (Figure 3a, FEL data). Its magnetic properties were period Si grating and protected by a 3 nm Al capping layer (Figure 3a, FEL data). Its magnetic characterized by MOKE using an optical laser (Figure 3b). The second is a 12.5 nm thick Ni-ferrite properties were characterized by MOKE using an optical laser (Figure 3b). The second is a 12.5 nm (NiFe O ) layer grown epitaxially on MgAl O (001) [28]. A 100  400 m area of the layer was ruled thick Ni-ferrite (NiFe2O4) layer grown epitaxially on MgAl2O4(001) [28]. A 100 × 400 µ m area of the 2 4 2 4 by focused ion beam (FIB) etching into a set of ~350 nm wide stripes with a ~600 nm period (Figure 3c). layer was ruled by focused ion beam (FIB) etching into a set of ~350 nm wide stripes with a ~600 nm Magnetization curves showed a coercive field of ~500 Oe with >80% remanence (Figure 3d, FEL data). period (Figure 3c). Magnetization curves showed a coercive field of ~500 Oe with >80% remanence It is worth stressing that the fraction of pump energy absorbed in the Ni-ferrite sample is the same (Figure 3d, FEL data). It is worth stressing that the fraction of pump energy absorbed in the (~78% for the 12.5 nm thick film at 46.5 incidence) for both resonant (22.3 nm) and non-resonant Ni-ferrite sample is the same (~78% for the 12.5 nm thick film at 46.5° incidence) for both resonant (25.5 nm) pump wavelengths. (22.3 nm) and non-resonant (25.5 nm) pump wavelengths. The Ni magnetic signal at different pump-probe delays was measured as a function of the The Ni magnetic signal at different pump-probe delays was measured as a function of the pump fluence F, for both Fe-3p resonant and non-resonant pump wavelengths, using the acquisition pump fluence F, for both Fe-3p resonant and non-resonant pump wavelengths, using the acquisition procedure sketched in Figure 4. procedure sketched in Figure 4. A reliable analysis of the demagnetization in a pump-probe experiment requires the sample to be A reliable analysis of the demagnetization in a pump-probe experiment requires the sample to be homogeneously pumped over the probed area. From the images in Figures 4 and 5, it is apparent that homogeneously pumped over the probed area. From the images in Figures 4 and 5, it is apparent that this is not the case if one integrates the scattered probe signal over the entire diffraction spot, since both the pump and the probe have an inhomogeneous spatial distribution of the intensity. In order to avoid Photonics 2017, 4, 6 5 of 10 this is not the case if one integrates the scattered probe signal over the entire diffraction spot, since Photonics 2017, 4, 6 5 of 10 Photonics 2017, 4, 6 5 of 10 both the pump and the probe have an inhomogeneous spatial distribution of the intensity. In order to averaging the probe signal over areas that are pumped differently, the magnetic signal reported in the avoid averaging the probe signal over areas that are pumped differently, the magnetic signal reported averaging the probe signal over areas that are pumped differently, the magnetic signal reported in the following corresponds to the probe intensity scattered within a detector portion of 7 × 7 pixels (Figure 5) in the following corresponds to the probe intensity scattered within a detector portion of 7  7 pixels following corresponds to the probe intensity scattered within a detector portion of 7 × 7 pixels (Figure 5) that measures the homogeneously pumped area. (Figure 5) that measures the homogeneously pumped area. that measures the homogeneously pumped area. Figure 3. Permalloy and Ni-ferrite samples. (a) Rocking scan of the Py grating sample, showing the Figure 3. Permalloy and Ni-ferrite samples. (a) Rocking scan of the Py grating sample, showing the Bragg peaks corresponding to the Fe-3p resonant pump and to the Ni-3p resonant probe FEL beams; Figure 3. Permalloy and Ni-ferrite samples. (a) Rocking scan of the Py grating sample, showing the Bragg peaks corresponding to the Fe-3p resonant pump and to the Ni-3p resonant probe FEL beams; (b) Hyster Bragg esis peaks loop corrof esponding to the Py magnetization the Fe-3p reso measur nant pu ed mp an by magneto-optical d to the Ni-3p resonan Kerr t probe effectFEL (MOKE), beams;with (b) Hysteresis loop of the Py magnetization measured by magneto-optical Kerr effect (MOKE), with (b) Hysteresis loop of the Py magnetization measured by magneto-optical Kerr effect (MOKE), with the magnetic the magneti field c fiel app d applie lied parallel d parallelto tothe the grating grating li lines; nes; (c (c ) ) Scan Scanning ning elec electr tron on mic micr roscopy oscopy image image of the of the the magnetic field applied parallel to the grating lines; (c) Scanning electron microscopy image of the Ni-ferrite Ni-ferrite sample. sampAn le. An ar ea area of of 100 100 ×400 400  µm m is ispatterned into patterned into a ~600 a ~600 nm nm peri period od grati grating ng by foc by used focused ion ion Ni-ferrite sample. An area of 100 × 400 µ m is patterned into a ~600 nm period grating by focused ion beam etching; (d) Field dependence of the FEL radiation intensity diffracted at the Fe-3p resonance, beam etching; (d) Field dependence of the FEL radiation intensity diffracted at the Fe-3p resonance, beam etching; (d) Field dependence of the FEL radiation intensity diffracted at the Fe-3p resonance, measuring the magnetic response over the ruled area of the Ni-ferrite. measuring the magnetic response over the ruled area of the Ni-ferrite. measuring the magnetic response over the ruled area of the Ni-ferrite. Figure 4. Data acquisition sequence. Sketch of the acquisition sequence used for determining the Figure Figure 4. Data 4. Data acquisition acquisitiosequence. n sequence.Sketch Sketch of of the the acquisi acquisition tion se sequence quence used used for for deter determining mining the the magnetic signal. (a) After a +800 Oe pulse of ~10 ms duration, data are collected in a +200 Oe applied magnetic signal. (a) After a +800 Oe pulse of ~10 ms duration, data are collected in a +200 Oe applied magnetic field; signal. (b) Same (a) as After (a), for a +800 negative Oe pulse field of va~10 lues. ms The duration, magnetic data signar al eis collected defined as in a the +200 diffOe erence applied field; (b) Same as (a), for negative field values. The magnetic signal is defined as the difference divided by the sum of the signals measured for opposite magnetization directions. Top panels: CCD field; (b) Same as (a), for negative field values. The magnetic signal is defined as the difference divided divided by the sum of the signals measured for opposite magnetization directions. Top panels: CCD images of the pump (left spot) and probe (right spot) scattered intensities. by the sum of the signals measured for opposite magnetization directions. Top panels: CCD images of images of the pump (left spot) and probe (right spot) scattered intensities. the pump (left spot) and probe (right spot) scattered intensities. Photonics 2017, 4, 6 6 of 10 Photonics 2017, 4, 6 6 of 10 Photonics 2017, 4, 6 6 of 10 Figure 5. Measurement area selection. Fe-3p resonant pump (a) and Ni-3p resonant probe (b) diffracted Figure Figure 5.5. Mea Mea surem surem ent ent area area se se le le ct ct io io n. n. Fe Fe--3p 3p resonant resonant p pum ump p ( (a a) ) and and Ni Ni--3p 3p re re so so nan nan t t probe probe (b (b ) ) intensity from the Ni-ferrite sample. The two images are shown on different color scales. The gray didi ffracte ffracte d d inte inte nsit nsit y y from from the the Ni Ni -ferrite -ferrite sample. sample. The The two two iim mages ages are are sho shown wn on on di di fferen fferen t t co co lo lo r rsc sc aa le le s.s . square represents the 7  7 pixels area used for Ni-magnetization analysis, where the pump fluence is The gray The gray square represe square represe nt nt s the 7 s the 7 × × 7 7 pixel pixel ss area use area used d for for Ni Ni- -m magn agneti etizatio zation n analysis analysis , ,wher wher e e the the pu pu m m p p assumed to be homogeneous. fluence is assumed to be homogeneous. fluence is assumed to be homogeneous. 3.3. Results Results 3. Results Figure 6 shows the dependence of the Ni magnetic signal on the pump fluence at a probe delay of Figure 6 shows the dependence of the Ni magnetic signal on the pump fluence at a probe delay Figure 6 shows the dependence of the Ni magnetic signal on the pump fluence at a probe delay of −2 420 fs in Py. The pump fluence at the sample (F, top scale in Figure 6, in mJ· cm −) 2 is obtained by of 420 fs in Py. The pump fluence at the sample (F, top scale in Figure 6, in mJcm ) is obtained by 420 fs in Py. The pump fluence at the sample (F, top scale in Figure 6, in mJ· cm ) is obtained by correcting the pump energy measured at the exit of the FEL source (bottom scale in Figure 6, in µ J) for correcting the pump energy measured at the exit of the FEL source (bottom scale in Figure 6, in J) for correcting the pump energy measured at the exit of the FEL source (bottom scale in Figure 6, in µ J) for the transport line transmission (a factor of 0.4, accounting for six reflections and a 200 nm Al filter), for the transport line transmission (a factor of 0.4, accounting for six reflections and a 200 nm Al filter), the transport line transmission (a factor of 0.4, accounting for six reflections and a 200 nm Al filter), for the focal spot size (~80 µ m), and for the angle of incidence (46.5°). The Ni magnetic signal is the for the focal spot size (~80 m), and for the angle of incidence (46.5 ). The Ni magnetic signal is the the focal spot size (~80 µ m), and for the angle of incidence (46.5°). The Ni magnetic signal is the asymmetry ratio in the Bragg peak intensity, i.e., the difference between the scattered intensities for asymmetry ratio in the Bragg peak intensity, i.e., the difference between the scattered intensities for asymmetry ratio in the Bragg peak intensity, i.e., the difference between the scattered intensities for opposite signs of the saturation magnetization direction, divided by their sum (see Figure 4). Figure 6 opposite signs of the saturation magnetization direction, divided by their sum (see Figure 4). Figure 6 opposite signs of the saturation magnetization direction, divided by their sum (see Figure 4). Figure 6 shows the loss in Ni magnetic signal with respect to the static value measured without the pump. shows the loss in Ni magnetic signal with respect to the static value measured without the pump. shows the loss in Ni magnetic signal with respect to the static value measured without the pump. The red and blue dots refer to the Fe-3p resonant (23.2 nm) and non-resonant (25.5 nm) pumping, The red and blue dots refer to the Fe-3p resonant (23.2 nm) and non-resonant (25.5 nm) pumping, The red and blue dots refer to the Fe-3p resonant (23.2 nm) and non-resonant (25.5 nm) pumping, respectively. The results do not evidence any clear dependence on the pump wavelength; both respectively. The results do not evidence any clear dependence on the pump wavelength; both curves respectively. The results do not evidence any clear dependence on the pump wavelength; both −2 curves show the same trend of the Ni demagnetization with F, attaining a ~50% reduction at 10 mJ· cm . −2 show the same trend of the Ni demagnetization with F, attaining a ~50% reduction at 10 mJcm . curves show the same trend of the Ni demagnetization with F, attaining a ~50% reduction at 10 mJ· cm . Figure 6. Ni magnetic response in permalloy. F dependence of the Ni magnetization loss in Py, 420 fs Figure 6. Ni magnetic response in permalloy. F dependence of the Ni magnetization loss in Py, 420 fs after the pump pulse. The red circles and blue squares refer to 53.5 eV Fe-3p resonant and to 48.6 eV after the pump pulse. The red circles and blue squares refer to 53.5 eV Fe-3p resonant and to 48.6 eV Figure 6. Ni magnetic response in permalloy. F dependence of the Ni magnetization loss in Py, 420 fs non-resonant pump pulses, respectively. The pump fluence at the sample (top scale) is estimated non-resonant pump pulses, respectively. The pump fluence at the sample (top scale) is estimated after the pump pulse. The red circles and blue squares refer to 53.5 eV Fe-3p resonant and to 48.6 eV from the pump intensity measured at the exit of the source (bottom scale). The Ni magnetization loss from the pump intensity measured at the exit of the source (bottom scale). The Ni magnetization non-resonant pump pulses, respectively. The pump fluence at the sample (top scale) is estimated represents the decrease in the asymmetry ratio signal at the Bragg peak with respect to the non-pumped represents the decrease in the asymmetry ratio signal at the Bragg peak with respect to the from the pump intensity measured at the exit of the source (bottom scale). The Ni magnetization condition (black triangles). The black vertical bar corresponds to one standard deviation in the value of non-pumped condition (black triangles). The black vertical bar corresponds to one standard represents the decrease in the asymmetry ratio signal at the Bragg peak with respect to the the measured magnetic signal. deviation in the value of the measured magnetic signal. non-pumped condition (black triangles). The black vertical bar corresponds to one standard deviation in the value of the measured magnetic signal. Photonics 2017, 4, 6 7 of 10 Photonics 2017, 4, 6 7 of 10 The same experiment was performed with the NiFe2O4 sample. First, we verified that the epitaxial oxide layer on a low thermal conductivity insulating substrate could stand our pump-probe The same experiment was performed with the NiFe O sample. First, we verified that the epitaxial 2 4 conditions. Then we checked that the chosen 12.5 nm thickness, ensuring homogeneous absorption of oxide layer on a low thermal conductivity insulating substrate could stand our pump-probe conditions. the pump energy, provided a convenient magnetic signal in the diffracted intensity. We found that Then we checked that the chosen 12.5 nm thickness, ensuring homogeneous absorption of the pump inverting the saturation magnetization direction provides an excellent 80% asymmetry ratio at Ni-3p energy, provided a convenient magnetic signal in the diffracted intensity. We found that inverting the resonance. saturation magnetization direction provides an excellent 80% asymmetry ratio at Ni-3p resonance. Figure 7a shows the Ni magnetization loss after 400 fs as a function of F. There is a clear Figure 7a shows the Ni magnetization loss after 400 fs as a function of F. There is a clear difference difference between the data obtained using a non-resonant (blue dots) and an Fe-3p resonant (red between the data obtained using a non-resonant (blue dots) and an Fe-3p resonant (red dots) pump dots) pump pulse, showing the influence of the pump wavelength on the Ni demagnetization. This pulse, showing the influence of the pump wavelength on the Ni demagnetization. This sensitivity of sensitivity of the Ni-ferrite sample to the Fe-3p resonant and off-resonant pumping is also confirmed the Ni-ferrite sample to the Fe-3p resonant and off-resonant pumping is also confirmed by the results by the results obtained as a function of the pump-probe delay reported in Figure 7b,c. Dependence obtained as a function of the pump-probe delay reported in Figure 7b,c. Dependence on the pump −2 on the pump wavelength is observed already at the lower fluence F = 4 mJ· cm (Figure 7b), and it wavelength is observed already at the lower fluence F = 4 mJcm (Figure 7b), and it becomes much 2 −2 becomes much more evident and well beyond experimental uncertainty at F = 10 mJ· cm (Figure 7c). more evident and well beyond experimental uncertainty at F = 10 mJcm (Figure 7c). Figure 7b,c Figure 7b,c shows that for both F values, the quenching of the Ni magnetization is initially rather shows that for both F values, the quenching of the Ni magnetization is initially rather slow and reaches slow and reaches a maximum after ~500 fs. a maximum after ~500 fs. Figure 7. Ni magnetic response in Ni-ferrite. (a) Relative Ni magnetization loss as a function of F for Figure 7. Ni magnetic response in Ni-ferrite. (a) Relative Ni magnetization loss as a function of Fe-3p resonant and non-resonant pumping, at a fixed delay of 400 fs; (b,c) Normalized Ni F for Fe-3p resonant and non-resonant pumping, at a fixed delay of 400 fs; (b,c) Normalized Ni −2 magnetization as a function of delay Δt for resonant and non-resonant pumping at F = 4 mJ· cm (b) magnetization as a function of delay Dt for resonant and non-resonant pumping at F = 4 mJcm (b) −2 and F = 10 mJ· cm (c). Error bars on Δt (±5 fs) are smaller than the symbol size. and F = 10 mJcm (c). Error bars on Dt (5 fs) are smaller than the symbol size. 4. Conclusions and Outlook 4. Conclusions and Outlook The observed different demagnetization behavior of the two Fe-Ni compounds under The observed different demagnetization behavior of the two Fe-Ni compounds under investigation can be qualitatively ascribed to the fundamental differences in their electronic investigation can be qualitatively ascribed to the fundamental differences in their electronic structures. structures. Permalloy is a ferromagnetic metallic alloy where Fe-3d and Ni-3d electrons, which Permalloy is a ferromagnetic metallic alloy where Fe-3d and Ni-3d electrons, which determine the determine the magnetic properties, pertain to strongly hybridized and delocalized orbitals featuring magnetic properties, pertain to strongly hybridized and delocalized orbitals featuring direct exchange. direct exchange. In the ferrimagnetic oxide NiFe2O4, the 3d electrons are much more localized onto In the ferrimagnetic oxide NiFe O , the 3d electrons are much more localized onto the respective 2 4 the respective atomic sites and the exchange is mainly mediated by oxygen. Releasing the pump atomic sites and the exchange is mainly mediated by oxygen. Releasing the pump energy to the energy to the ensemble of the 3d electrons (off-resonant pumping) or more selectively to the Fe site ensemble of the 3d electrons (off-resonant pumping) or more selectively to the Fe site via 3p-3d via 3p-3d core excitations (Fe-3p resonant pumping) is likely to make a difference in the Ni magnetic core excitations (Fe-3p resonant pumping) is likely to make a difference in the Ni magnetic response response when the 3d electrons are localized and interact via an indirect exchange, as it is the case when the 3d electrons are localized and interact via an indirect exchange, as it is the case for the for the Ni-ferrite. Our results show that the resonant/non-resonant character of the pump affects the Ni-ferrite. Our results show that the resonant/non-resonant character of the pump affects the degree degree of magnetization loss but not its delay dependence. Since no experimental or theoretical of magnetization loss but not its delay dependence. Since no experimental or theoretical studies have studies have addressed this kind of problem yet, we are not in a condition of interpreting this result addressed this kind of problem yet, we are not in a condition of interpreting this result at a more at a more fundamental level. fundamental level. The two-color mode developed at FERMI provides a versatile source of FEL twin-pulses with controlled wavelength tunability and time separation. The use of two optical laser seeds allows for the fast and independent control of the intensity and time separation of the two FEL pulses. Additionally, the inversion of the pump and probe wavelengths can be performed in a reasonable Photonics 2017, 4, 6 8 of 10 time lapse: the reconfiguration of the radiator modules for probing the Fe magnetic signal while pumping the Ni resonantly (Figure 8) took less than one hour. The red circles in Figure 8 represent the Fe demagnetization in the Ni-ferrite as a function of the Ni-3p resonant pump fluence. The blue circles represent the Fe-3p resonant probe fluence, showing that it is not affected by varying the Ni-resonant Photonics 2017, 4, 6 8 of 10 pump intensity over the same F range. Figure Figure 8. 8. Ni-3p Ni-3p pump/Fe-3p pump/Fe-3p p pr roobe be rr eesonant sonant m magnetic agnetic sscattering cattering in in N NiFe iFe2OO 4. T . h The e FeFe mmagnetic agnetic sig signal nal is 2 4 is measured 530 fs after the pump pulse as a function of the pump fluence (red circles, left axis). measured 530 fs after the pump pulse as a function of the pump fluence (red circles, left axis). The The Fe-resonant probe fluence (blue squares, right axis) is unaffected over the spanned range of Fe-resonant probe fluence (blue squares, right axis) is unaffected over the spanned range of pump pump fluence. fluence. Our first results with two magnetic compounds containing Fe and Ni illustrate the potential of the The two-color mode developed at FERMI provides a versatile source of FEL twin-pulses with on/off resonance FEL pump-probe experiments. In its present version, the twin-seeded two-color FEL controlled wavelength tunability and time separation. The use of two optical laser seeds allows for source at FERMI can cover the 3p resonances of Mn, Fe, Co, and Ni, making a wide class of important the fast and independent control of the intensity and time separation of the two FEL pulses. magnetic materials accessible for time resolved experiments. We stress that this kind of source can Additionally, the inversion of the pump and probe wavelengths can be performed in a reasonable find original applications well beyond ultrafast demagnetization studies, in many other fields of time lapse: the reconfiguration of the radiator modules for probing the Fe magnetic signal while condensed matter and of atomic and molecular physics. In general, it enables the excitation of a pumping the Ni resonantly (Figure 8) took less than one hour. The red circles in Figure 8 represent core resonance on a specific atomic site in a complex system with pulses of selected wavelength, the Fe demagnetization in the Ni-ferrite as a function of the Ni-3p resonant pump fluence. The blue intensity, and polarization, and makes it possible to probe the dynamic response at another atomic site circles represent the Fe-3p resonant probe fluence, showing that it is not affected by varying the with a second FEL pulse. Such multi-colour time-resolved experiments open unique opportunities Ni-resonant pump intensity over the same F range. for exploring complex relaxation processes, such as sequential multiple ionizations, multi-electron Our first results with two magnetic compounds containing Fe and Ni illustrate the potential of cascades, and charge transfer dynamics. the on/off resonance FEL pump-probe experiments. In its present version, the twin-seeded two-color FEL source at FERMI can cover the 3p resonances of Mn, Fe, Co, and Ni, making a wide class of Acknowledgments: We are grateful to Jan Vogel (Institut Néel, Grenoble), Giancarlo Panaccione (CNR-IOM, important magnetic materials accessible for time resolved experiments. We stress that this kind of Trieste), Fausto Sirotti (Synchrotron SOLEIL), Nicolas Moisan (LPS, Orsay), Michael Meyer (European XFEL, Hamburg), and Coryn F. Hague (LCPMR, Paris) for useful discussions and suggestions. This research received source can find original applications well beyond ultrafast demagnetization studies, in many other financial support from the European Community 7th Framework Programme under grant agreement No. 312284, fields of condensed matter and of atomic and molecular physics. In general, it enables the excitation and from CNRS (France) via the PEPS_SASLELX program. The FERMI project at Elettra—Sincrotrone Trieste is of a core resonance on a specific atomic site in a complex system with pulses of selected wavelength, supported by MIUR under grants FIRB-RBAP045JF2 and FIRB-RBAP06AWK3. intensity, and polarization, and makes it possible to probe the dynamic response at another atomic Author Contributions: E.F., C. Spezzani, G.D.N., M.B.D., E.A., and M.S. conceived and coordinated the site with a second FEL pulse. Such multi-colour time-resolved experiments open unique experiment. E.F., C. Spezzani, L.G., G.D.N., M.B.D., and E.A. designed the two-colour scheme. I.N., P.C., A.D., and M.B.D. operated the laser source. E.F., B.D., D.G., G.P., P.R.R., E.R., M.T., L.G., G.D.N., and E.A. operated opportunities for exploring complex relaxation processes, such as sequential multiple ionizations, the F.E.L. source. F.F., R.D., F.V., J.-B.M., and L.L. fabricated and characterized the samples. C. Svetina, M.Z., N.M., multi-electron cascades, and charge transfer dynamics. L.R., M.M., E.P., F.C., and M.K. contributed to the DIPROI setup. C. Spezzani, F.F., R.D., T.P., and M.S. prepared and performed the scattering experiment. E.F., C. Spezzani, F.V., M.K., G.D.N., M.B.D., E.A., and M.S. analysed the Ack data nowand ledgwr meote nts:the We manuscript, are gratefuwith l to Jcontributions an Vogel (Insfr tiom tut all Née authors. l, Grenoble), Giancarlo Panaccione (CNR-IOM, Trieste), Fausto Sirotti (Synchrotron SOLEIL), Nicolas Moisan (LPS, Orsay), Michael Meyer (European XFEL, Conflicts of Interest: The authors declare no conflict of interest. Hamburg), and Coryn F. Hague (LCPMR, Paris) for useful discussions and suggestions. This research received financial support from the European Community 7th Framework Programme under grant agreement No. 312284, and from CNRS (France) via the PEPS_SASLELX program. The FERMI project at Elettra—Sincrotrone Trieste is supported by MIUR under grants FIRB-RBAP045JF2 and FIRB-RBAP06AWK3. Author Contributions: E.F., C. Spezzani, G.D.N., M.B.D., E.A., and M.S. conceived and coordinated the experiment. E.F., C. Spezzani, L.G., G.D.N., M.B.D., and E.A. designed the two-colour scheme. I.N., P.C., A.D., and M.B.D. operated the laser source. E.F., B.D., D.G., G.P., P.R.R., E.R., M.T., L.G., G.D.N., and E.A. operated the F.E.L. source. F.F., R.D., F.V., J.-B.M., and L.L. fabricated and characterized the samples. C. Svetina, M.Z., N.M., L.R., M.M., E.P., F.C., and M.K. contributed to the DIPROI setup. C. Spezzani, F.F., R.D., T.P., and M.S. prepared and performed the scattering experiment. E.F., C. Spezzani, F.V., M.K., G.D.N., M.B.D., E.A., and M.S. analysed the data and wrote the manuscript, with contributions from all authors. Photonics 2017, 4, 6 9 of 10 References 1. Beaurepaire, E.; Merle, J.-C.; Daunois, A.; Bigot, J.-Y. 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hv photonics Article Element Selective Probe of the Ultra-Fast Magnetic Response to an Element Selective Excitation in Fe-Ni Compounds Using a Two-Color FEL Source 1 , 2 , † 1 , 3 4 5 6 Eugenio Ferrari , Carlo Spezzani , Franck Fortuna , Renaud Delaunay , Franck Vidal , 1 1 1 1 1 Ivaylo Nikolov , Paolo Cinquegrana , Bruno Diviacco , David Gauthier , Giuseppe Penco , 1 1 1 7 Primož Rebernik Ribic ˇ , Eléonore Roussel , Mauro Trovò , Jean-Baptiste Moussy , 8 6 , 9 1 , 10 1 , 11 Tommaso Pincelli , Lounès Lounis , Cristian Svetina , Marco Zangrando , 1 1 1 1 Nicola Mahne , Lorenzo Raimondi , Michele Manfredda , Emanuele Pedersoli , 1 1 1 , 12 1 Flavio Capotondi , Alexander Demidovich , Luca Giannessi , Maya Kiskinova , 1 , 13 1 1 , 6 , 14 , Giovanni De Ninno , Miltcho Boyanov Danailov , Enrico Allaria * and Maurizio Sacchi * ELETTRA—Sincrotrone Trieste, Area Science Park, 34149 Trieste, Italy; eugenio.ferrari@psi.ch (E.F.); carlo.spezzani@elettra.eu (C.S.); ivaylo.nikolov@elettra.eu (I.N.); paolo.cinquegrana@elettra.eu (P.C.); bruno.diviacco@elettra.eu (B.D.); david.gauthier@elettra.eu (D.G.); giuseppe.penco@elettra.eu (G.P.); primoz.rebernik@elettra.eu (P.R.R.); eleonore.roussel@elettra.eu (E.R.); mauro.trovo@elettra.eu (M.T.); Cristian.Svetina@elettra.eu (C.S.); Marco.Zangrando@elettra.eu (M.Z.); Nicola.Mahne@elettra.eu (N.M.); Lorenzo.Raimondi@elettra.eu (L.R.); michele.manfredda@elettra.eu (M.M.); emanuele.pedersoli@elettra.eu (E.P.); flavio.capotondi@elettra.eu (F.C.); alexander.demidovich@elettra.eu (A.D.); lucagiannessi@gmail.com (L.G.); maya.kiskinova@elettra.eu (M.K.); giovanni.deninno@elettra.eu (G.D.N.); miltcho.danailov@elettra.eu (M.B.D.) Dipartimento di Fisica, Università degli Studi di Trieste, 34127 Trieste, Italy Laboratoire de Physique des Solides, Université Paris-Sud, CNRS-UMR 8502, Bât. 510, 91405 Orsay, France Centre de Sciences Nucléaires et de Sciences de la Matière, Université Paris-Sud, CNRS UMR 8609, Bât. 104-108, 91405 Orsay, France; fortuna@csnsm.in2p3.fr Laboratoire de Chimie Physique Matière et Rayonnement, Sorbonne Universités, UPMC Univ Paris 06, CNRS UMR 7614, 75005 Paris, France; renaud.delaunay@upmc.fr Institut des NanoSciences de Paris, Sorbonne Universités, UPMC Univ Paris 06, CNRS UMR 7588, 75005 Paris, France; franck.vidal@insp.jussieu.fr (F.V.); lounis@insp.jussieu.fr (L.L.) Service de Physique de l’Etat Condensé, DSM/IRAMIS/SPEC, CNRS UMR 3680, CEA Saclay, 91191 Gif-sur-Yvette, France; jean-baptiste.moussy@cea.fr Dipartimento di Fisica, Università degli Studi di Milano, 20133 Milano, Italy; tommaso.pincelli@gmail.com Ecole Normale Supérieure, PSL Research University, 75231 Paris, France Graduate School of Nanotechnology, Università degli Studi di Trieste, 34127 Trieste, Italy Istituto Officina dei Materiali, Consiglio Nazionale delle Ricerche, 34149 Trieste, Italy ENEA, Centro Ricerche Frascati, Via E. Fermi 45, 00044 Frascati, Italy Laboratory of Quantum Optics, University of Nova Gorica, 5001 Nova Gorica, Slovenia Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, B.P. 48, 91192 Gif-sur-Yvette, France * Correspondence: enrico.allaria@elettra.eu (E.A.); maurizio.sacchi@synchrotron-soleil.fr (M.S.) † Current Address: Particle Accelerator Physics Laboratory, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland. Received: 18 December 2016; Accepted: 20 January 2017; Published: 26 January 2017 Abstract: The potential of the two-color mode implemented at the FERMI free-electron laser (FEL) source for pumping and probing selectively different atomic species has been demonstrated by time-resolved scattering experiments with permalloy (FeNi alloy) and NiFe O samples. 2 4 We monitored the ultra-fast demagnetization of Ni induced by the pump FEL pulse, by tuning the linearly-polarized FEL probe pulse to the Ni-3p resonance and measuring the scattered intensity in the transverse magneto-optical Kerr effect geometry. The measurements were performed by varying the intensity of the FEL pump pulse, tuning its wavelength to and off of the Fe-3p resonance, and by spanning the FEL probe pulse delays across the 300–900 fs range. The obtained results have Photonics 2017, 4, 6; doi:10.3390/photonics4010006 www.mdpi.com/journal/photonics Photonics 2017, 4, 6 2 of 10 evidenced that for the case of NiFe O , there is a sensible difference in the magnetic response at the 2 4 Ni site when the pump pulse causes electronic excitations at the Fe site. Keywords: free electron laser; two-color source; ultra-fast dynamics 1. Introduction The most common approach to ultra-fast demagnetization relies on optical-laser based time-resolved magneto-optical Kerr effect (MOKE) studies [1,2]. Modern pulsed X-ray sources, notably free-electron lasers (FEL) and high-harmonic generation sources, have introduced the use of X-rays as a complement to optical lasers. The most common set-up makes use of an optical laser as the pump and of an X-ray beam as the probe, providing element selectivity by fine-tuning the X-ray wavelength to a core resonance [3–6]. We developed a new scheme where, in a given multi-element magnetic sample, we pump one selected element and probe another element, also selectively, using two FEL pulses of different wavelengths [7]. The interest of this kind of experiment resides in the possibility of associating the pump energy with a specific electronic excitation of one component of a complex system. The combination of an element selective excitation with an element selective probe offers new paths for unraveling the fundamental mechanisms that drive magnetization loss by separately addressing the weight of possible contributions in this complex problem. Important classes of magnetic materials, in this respect, are ferrites [8,9] and transition metal (TM) rare-earth (RE) compounds [10–13]. Beyond their applicative interest, the (O-mediated) TM-TM coupling in magnetic oxides and the (5d-mediated) 3d-4f coupling in TM-RE compounds make these materials ideal candidates for resonant-pump resonant-probe magnetization dynamics studies. The highly localized 3d and 4f orbitals and the mediated coupling make it plausible that associating the pump energy with a specific electronic excitation can influence the magnetization dynamics profoundly, compared to using a non-resonant pump. 2. Materials and Methods 2.1. Source Design The conceptual and experimental development of two-color schemes prompted major research efforts at all FEL facilities worldwide [14–18], with the ambition of optimizing wavelength and timing control. In general, two-color sources are constrained by limited wavelength tunability. Recently, a new configuration of the FERMI FEL-1 seeded source [19] was implemented that delivers two time-delayed FEL pulses with different wavelengths, each independently tunable over a broad spectral range. The principle of the two-color double-resonant source was set out in reference [7]. Here we summarize its most relevant features. The twin-seed layout starts from a common ~780 nm laser source that produces two ultraviolet (UV) pulses along two distinct optical paths. One comprises a third-harmonic generator and a delay-line for adjusting the delay Dt between the two pulses. The other features an optical parametric amplifier (OPA) for adjusting the pulse wavelength over the 230–260 nm range. The former, which has a fixed 261.5 nm wavelength and a FWHM pulse duration t of ~120 fs, serves as a UV seed for seed producing the Ni-3p resonant FEL pulse. The latter, with t ~150 fs, is used for seeding the Fe-3p seed resonant FEL emission by setting its wavelength to 255 nm. The two UV seeds are recombined and kept along the same optical path by using a feed-back system based on multiple motorized piezoelectric tip-tilt devices. Figure 1 sketches the principle of the two-color resonant FEL mode used in our experiment. The two UV laser seeds are aligned with the electron beam trajectory within the FEL modulator section, where they interact with the electron bunch, stretched in time to ~1 ps length. The wavelength Photonics 2017, 4, 6 3 of 10 separation between the two seeds is smaller than the bandwidth accepted by the modulator (~3%), but larger than that accepted by the radiator (~0.7%). The FERMI FEL-1 radiator section is composed of six undulator modules. For most of the measurements, five modules (Rad_2 in Figure 1a) are tuned to the 11th harmonic of the 255 nm seed laser, generating a FEL pump pulse at 23.2 nm (53.5 eV), i.e., Photonics 2017, 4, 6 3 of 10 at the Fe-3p resonance (Figure 1a). By controlling the seed laser intensity via cross-polarizers, the of six undulator modules. For most of the measurements, five modules (Rad_2 in Figure 1a) are energy per FEL pulse spans the 0 to 10 J range, corresponding to a pump fluence (F) of 0–40 mJcm tuned to the 11th harmonic of the 255 nm seed laser, generating a FEL pump pulse at 23.2 nm (53.5 eV), at the sample position. Off-resonance pumping (Figure 1b) is obtained by tuning Rad_2 to the i.e., at the Fe-3p resonance (Figure 1a). By controlling the seed laser intensity via cross-polarizers, the −2 10th harmonic, corresponding to 25.5 nm (48.6 eV). The remaining radiator module (Rad_1) is tuned energy per FEL pulse spans the 0 to 10 µ J range, corresponding to a pump fluence (F) of 0–40 mJ· cm at the sample position. Off-resonance pumping (Figure 1b) is obtained by tuning Rad_2 to the 10th to the 14th harmonic of the 261.5 nm seed pulse, resulting in a FEL emission at 18.6 nm (67 eV, Ni-3p harmonic, corresponding to 25.5 nm (48.6 eV). The remaining radiator module (Rad_1) is tuned to resonance), with a maximum energy of ~0.8 J per pulse. We also tested a different source scheme the 14th harmonic of the 261.5 nm seed pulse, resulting in a FEL emission at 18.6 nm (67 eV, Ni-3p with inverted pump and probe wavelengths and an equal distribution of the radiator modules on resonance), with a maximum energy of ~0.8 µ J per pulse. We also tested a different source scheme the Rad_1 and Rad_2 sub-sections (Figure 1c). The delay Dt between the two UV seeds, measured with inverted pump and probe wavelengths and an equal distribution of the radiator modules on by a cross-corr the Rad_1 elator and ,Rcan ad_2 be sub adjusted -sections (F over igure the 1c). Th 300–900 e delay Δ fs t bet range weenwith the twnegligible o UV seeds, (<5 measu fs) red jitter by [20,21]. a cross-correlator, can be adjusted over the 300–900 fs range with negligible (<5 fs) jitter [20,21]. The The upper limit is imposed by the electron bunch length of ~1 ps. Cross-talk and instabilities are upper limit is imposed by the electron bunch length of ~1 ps. Cross-talk and instabilities are observed in the double FEL emission below 300 fs separation, hampering the collection of reliable data observed in the double FEL emission below 300 fs separation, hampering the collection of reliable at shorter delays. The duration t of each FEL pulse can be conservatively estimated from t by FEL seed data at shorter delays. The duration tFEL of each FEL pulse can be conservatively estimated from tseed 0.3 the scaling law t = t  h ,−0h .3 being the radiator harmonic. Therefore the FEL pulses in our FEL by the scaling law seed tFEL = tseed × h , h being the radiator harmonic. Therefore the FEL pulses in our experiment expe ar rim e e expected nt are expeto ctehave d to ha a ve 50 a 5 to 0 t70 o 70 fs fsduration, duration, fo for r an an ovoverall erall timetime resolu resolution tion better th better an 100 than fs. 100 fs. Figure 1. Twin-seed split-radiator source scheme. (a) Ni-3p resonant probe, Fe-3p resonant pump. Figure 1. Twin-seed split-radiator source scheme. (a) Ni-3p resonant probe, Fe-3p resonant pump. The two UV seed pulses, controlled in intensity and separated by a well-defined time delay Δt, The two UV seed pulses, controlled in intensity and separated by a well-defined time delay Dt, interact interact with the electron bunch within the modulator (Mod) section. After going through the with the electron bunch within the modulator (Mod) section. After going through the dispersive section dispersive section (DS), the electron excitation induced by the probe seed pulse (λseed_1 = 261.5 nm) is (DS), theaelectr mplifieon d bexcitation y the first oinduced f the six ra by diathe tor m pr od obe ulesseed (Rad_pulse 1) tune(dl to harm =on 261.5 ic 14 nm) (h_14)is , g amplifi enerating ed by the seed_1 18.6 nm free-electron laser (FEL) radiation (67 eV, Ni-3p resonance). This excitation is not amplified first of the six radiator modules (Rad_1) tuned to harmonic 14 (h_14), generating 18.6 nm free-electron by the five other radiator modules (Rad_2). Conversely, the UV pump seed (λseed_2 = 255 nm) induces laser (FEL) radiation (67 eV, Ni-3p resonance). This excitation is not amplified by the five other radiator an excitation of the electron bunch that is amplified by tuning Rad_2 to the 11th harmonic, modules (Rad_2). Conversely, the UV pump seed (l = 255 nm) induces an excitation of the seed_2 generating a 23.2 nm FEL pulse (53.5 eV, Fe-3p resonance), but this excitation is not amplified by electron bunch that is amplified by tuning Rad_2 to the 11th harmonic, generating a 23.2 nm FEL Rad_1. The source delivers two FEL pulses tuned in energy to the Fe-3p and Ni-3p resonances, with pulse (53.5 eV, Fe-3p resonance), but this excitation is not amplified by Rad_1. The source delivers two controlled intensities and time separation; (b) Ni-3p resonant probe, non-resonant pump. The same FEL pulses seed tuned ing schem in e ener as in gy (a) to appl the ieFe-3p s, but the and Rad_2 Ni-3p radia resonances, tor sub-sectiowith n is tucontr ned to olled h_10 of intensities the 255 nm and time pump seed, generating 25.5 nm FEL radiation, whose photon energy is insufficient to photo-excite separation; (b) Ni-3p resonant probe, non-resonant pump. The same seeding scheme as in (a) applies, the core electrons of either Fe or Ni; (c) Fe-3p resonant probe, Ni-3p resonant pump. The but the Rad_2 radiator sub-section is tuned to h_10 of the 255 nm pump seed, generating 25.5 nm FEL wavelengths of the two UV seeds are inverted (λseed_1 = 255 nm, λseed_2 = 261.5 nm) and the radiator is radiation, whose photon energy is insufficient to photo-excite the core electrons of either Fe or Ni; split in two sub-sections of three modules each. Rad_1 provides the 23.3 nm FEL probe pulse tuned (c) Fe-3p resonant probe, Ni-3p resonant pump. The wavelengths of the two UV seeds are inverted to the Fe-3p resonance, while Rad_2 generates the Ni-3p resonant 18.6 nm pump pulse. (l = 255 nm, l = 261.5 nm) and the radiator is split in two sub-sections of three modules seed_1 seed_2 each. Rad_1 provides the 23.3 nm FEL probe pulse tuned to the Fe-3p resonance, while Rad_2 generates the Ni-3p resonant 18.6 nm pump pulse. Photonics 2017, 4, 6 4 of 10 Photonics 2017, 4, 6 4 of 10 2.2. Experimental Details 2.2. Experimental Details The measurements were performed at the DiProI beamline [22,23] of the FERMI FEL source, The measurements were performed at the DiProI beamline [22,23] of the FERMI FEL source, using the IRMA (Instrument pour la Réfléctivité MAgnétique) scattering chamber [24]. Figure 2 using the IRMA (Instrument pour la Réfléctivité MAgnétique) scattering chamber [24]. Figure 2 shows shows a sketch of the experimental set-up and of the data collection method. After aligning the a sketch of the experimental set-up and of the data collection method. After aligning the sample sample using the two-axis goniometer, the pump-probe results were monitored using a using the two-axis goniometer, the pump-probe results were monitored using a charge-coupled device charge-coupled device (CCD) detector. The grating-like structure of the samples separated the pump (CCD) detector. The grating-like structure of the samples separated the pump and probe contributions and probe contributions spatially, allowing us to discriminate the diffracted intensities at the two spatially, allowing us to discriminate the diffracted intensities at the two wavelengths. The diffraction wavelengths. The diffraction peaks corresponding to the pump and probe FEL wavelengths were peaks corresponding to the pump and probe FEL wavelengths were collected simultaneously on collected simultaneously on the CCD. Blanking out one of the FEL beams allowed us to check for the CCD. Blanking out one of the FEL beams allowed us to check for cross-talking between them. cross-talking between them. We confirmed its absence for delays exceeding 300 fs, while interference We confirmed its absence for delays exceeding 300 fs, while interference between the two sources was between the two sources was observed at shorter delays. In order to maximize the magnetic signal, observed at shorter delays. In order to maximize the magnetic signal, we worked in transverse-MOKE we worked in transverse-MOKE geometry [25,26], setting the incidence angle to 46.5°, i.e., close to geometry [25,26], setting the incidence angle to 46.5 , i.e., close to the Brewster angle. We used linear the Brewster angle. We used linear vertical polarization of the FEL pulses, taking advantage of the vertical polarization of the FEL pulses, taking advantage of the Apple-II undulators in the FERMI Apple-II undulators in the FERMI radiator [27]. radiator [27]. Figure 2. Schematics of the scattering measurement setup. The grating samples are mounted Figure 2. Schematics of the scattering measurement setup. The grating samples are mounted in a in a reflectometer featuring a vertical scattering plane. A magnetic field (up to 1.5 kOe pulsed, reflectometer featuring a vertical scattering plane. A magnetic field (up to 1.5 kOe pulsed, 500 Oe 500 Oe permanent) can be applied normal to the scattering plane and parallel to the sample permanent) can be applied normal to the scattering plane and parallel to the sample surface by a surface by a horseshoe electromagnet. The FEL pulses with linear vertical polarization impinge horseshoe electromagnet. The FEL pulses with linear vertical polarization impinge on the sample on the sample grating at 46.5 and are diffracted at different angles according to their wavelengths. grating at 46.5° and are diffracted at different angles according to their wavelengths. Diffracted Diffracted intensities from the pump and probe FEL pulses are collected simultaneously by using a intensities from the pump and probe FEL pulses are collected simultaneously by using a two-dimensional charge-coupled device detector. two-dimensional charge-coupled device detector. We measured two samples. The first is a 20 nm permalloy (Py) film, deposited on a 605 nm period We measured two samples. The first is a 20 nm permalloy (Py) film, deposited on a 605 nm Si grating and protected by a 3 nm Al capping layer (Figure 3a, FEL data). Its magnetic properties were period Si grating and protected by a 3 nm Al capping layer (Figure 3a, FEL data). Its magnetic characterized by MOKE using an optical laser (Figure 3b). The second is a 12.5 nm thick Ni-ferrite properties were characterized by MOKE using an optical laser (Figure 3b). The second is a 12.5 nm (NiFe O ) layer grown epitaxially on MgAl O (001) [28]. A 100  400 m area of the layer was ruled thick Ni-ferrite (NiFe2O4) layer grown epitaxially on MgAl2O4(001) [28]. A 100 × 400 µ m area of the 2 4 2 4 by focused ion beam (FIB) etching into a set of ~350 nm wide stripes with a ~600 nm period (Figure 3c). layer was ruled by focused ion beam (FIB) etching into a set of ~350 nm wide stripes with a ~600 nm Magnetization curves showed a coercive field of ~500 Oe with >80% remanence (Figure 3d, FEL data). period (Figure 3c). Magnetization curves showed a coercive field of ~500 Oe with >80% remanence It is worth stressing that the fraction of pump energy absorbed in the Ni-ferrite sample is the same (Figure 3d, FEL data). It is worth stressing that the fraction of pump energy absorbed in the (~78% for the 12.5 nm thick film at 46.5 incidence) for both resonant (22.3 nm) and non-resonant Ni-ferrite sample is the same (~78% for the 12.5 nm thick film at 46.5° incidence) for both resonant (25.5 nm) pump wavelengths. (22.3 nm) and non-resonant (25.5 nm) pump wavelengths. The Ni magnetic signal at different pump-probe delays was measured as a function of the The Ni magnetic signal at different pump-probe delays was measured as a function of the pump fluence F, for both Fe-3p resonant and non-resonant pump wavelengths, using the acquisition pump fluence F, for both Fe-3p resonant and non-resonant pump wavelengths, using the acquisition procedure sketched in Figure 4. procedure sketched in Figure 4. A reliable analysis of the demagnetization in a pump-probe experiment requires the sample to be A reliable analysis of the demagnetization in a pump-probe experiment requires the sample to be homogeneously pumped over the probed area. From the images in Figures 4 and 5, it is apparent that homogeneously pumped over the probed area. From the images in Figures 4 and 5, it is apparent that this is not the case if one integrates the scattered probe signal over the entire diffraction spot, since both the pump and the probe have an inhomogeneous spatial distribution of the intensity. In order to avoid Photonics 2017, 4, 6 5 of 10 this is not the case if one integrates the scattered probe signal over the entire diffraction spot, since Photonics 2017, 4, 6 5 of 10 Photonics 2017, 4, 6 5 of 10 both the pump and the probe have an inhomogeneous spatial distribution of the intensity. In order to averaging the probe signal over areas that are pumped differently, the magnetic signal reported in the avoid averaging the probe signal over areas that are pumped differently, the magnetic signal reported averaging the probe signal over areas that are pumped differently, the magnetic signal reported in the following corresponds to the probe intensity scattered within a detector portion of 7 × 7 pixels (Figure 5) in the following corresponds to the probe intensity scattered within a detector portion of 7  7 pixels following corresponds to the probe intensity scattered within a detector portion of 7 × 7 pixels (Figure 5) that measures the homogeneously pumped area. (Figure 5) that measures the homogeneously pumped area. that measures the homogeneously pumped area. Figure 3. Permalloy and Ni-ferrite samples. (a) Rocking scan of the Py grating sample, showing the Figure 3. Permalloy and Ni-ferrite samples. (a) Rocking scan of the Py grating sample, showing the Bragg peaks corresponding to the Fe-3p resonant pump and to the Ni-3p resonant probe FEL beams; Figure 3. Permalloy and Ni-ferrite samples. (a) Rocking scan of the Py grating sample, showing the Bragg peaks corresponding to the Fe-3p resonant pump and to the Ni-3p resonant probe FEL beams; (b) Hyster Bragg esis peaks loop corrof esponding to the Py magnetization the Fe-3p reso measur nant pu ed mp an by magneto-optical d to the Ni-3p resonan Kerr t probe effectFEL (MOKE), beams;with (b) Hysteresis loop of the Py magnetization measured by magneto-optical Kerr effect (MOKE), with (b) Hysteresis loop of the Py magnetization measured by magneto-optical Kerr effect (MOKE), with the magnetic the magneti field c fiel app d applie lied parallel d parallelto tothe the grating grating li lines; nes; (c (c ) ) Scan Scanning ning elec electr tron on mic micr roscopy oscopy image image of the of the the magnetic field applied parallel to the grating lines; (c) Scanning electron microscopy image of the Ni-ferrite Ni-ferrite sample. sampAn le. An ar ea area of of 100 100 ×400 400  µm m is ispatterned into patterned into a ~600 a ~600 nm nm peri period od grati grating ng by foc by used focused ion ion Ni-ferrite sample. An area of 100 × 400 µ m is patterned into a ~600 nm period grating by focused ion beam etching; (d) Field dependence of the FEL radiation intensity diffracted at the Fe-3p resonance, beam etching; (d) Field dependence of the FEL radiation intensity diffracted at the Fe-3p resonance, beam etching; (d) Field dependence of the FEL radiation intensity diffracted at the Fe-3p resonance, measuring the magnetic response over the ruled area of the Ni-ferrite. measuring the magnetic response over the ruled area of the Ni-ferrite. measuring the magnetic response over the ruled area of the Ni-ferrite. Figure 4. Data acquisition sequence. Sketch of the acquisition sequence used for determining the Figure Figure 4. Data 4. Data acquisition acquisitiosequence. n sequence.Sketch Sketch of of the the acquisi acquisition tion se sequence quence used used for for deter determining mining the the magnetic signal. (a) After a +800 Oe pulse of ~10 ms duration, data are collected in a +200 Oe applied magnetic signal. (a) After a +800 Oe pulse of ~10 ms duration, data are collected in a +200 Oe applied magnetic field; signal. (b) Same (a) as After (a), for a +800 negative Oe pulse field of va~10 lues. ms The duration, magnetic data signar al eis collected defined as in a the +200 diffOe erence applied field; (b) Same as (a), for negative field values. The magnetic signal is defined as the difference divided by the sum of the signals measured for opposite magnetization directions. Top panels: CCD field; (b) Same as (a), for negative field values. The magnetic signal is defined as the difference divided divided by the sum of the signals measured for opposite magnetization directions. Top panels: CCD images of the pump (left spot) and probe (right spot) scattered intensities. by the sum of the signals measured for opposite magnetization directions. Top panels: CCD images of images of the pump (left spot) and probe (right spot) scattered intensities. the pump (left spot) and probe (right spot) scattered intensities. Photonics 2017, 4, 6 6 of 10 Photonics 2017, 4, 6 6 of 10 Photonics 2017, 4, 6 6 of 10 Figure 5. Measurement area selection. Fe-3p resonant pump (a) and Ni-3p resonant probe (b) diffracted Figure Figure 5.5. Mea Mea surem surem ent ent area area se se le le ct ct io io n. n. Fe Fe--3p 3p resonant resonant p pum ump p ( (a a) ) and and Ni Ni--3p 3p re re so so nan nan t t probe probe (b (b ) ) intensity from the Ni-ferrite sample. The two images are shown on different color scales. The gray didi ffracte ffracte d d inte inte nsit nsit y y from from the the Ni Ni -ferrite -ferrite sample. sample. The The two two iim mages ages are are sho shown wn on on di di fferen fferen t t co co lo lo r rsc sc aa le le s.s . square represents the 7  7 pixels area used for Ni-magnetization analysis, where the pump fluence is The gray The gray square represe square represe nt nt s the 7 s the 7 × × 7 7 pixel pixel ss area use area used d for for Ni Ni- -m magn agneti etizatio zation n analysis analysis , ,wher wher e e the the pu pu m m p p assumed to be homogeneous. fluence is assumed to be homogeneous. fluence is assumed to be homogeneous. 3.3. Results Results 3. Results Figure 6 shows the dependence of the Ni magnetic signal on the pump fluence at a probe delay of Figure 6 shows the dependence of the Ni magnetic signal on the pump fluence at a probe delay Figure 6 shows the dependence of the Ni magnetic signal on the pump fluence at a probe delay of −2 420 fs in Py. The pump fluence at the sample (F, top scale in Figure 6, in mJ· cm −) 2 is obtained by of 420 fs in Py. The pump fluence at the sample (F, top scale in Figure 6, in mJcm ) is obtained by 420 fs in Py. The pump fluence at the sample (F, top scale in Figure 6, in mJ· cm ) is obtained by correcting the pump energy measured at the exit of the FEL source (bottom scale in Figure 6, in µ J) for correcting the pump energy measured at the exit of the FEL source (bottom scale in Figure 6, in J) for correcting the pump energy measured at the exit of the FEL source (bottom scale in Figure 6, in µ J) for the transport line transmission (a factor of 0.4, accounting for six reflections and a 200 nm Al filter), for the transport line transmission (a factor of 0.4, accounting for six reflections and a 200 nm Al filter), the transport line transmission (a factor of 0.4, accounting for six reflections and a 200 nm Al filter), for the focal spot size (~80 µ m), and for the angle of incidence (46.5°). The Ni magnetic signal is the for the focal spot size (~80 m), and for the angle of incidence (46.5 ). The Ni magnetic signal is the the focal spot size (~80 µ m), and for the angle of incidence (46.5°). The Ni magnetic signal is the asymmetry ratio in the Bragg peak intensity, i.e., the difference between the scattered intensities for asymmetry ratio in the Bragg peak intensity, i.e., the difference between the scattered intensities for asymmetry ratio in the Bragg peak intensity, i.e., the difference between the scattered intensities for opposite signs of the saturation magnetization direction, divided by their sum (see Figure 4). Figure 6 opposite signs of the saturation magnetization direction, divided by their sum (see Figure 4). Figure 6 opposite signs of the saturation magnetization direction, divided by their sum (see Figure 4). Figure 6 shows the loss in Ni magnetic signal with respect to the static value measured without the pump. shows the loss in Ni magnetic signal with respect to the static value measured without the pump. shows the loss in Ni magnetic signal with respect to the static value measured without the pump. The red and blue dots refer to the Fe-3p resonant (23.2 nm) and non-resonant (25.5 nm) pumping, The red and blue dots refer to the Fe-3p resonant (23.2 nm) and non-resonant (25.5 nm) pumping, The red and blue dots refer to the Fe-3p resonant (23.2 nm) and non-resonant (25.5 nm) pumping, respectively. The results do not evidence any clear dependence on the pump wavelength; both respectively. The results do not evidence any clear dependence on the pump wavelength; both curves respectively. The results do not evidence any clear dependence on the pump wavelength; both −2 curves show the same trend of the Ni demagnetization with F, attaining a ~50% reduction at 10 mJ· cm . −2 show the same trend of the Ni demagnetization with F, attaining a ~50% reduction at 10 mJcm . curves show the same trend of the Ni demagnetization with F, attaining a ~50% reduction at 10 mJ· cm . Figure 6. Ni magnetic response in permalloy. F dependence of the Ni magnetization loss in Py, 420 fs Figure 6. Ni magnetic response in permalloy. F dependence of the Ni magnetization loss in Py, 420 fs after the pump pulse. The red circles and blue squares refer to 53.5 eV Fe-3p resonant and to 48.6 eV after the pump pulse. The red circles and blue squares refer to 53.5 eV Fe-3p resonant and to 48.6 eV Figure 6. Ni magnetic response in permalloy. F dependence of the Ni magnetization loss in Py, 420 fs non-resonant pump pulses, respectively. The pump fluence at the sample (top scale) is estimated non-resonant pump pulses, respectively. The pump fluence at the sample (top scale) is estimated after the pump pulse. The red circles and blue squares refer to 53.5 eV Fe-3p resonant and to 48.6 eV from the pump intensity measured at the exit of the source (bottom scale). The Ni magnetization loss from the pump intensity measured at the exit of the source (bottom scale). The Ni magnetization non-resonant pump pulses, respectively. The pump fluence at the sample (top scale) is estimated represents the decrease in the asymmetry ratio signal at the Bragg peak with respect to the non-pumped represents the decrease in the asymmetry ratio signal at the Bragg peak with respect to the from the pump intensity measured at the exit of the source (bottom scale). The Ni magnetization condition (black triangles). The black vertical bar corresponds to one standard deviation in the value of non-pumped condition (black triangles). The black vertical bar corresponds to one standard represents the decrease in the asymmetry ratio signal at the Bragg peak with respect to the the measured magnetic signal. deviation in the value of the measured magnetic signal. non-pumped condition (black triangles). The black vertical bar corresponds to one standard deviation in the value of the measured magnetic signal. Photonics 2017, 4, 6 7 of 10 Photonics 2017, 4, 6 7 of 10 The same experiment was performed with the NiFe2O4 sample. First, we verified that the epitaxial oxide layer on a low thermal conductivity insulating substrate could stand our pump-probe The same experiment was performed with the NiFe O sample. First, we verified that the epitaxial 2 4 conditions. Then we checked that the chosen 12.5 nm thickness, ensuring homogeneous absorption of oxide layer on a low thermal conductivity insulating substrate could stand our pump-probe conditions. the pump energy, provided a convenient magnetic signal in the diffracted intensity. We found that Then we checked that the chosen 12.5 nm thickness, ensuring homogeneous absorption of the pump inverting the saturation magnetization direction provides an excellent 80% asymmetry ratio at Ni-3p energy, provided a convenient magnetic signal in the diffracted intensity. We found that inverting the resonance. saturation magnetization direction provides an excellent 80% asymmetry ratio at Ni-3p resonance. Figure 7a shows the Ni magnetization loss after 400 fs as a function of F. There is a clear Figure 7a shows the Ni magnetization loss after 400 fs as a function of F. There is a clear difference difference between the data obtained using a non-resonant (blue dots) and an Fe-3p resonant (red between the data obtained using a non-resonant (blue dots) and an Fe-3p resonant (red dots) pump dots) pump pulse, showing the influence of the pump wavelength on the Ni demagnetization. This pulse, showing the influence of the pump wavelength on the Ni demagnetization. This sensitivity of sensitivity of the Ni-ferrite sample to the Fe-3p resonant and off-resonant pumping is also confirmed the Ni-ferrite sample to the Fe-3p resonant and off-resonant pumping is also confirmed by the results by the results obtained as a function of the pump-probe delay reported in Figure 7b,c. Dependence obtained as a function of the pump-probe delay reported in Figure 7b,c. Dependence on the pump −2 on the pump wavelength is observed already at the lower fluence F = 4 mJ· cm (Figure 7b), and it wavelength is observed already at the lower fluence F = 4 mJcm (Figure 7b), and it becomes much 2 −2 becomes much more evident and well beyond experimental uncertainty at F = 10 mJ· cm (Figure 7c). more evident and well beyond experimental uncertainty at F = 10 mJcm (Figure 7c). Figure 7b,c Figure 7b,c shows that for both F values, the quenching of the Ni magnetization is initially rather shows that for both F values, the quenching of the Ni magnetization is initially rather slow and reaches slow and reaches a maximum after ~500 fs. a maximum after ~500 fs. Figure 7. Ni magnetic response in Ni-ferrite. (a) Relative Ni magnetization loss as a function of F for Figure 7. Ni magnetic response in Ni-ferrite. (a) Relative Ni magnetization loss as a function of Fe-3p resonant and non-resonant pumping, at a fixed delay of 400 fs; (b,c) Normalized Ni F for Fe-3p resonant and non-resonant pumping, at a fixed delay of 400 fs; (b,c) Normalized Ni −2 magnetization as a function of delay Δt for resonant and non-resonant pumping at F = 4 mJ· cm (b) magnetization as a function of delay Dt for resonant and non-resonant pumping at F = 4 mJcm (b) −2 and F = 10 mJ· cm (c). Error bars on Δt (±5 fs) are smaller than the symbol size. and F = 10 mJcm (c). Error bars on Dt (5 fs) are smaller than the symbol size. 4. Conclusions and Outlook 4. Conclusions and Outlook The observed different demagnetization behavior of the two Fe-Ni compounds under The observed different demagnetization behavior of the two Fe-Ni compounds under investigation can be qualitatively ascribed to the fundamental differences in their electronic investigation can be qualitatively ascribed to the fundamental differences in their electronic structures. structures. Permalloy is a ferromagnetic metallic alloy where Fe-3d and Ni-3d electrons, which Permalloy is a ferromagnetic metallic alloy where Fe-3d and Ni-3d electrons, which determine the determine the magnetic properties, pertain to strongly hybridized and delocalized orbitals featuring magnetic properties, pertain to strongly hybridized and delocalized orbitals featuring direct exchange. direct exchange. In the ferrimagnetic oxide NiFe2O4, the 3d electrons are much more localized onto In the ferrimagnetic oxide NiFe O , the 3d electrons are much more localized onto the respective 2 4 the respective atomic sites and the exchange is mainly mediated by oxygen. Releasing the pump atomic sites and the exchange is mainly mediated by oxygen. Releasing the pump energy to the energy to the ensemble of the 3d electrons (off-resonant pumping) or more selectively to the Fe site ensemble of the 3d electrons (off-resonant pumping) or more selectively to the Fe site via 3p-3d via 3p-3d core excitations (Fe-3p resonant pumping) is likely to make a difference in the Ni magnetic core excitations (Fe-3p resonant pumping) is likely to make a difference in the Ni magnetic response response when the 3d electrons are localized and interact via an indirect exchange, as it is the case when the 3d electrons are localized and interact via an indirect exchange, as it is the case for the for the Ni-ferrite. Our results show that the resonant/non-resonant character of the pump affects the Ni-ferrite. Our results show that the resonant/non-resonant character of the pump affects the degree degree of magnetization loss but not its delay dependence. Since no experimental or theoretical of magnetization loss but not its delay dependence. Since no experimental or theoretical studies have studies have addressed this kind of problem yet, we are not in a condition of interpreting this result addressed this kind of problem yet, we are not in a condition of interpreting this result at a more at a more fundamental level. fundamental level. The two-color mode developed at FERMI provides a versatile source of FEL twin-pulses with controlled wavelength tunability and time separation. The use of two optical laser seeds allows for the fast and independent control of the intensity and time separation of the two FEL pulses. Additionally, the inversion of the pump and probe wavelengths can be performed in a reasonable Photonics 2017, 4, 6 8 of 10 time lapse: the reconfiguration of the radiator modules for probing the Fe magnetic signal while pumping the Ni resonantly (Figure 8) took less than one hour. The red circles in Figure 8 represent the Fe demagnetization in the Ni-ferrite as a function of the Ni-3p resonant pump fluence. The blue circles represent the Fe-3p resonant probe fluence, showing that it is not affected by varying the Ni-resonant Photonics 2017, 4, 6 8 of 10 pump intensity over the same F range. Figure Figure 8. 8. Ni-3p Ni-3p pump/Fe-3p pump/Fe-3p p pr roobe be rr eesonant sonant m magnetic agnetic sscattering cattering in in N NiFe iFe2OO 4. T . h The e FeFe mmagnetic agnetic sig signal nal is 2 4 is measured 530 fs after the pump pulse as a function of the pump fluence (red circles, left axis). measured 530 fs after the pump pulse as a function of the pump fluence (red circles, left axis). The The Fe-resonant probe fluence (blue squares, right axis) is unaffected over the spanned range of Fe-resonant probe fluence (blue squares, right axis) is unaffected over the spanned range of pump pump fluence. fluence. Our first results with two magnetic compounds containing Fe and Ni illustrate the potential of the The two-color mode developed at FERMI provides a versatile source of FEL twin-pulses with on/off resonance FEL pump-probe experiments. In its present version, the twin-seeded two-color FEL controlled wavelength tunability and time separation. The use of two optical laser seeds allows for source at FERMI can cover the 3p resonances of Mn, Fe, Co, and Ni, making a wide class of important the fast and independent control of the intensity and time separation of the two FEL pulses. magnetic materials accessible for time resolved experiments. We stress that this kind of source can Additionally, the inversion of the pump and probe wavelengths can be performed in a reasonable find original applications well beyond ultrafast demagnetization studies, in many other fields of time lapse: the reconfiguration of the radiator modules for probing the Fe magnetic signal while condensed matter and of atomic and molecular physics. In general, it enables the excitation of a pumping the Ni resonantly (Figure 8) took less than one hour. The red circles in Figure 8 represent core resonance on a specific atomic site in a complex system with pulses of selected wavelength, the Fe demagnetization in the Ni-ferrite as a function of the Ni-3p resonant pump fluence. The blue intensity, and polarization, and makes it possible to probe the dynamic response at another atomic site circles represent the Fe-3p resonant probe fluence, showing that it is not affected by varying the with a second FEL pulse. Such multi-colour time-resolved experiments open unique opportunities Ni-resonant pump intensity over the same F range. for exploring complex relaxation processes, such as sequential multiple ionizations, multi-electron Our first results with two magnetic compounds containing Fe and Ni illustrate the potential of cascades, and charge transfer dynamics. the on/off resonance FEL pump-probe experiments. In its present version, the twin-seeded two-color FEL source at FERMI can cover the 3p resonances of Mn, Fe, Co, and Ni, making a wide class of Acknowledgments: We are grateful to Jan Vogel (Institut Néel, Grenoble), Giancarlo Panaccione (CNR-IOM, important magnetic materials accessible for time resolved experiments. We stress that this kind of Trieste), Fausto Sirotti (Synchrotron SOLEIL), Nicolas Moisan (LPS, Orsay), Michael Meyer (European XFEL, Hamburg), and Coryn F. Hague (LCPMR, Paris) for useful discussions and suggestions. This research received source can find original applications well beyond ultrafast demagnetization studies, in many other financial support from the European Community 7th Framework Programme under grant agreement No. 312284, fields of condensed matter and of atomic and molecular physics. In general, it enables the excitation and from CNRS (France) via the PEPS_SASLELX program. The FERMI project at Elettra—Sincrotrone Trieste is of a core resonance on a specific atomic site in a complex system with pulses of selected wavelength, supported by MIUR under grants FIRB-RBAP045JF2 and FIRB-RBAP06AWK3. intensity, and polarization, and makes it possible to probe the dynamic response at another atomic Author Contributions: E.F., C. Spezzani, G.D.N., M.B.D., E.A., and M.S. conceived and coordinated the site with a second FEL pulse. Such multi-colour time-resolved experiments open unique experiment. E.F., C. Spezzani, L.G., G.D.N., M.B.D., and E.A. designed the two-colour scheme. I.N., P.C., A.D., and M.B.D. operated the laser source. E.F., B.D., D.G., G.P., P.R.R., E.R., M.T., L.G., G.D.N., and E.A. operated opportunities for exploring complex relaxation processes, such as sequential multiple ionizations, the F.E.L. source. F.F., R.D., F.V., J.-B.M., and L.L. fabricated and characterized the samples. C. Svetina, M.Z., N.M., multi-electron cascades, and charge transfer dynamics. L.R., M.M., E.P., F.C., and M.K. contributed to the DIPROI setup. C. Spezzani, F.F., R.D., T.P., and M.S. prepared and performed the scattering experiment. E.F., C. Spezzani, F.V., M.K., G.D.N., M.B.D., E.A., and M.S. analysed the Ack data nowand ledgwr meote nts:the We manuscript, are gratefuwith l to Jcontributions an Vogel (Insfr tiom tut all Née authors. l, Grenoble), Giancarlo Panaccione (CNR-IOM, Trieste), Fausto Sirotti (Synchrotron SOLEIL), Nicolas Moisan (LPS, Orsay), Michael Meyer (European XFEL, Conflicts of Interest: The authors declare no conflict of interest. Hamburg), and Coryn F. Hague (LCPMR, Paris) for useful discussions and suggestions. This research received financial support from the European Community 7th Framework Programme under grant agreement No. 312284, and from CNRS (France) via the PEPS_SASLELX program. The FERMI project at Elettra—Sincrotrone Trieste is supported by MIUR under grants FIRB-RBAP045JF2 and FIRB-RBAP06AWK3. Author Contributions: E.F., C. Spezzani, G.D.N., M.B.D., E.A., and M.S. conceived and coordinated the experiment. E.F., C. Spezzani, L.G., G.D.N., M.B.D., and E.A. designed the two-colour scheme. I.N., P.C., A.D., and M.B.D. operated the laser source. E.F., B.D., D.G., G.P., P.R.R., E.R., M.T., L.G., G.D.N., and E.A. operated the F.E.L. source. F.F., R.D., F.V., J.-B.M., and L.L. fabricated and characterized the samples. C. Svetina, M.Z., N.M., L.R., M.M., E.P., F.C., and M.K. contributed to the DIPROI setup. C. Spezzani, F.F., R.D., T.P., and M.S. prepared and performed the scattering experiment. E.F., C. Spezzani, F.V., M.K., G.D.N., M.B.D., E.A., and M.S. analysed the data and wrote the manuscript, with contributions from all authors. Photonics 2017, 4, 6 9 of 10 References 1. Beaurepaire, E.; Merle, J.-C.; Daunois, A.; Bigot, J.-Y. 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Published: Jan 26, 2017

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