© 2017 Author(s). Using THz emission spectroscopy, we detect spin-photo-currents from a ferrimagnetic amorphous alloy GdFeCo to an adjacent Pt capping layer. The currents are generated upon excitation of a GdFeCo/Pt heterostructure with femtosecond laser pulses. It is found that the polarization of the spin-polarized current is determined by magnetic sublattice sensitivity rather than the total magnetization, allowing for spin-polarized current generation when the net magnetization is zero

Ciccarelli, C

Huisman, TJ

Kimel, AV

Mikhaylovskiy, RV

Rasing, T

Tsukamoto, A

English

Huisman, T.J.

Ciccarelli, C.

Mikhaylovskiy, R.

Rasing, T.H.M.

Kimel, A.V.

NARCIS 

Spin-photo-currents generated by femtosecond laser pulses in a ferrimagnetic gdfeco/pt bilayer

Apollo (Cambridge)

Appl. Phys. Lett. 110, 072402 (2017); https://doi.org/10.1063/1.4976202 110, 072402© 2017 Author(s).Spin-photo-currents generated byfemtosecond laser pulses in a ferrimagneticGdFeCo/Pt bilayerCite as: Appl. Phys. Lett. 110, 072402 (2017); https://doi.org/10.1063/1.4976202Submitted: 13 December 2016 . Accepted: 30 January 2017 . Published Online: 13 February 2017T. J. Huisman, C. Ciccarelli, A. Tsukamoto, R. V. Mikhaylovskiy, Th. Rasing, and A. V. KimelARTICLES YOU MAY BE INTERESTED IN Perspective: Ultrafast magnetism and THz spintronicsJournal of Applied Physics 120, 140901 (2016); https://doi.org/10.1063/1.4958846 Ultrabroadband single-cycle terahertz pulses with peak fields of 300 kV cm−1 from a metallicspintronic emitterApplied Physics Letters 110, 252402 (2017); https://doi.org/10.1063/1.4986755Conversion of spin current into charge current at room temperature: Inverse spin-Hall effectApplied Physics Letters 88, 182509 (2006); https://doi.org/10.1063/1.2199473Spin-photo-currents generated by femtosecond laser pulsesin a ferrimagnetic GdFeCo/Pt bilayerT. J. Huisman,1 C. Ciccarelli,2 A. Tsukamoto,3 R. V. Mikhaylovskiy,1 Th. Rasing,1and A. V. Kimel11Institute for Molecules and Materials, Radboud University Nijmegen, 6525 AJ Nijmegen, The Netherlands2Cavendish Laboratory, University of Cambridge, CB3 0HE Cambridge, United Kingdom3College of Science and Technology, Nihon University, 7-24-1 Funabashi, Chiba, Japan(Received 13 December 2016; accepted 30 January 2017; published online 13 February 2017)Using THz emission spectroscopy, we detect spin-photo-currents from a ferrimagnetic amorphousalloy GdFeCo to an adjacent Pt capping layer. The currents are generated upon excitation of aGdFeCo/Pt heterostructure with femtosecond laser pulses. It is found that the polarization of thespin-polarized current is determined by magnetic sublattice sensitivity rather than the total magne-tization, allowing for spin-polarized current generation when the net magnetization is zero.Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4976202]Spintronics, i.e., the functional use of the coupling ofspins and movement of electrons, has developed in the lastdecades from a scientific discovery to a standard solution indata memory and logic technology.1 Very recently, the ideaof antiferromagnetic spintronics has been suggested, withattractive features including insensitivity to disturbing mag-netic fields, absence of parasitic interactions between neigh-boring antiferromagnetic elements, and intrinsically fasterspin dynamics compared to ferromagnets.2 Despite severaltheoretical predictions, experimental verification of spin-currents from antiferromagnets has so far been very chal-lenging. As a result, the feasibility of an effective generationof spin-currents from a material with no net magnetization isstill questionable.A new approach towards antiferromagnetic spintronicsmay come from metallic heterostructures, in which a ferro-magnetic layer is brought in contact with a metallic non-magnetic layer. In such structures, a spin-polarized currentcan be generated upon femtosecond laser excitation.3–6 Theultrashort pulse launches an intense diffusion of spins fromthe magnetic to the non-magnetic layer due to the excitationof hot electrons with spin-dependent velocities.7 Here,we study the effect of the net magnetic moment on laser-induced spin-polarized currents. For this, we have chosenthe ferrimagnetic metallic alloy GdFeCo. This alloy hastwo magnetic sublattices (Gd and FeCo), which are coupledantiferromagnetically. Due to different Gd-Gd and Fe-Feexchange constants, the magnetizations of these two sublatti-ces have different temperature dependences. As a result, thenet magnetization of the material can be effectively tunedby changing the temperature of the alloy. At the so-calledmagnetization compensation temperature (Tcomp), the mag-netization of the FeCo magnetic sublattice exactly balancesthat of the Gd and the net magnetization becomes zero. ForT>Tcomp, the net magnetization is dominated by FeCo,while for T<Tcomp, the main contribution to the netmagnetization is given by the magnetic sublattice of Gd.The goal of this work is to reveal how the spin-photo-current from a GdFeCo alloy changes upon changing thenet magnetization.To achieve this goal, we employed the fact that a femto-second laser excitation of a GdFeCo/Pt heterostructure leadsto a spin diffusion from GdFeCo to Pt on a picosecond tosub-picosecond timescale, i.e., at THz frequencies.3–11 Dueto the strong spin-orbit interaction in Pt, this layer works as atransformer of the pulse of spin-polarized electrons into acharge current pulse in the plane of the structure. The latteris a source of THz radiation in accordance with the Maxwellequations. By performing THz emission spectroscopy, onethus acquires the mean for detection of the femtosecondspin-photo-currents from the GdFeCo to the Pt layer.The sample that we have used in our experiments isshown in Fig. 1(a). It consists of a 20 nm thick layer ofGd0.25(Fe0.875Co0.125)0.75 grown in contact to a 2 nm layer ofPt. This multilayer sample is grown on a glass substrate bymagnetron sputtering, using a 5 nm thick SiN layer as seed,and a 5 nm thick layer of SiN layer as capping to prevent oxi-dation. The particular composition of our alloy results in auniaxial anisotropy with the out-of-plane z^ direction beingthe easy axis. Therefore, when no magnetic field is applied,the magnetic moments of both FeCo and Gd lay along theout-of-plane easy axis.The full setup that we used for our experiment isdepicted in Fig. 1(a). An amplified laser system produces 50fs short laser pulses, which are split into two beam paths.One beam path is used to stimulate a spin-photo-currentfrom the alloy. The spin-photo-current in the Pt layer resultsin the emission of THz radiation, which is collected andrefocused using parabolic mirrors onto a 1mm thick ZnTecrystal. By using electro-optic sampling,12 we detected thetime-dependence of the electric field of this radiation. Thepolarization axis of the emitted THz signal is given bythe vector product z^  rip, where rip is the projection of theinjected spin-polarization on the plane of the sample. Togain nonzero rip in the studied structures, we apply an in-plane magnetic field Bip that tilts the magnetic momentsaway from the out-of-plane easy axis.In Fig. 1(b), we show examples of the measured electricfield as a function of time. It is seen that turning the samplearound the vertical axis over 180 results in the reversal of0003-6951/2017/110(7)/072402/4/$30.00 Published by AIP Publishing.110, 072402-1APPLIED PHYSICS LETTERS 110, 072402 (2017)the sign of the emitted THz traces. Furthermore, it is seen thatthe emitted THz traces also reverse the sign upon reversal ofthe applied field Bip, confirming its magnetic origin. All theseobservations are in agreement with the detected THz tracesgenerated as a consequence of spin-diffusion from GdFeCo toPt. Note that the laser induced demagnetization dynamics canalso give rise to the emission of THz radiation depending onthe magnetization direction,13,14 which is fixed by the exter-nally applied magnetic field. As a consequence, the THz elec-tric field resulting from demagnetization dynamics should notchange sign with the 180 reversal of the sample around itsvertical axis and we can therefore exclude it as the dominantsource of THz emission for our sample.Next in Fig. 2(a), we show the temperature dependenceof the peak electric field with an in-plane magnetic field of60.5 T. It is seen that around 200K, the sign of the electricfield reverses showing that the spin-polarization injected intoPt also reverses its projection on the applied magnetic field.This unambiguously shows that it is not the net magnetiza-tion that determines the polarization of the spin-photo-current from GdFeCo to Pt. For completeness, we includedin Fig. 2(b) the temperature dependence of the amplitudeand phase of the THz electric field odd in magnetic field,integrated between 0.8 and 1.2 THz. We can see that around200K, there is a sharp change in the phase, indicating thesign reversal of the electric field.FIG. 2. (a) Temperature dependenceof the emitted electric field of theGdFeCo/Pt sample, as a function oftemperature. The closed symbols cor-respond to an applied field of 0.5 T,while the open symbols correspond toan applied field of 0.5 T. For the bluedata set, the fluence of the pump wasroughly twice (about 3 mJ/cm2) thepower used for the red data. The linearregression overlaying the data sets isused to guide the eye. (b) Amplitudeand phase of the emitted electric fieldshown in (a), odd in applied magneticfield. The spectra are integratedbetween 0.8 and 1.2 THz. The solidlines overlaying the data sets are usedto guide the eye. (c) The magnetizationas measured with a vibrating samplemagnetometer. (d) The temperaturedependence of the Faraday rotation,measured using THz radiation andintegrated between 0.5 and 0.9 THz.The solid lines overlaying the data setare used to guide the eye.FIG. 1. Schematics of the experiment and room temperature measurements. (a) Intense laser pulses, the pump, are incident on the GdFeCo/Pt sample placed ina 7 T cryostat. The sample emits THz radiation, which is collected using parabolic mirrors and measured using electro-optic sampling and a polarization bal-anced detector scheme. The sample structure is shown left at the bottom. (b) Observed traces of the electric field of the emitted THz radiation. The blue linecorresponds to the electric field of the emitted THz radiation when the pump is incident from the glass side of the substrate. The black traces (shown with a ver-tical offset) correspond to the electric field of the emitted THz radiation when the pump is incident from the side of the metallic layers. B and Bþ correspondto an in-plane applied field of 0.5 and 0.5 T, respectively.072402-2 Huisman et al. Appl. Phys. Lett. 110, 072402 (2017)Another interesting observation is that the electric fieldof the THz radiation crosses zero around 200K, which isaround 25K below the magnetization compensation point asmeasured by a vibrating sample magnetometer (VSM)shown in Fig. 2(c). In order to exclude any effect induced bycumulative heating from the laser, we have repeated themeasurement for a pump power reduced by a half as shownin Fig. 2(a), which showed that the zero crossing of the THzemission is nearly pump intensity independent. However,Fig. 2(d) shows that the measured magneto-optical Faradayeffect in the THz spectral range for this sample is character-ized by an abrupt change near Tcomp as measured with theVSM, similar to what we show in Ref. 15. Since both theTHz emission and THz Faraday rotation are measured in thesame setup, we can exclude the influence of errors in thetemperature determination. This indicates that the apparentshift of Tcomp is an inherent effect of the THz generationmechanism.One can write the THz amplitude as being proportionalto a linear combination of the in-plane components of the Gdand FeCo magnetic sublattices, aMGdþ bMFe. In this case,one expects zero THz emission when aMGdþ bMFe ¼ 0,which does not necessarily correspond to the case in whichMGdþMFe ¼ 0, because the sign or value of a and b maydiffer. By assuming that the magnetic anisotropy of bothmagnetic sublattices near the magnetization compensationtemperature is similar, the fact that we see a sign change inthe THz emission below the compensation temperature indi-cates that jaj < jbj. This means that the FeCo magnetic sub-lattice gives a larger contribution to the THz emission. Thiscan be understood because the THz emission comes from alaser induced spin-polarized current and the magnetizationof the FeCo magnetic sublattice comes from the itinerant 3delectrons, while the magnetization of the Gd magnetic sub-lattices comes from the buried 4f electrons.Fig. 3 shows the peak THz emission for different mag-netic fields and two indicative temperatures. The magneticfield dependence of the THz peak is reversed in sign as wecross the compensation point. Also, we note that the peakTHz emission tends to decrease with an applied field strongerthan 2 T. While the magnetization always tends to increasewith increasing magnetic field, the amount of rotation fromout-of-plane to in-plane of the magnetic sublattices maychange at finite applied magnetic fields. For small appliedfields, the magnetic sublattices will typically start to alignwith the orientation of the applied magnetic field, whileremaining more or less anti-parallel with respect to eachother. However, for higher applied magnetic fields, a signifi-cant canting between the magnetic sublattices may be inducedwith ultimately the magnetic sublattices aligning parallel toeach other and the applied magnetic field. The non-linearrotation of the magnetic sublattices as a function of field com-bined with magnetic sublattice sensitivity for the THz emis-sion explains why for higher fields the amplitude of the THzelectric field can decrease or even reverse sign.In summary, we have shown THz emission arising fromlaser induced spin-diffusion from ferrimagnetic GdFeCo tononmagnetic Pt, followed by electric current generationinside the Pt capping layer by means of the inverse spin-Halleffect. Using temperature and field dependent measurements,we have shown that this emission does not follow the netmagnetization but rather is dominated by a single, more itin-erant, magnetic sublattice. We foresee that this magneticsublattice sensitivity could be used as a way to generate acurrent ultrafast via the inverse spin-Hall effect in fully com-pensated metallic antiferromagnets. One may even obtaincontrol over the magnetic sublattice sensitivity by tuning thewavelength of an optical excitation resonant to the differentelements making up for the different magnetic sublattices.Moreover, particularly GdFeCo is an interesting material forultrafast spintronics, as ultrafast all-optical magnetizationreversal has been demonstrated with this material.16–20 Ourdemonstration of an ultrafast current with this materialcapped with Pt and in combination with all-optical magneti-zation reversal allows for new concepts to realize ultrafastspintronics.We would like to thank T. Toonen and S. Semin fortechnical support. This work was supported by theFoundation for Fundamental Research on Matter (FOM), theEuropean Unions Seventh Framework Program (FP7/2007-2013) Grant No. 280555 (Go-Fast) and No. 281043(FemtoSpin), European Research Council Grant No. 257280(Femtomagnetism) and Grant No. 339813 (Exchange), theprogram “Leading Scientist” of the Russian Ministry ofEducation and Science (14.Z50.31.0034), and a LeverhulmeTrust (IN-2015-022).1A. Fert, “Nobel Lecture: Origin, development, and future of spintronics,”Rev. Mod. Phys. 80, 1517 (2008).2T. Jungwirth, X. Marti, P. Wadley, and J. Wunderlich, “Antiferromagneticspintronics,” Nat. Nanotechnol. 11, 231 (2016).3T. Kampfrath, M. Battiato, P. Maldonado, G. Eilers, J. N€otzold, S.M€ahrlein, V. Zbarsky, F. Freimuth, Y. Mokrousov, S. Bl€ugel, M. Wolf, I.Radu, P. M. Oppeneer, and M. M€unzenberg, “Terahertz spin current pulsescontrolled by magnetic heterostructures,” Nat. Nanotechnol. 8, 256(2013).4T. J. Huisman, R. V. Mikhaylovskiy, J. D. Costa, F. Freimuth, E. Pa, J.Ventura, P. P. Freitas, S. Bl€ugel, Y. Mokrousov, Th. Rasing, and A. V.Kimel, “Femtosecond control of electric currents in metallic ferromagneticheterostructures,” Nat. Nanotechnol. 11, 455 (2016).5T. Seifert, S. Jaiswal, U. Martens, J. Hannegan, L. Braun, P. Maldonado,F. Freimuth, A. Kronenberg, J. Henrizi, I. Radu, E. Beaurepaire, Y.Mokrousov, P. M. Oppeneer, M. Jourdan, G. Jakob, D. Turchinovich,L. M. Hayden, M. Wolf, M. M€unzenberg, M. Kl€aui, and T. Kampfrath,FIG. 3. Field dependencies of the GdFeCo/Pt sample. The measured electricfield of the sample is observed as a function of applied field for twotemperatures.072402-3 Huisman et al. Appl. Phys. Lett. 110, 072402 (2017)“Efficient metallic spintronic emitters of ultrabroadband terahertz radi-ation,” Nat. Photonics 10, 483 (2016).6D. Yang, J. Liang, C. Zhou, L. Sun, R. Zheng, S. Luo, Y. Wu, and J. Qui,“Powerful and tunable THz emitters based on the Fe/Pt magnetic hetero-structure,” Adv. Opt. 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M.Oppeneer, “Ultrafast magnetization enhancement in metallic multilayersdriven by superdiffusive spin current,” Nat. Commun. 3, 1037 (2012).11G.-M. Choi, B.-C. Min, K.-J. Lee, and D. G. Cahill, “Spin current gener-ated by thermally driven ultrafast demagnetization,” Nat. Commun. 5,4334 (2014).12G. Gallot and Grischkowsky, “Electro-optic detection of terahertz radi-ation,” J. Opt. Soc. Am. B 16, 1204 (1999).13E. Beaurepaire, G. M. Turner, S. M. Harrel, M. C. Beard, J.-Y. Bigot, andC. A. Schmuttenmaer, “Coherent terahertz emission from ferromagnetic filmsexcited by femtosecond laser pulses,” Appl. Phys. Lett. 84, 3465 (2004).14T. J. Huisman, R. V. Mikhaylovskiy, A. Tsukamoto, T. Rasing, and A. V.Kimel, “Simultaneous measurements of terahertz emission and magneto-optical Kerr effect for resolving ultrafast laser-induced demagnetizationdynamics,” Phys. Rev. B 92, 104419 (2015).15T. J. Huisman, R. V. Mikhaylovskiy, A. Tsukamoto, L. Ma, W. J. Fan,S. M. Zhou, Th. Rasing, and A. V. 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Kirilyuk, A. V.Kimel, and Th. Rasing, “Highly efficient all-optical switching of magneti-zation in GdFeCo microstructures by interference-enhanced absorption oflight,” Phys. Rev. B 86, 140404 (2012).20C. E. Graves, A. H. Reid, T. Wang, B. Wu, S. de Jong, K. Vahaplar, I.Radu, D. P. Bernstein, M. Messerschmidt, L. M€uller et al., “Nanoscalespin reversal by non-local angular momentum transfer following ultra-fast laser excitation in ferrimagnetic GdFeCo,” Nat. Mater. 12, 293(2013).072402-4 Huisman et al. Appl. Phys. Lett. 110, 072402 (2017)

Spin-photo-currents generated by femtosecond laser pulses in a ferrimagnetic GdFeCo/Pt bilayer

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Spin-photo-currents generated by femtosecond laser pulses in a ferrimagnetic GdFeCo/Pt bilayer

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