Li Wang(王力), Yangtao Su(蘇仰濤), Yang Meng(孟洋),Haibin Shi(石海濱), Xinyu Cao(曹昕宇), and Hongwu Zhao(趙宏武),3,?

1Beijing National Laboratory for Condensed Matter Physics,Institute of Physics,Chinese Academy of Sciences,Beijing 100190,China

2School of Physical Sciences,University of Chinese Academy of Sciences,Beijing 100049,China

3Songshan Lake Materials Laboratory,Dongguan 523808,China

We investigate the spin to charge conversion phenomena in Y3Fe5O12/Pt/Co1?xTbx/Pt multilayers by both the spin pumping and spin Seebeck effects.We find that the spin transport efficiency is irrelevant to magnetization states of the perpendicular magnetized Co1?xTbx films,which can be attributed to the symmetry requirement of the inverse transverse spin Hall effect.Furthermore,the spin transmission efficiency is significantly affected by the film concentration,revealing the dominant role of extrinsic impurity scattering caused by Tb impurity.The present results provide further guidance for enhancing the spin transport efficiency and developing spintronic devices.

Keywords: spin to charge conversion,perpendicular magnetized ferrimagnetic alloy,inverse spin Hall effect

1.Introduction

In spintronics, the interconversion between spin current and charge current arising from spin–orbit coupling (SOC)has received considerable attention as an effective way to manipulate spins by electrical means.[1,2]In a typical ferromagnetic/nonmagnetic metal(FM/NM)bilayer structure,pure spin currents are generated commonly by the spin pumping effect (SPE) and the longitudinal spin Seebeck effect (SSE) in the FM layer, then injected into the adjacent NM layer and converted into charge currents by the inverse spin Hall effect(ISHE).[3,4]Due to their strong SOC,5d heavy metals,such as Pt, W, Ta, have been demonstrated to reveal appreciable spin to charge (S-C) conversion efficiency.[5–9]Recently, experimental studies show that the FMs also exhibit significant S-C conversion efficiency which is generally dependent on the orientation of the FM magnetization.[10–16]For example, based on an analysis of the ISHE characteristics, Das and Crameret al.claimed the remarkable difference between the occasions of FM magnetization parallel and perpendicular to the spin polarization in permalloy (Py) and Co60Fe20B20films,respectively.[13,14]Zhuet al.further demonstrated that the Py magnetization greatly affects the spin relaxation process and interfacial spin injection efficiency.[15]However, a recent report by Tianet al.showed that the ISHE in pure Co is independent of the Co magnetization direction.[16]Nevertheless,the dependence of ISHE upon magnetization state reported by different groups still seems to be controversial.Moreover, in FMs,the additional symmetry breaking due to the presence of FM magnetization leads to generation of extra spin currents with different spin polarizations from those observed in nonmagnetic materials.[17–21]Therein, a magnetization-related ISHE, referred to the inverse transverse spin Hall effect (ITSHE)where spin current flows perpendicularly into magnetic layer with its spin polarization perpendicular to the magnetization,was proposed theoretically.[17]Until recently,experimental demonstrations of the ITSHE associated with symmetry properties of magnetization distribution remain rare.Therefore,a complete understanding of spin current transport in ferromagnets remains largely ambiguous.Experimental studies on fabricating various magnetic ordering structures and probing the impact of their magnetization process on the spin transport efficiency can help to further advance the development of spintronic devices.

For rare earth–transition metal(RE-TM)ferrimagnetic alloys, such as CoTb and CoGd,[22,23]the magnetic moments are antiferromagnetically coupled and uncompensated, thus can be tailored by varying the composition ratio or temperature.In addition, due to the strong SOC of the rare-earth element,RE-TM alloys are expected to have relatively high SC converting efficiency.Hence, RE-TM ferrimagnetic alloys are endued with more versatility as S-C converting material.In this work, we demonstrate experimentally that the ITSHE is irrelevant to the magnetization states in the perpendicular magnetized system, and investigate the effect of SOC on the spin transport efficiency of Co1?xTbxalloys.We fabricate a special Y3Fe5O12(YIG)/Pt/Co1?xTbx/Pt multilayered structure,where the magnetization of the spin injecting YIG layer and that of spin detecting Co1?xTbxlayer are perpendicular to each other,while the Pt interlayer actually suppresses the exchange coupling between YIG and Co1?xTbx.We show that the ITSHE signal remains unchanged under different magnetization states of Co1?xTbx,consistent with the theoretical expectation.By varying the Co1?xTbxcomposition, we further show that the spin–charge conversion is significantly changed,demonstrating that the extrinsic impurity scattering dominates the transport process due to Tb impurity.

2.Method

A schematic diagram of the sample structure and measurement configuration is shown in Fig.1(a).The YIG films(120 nm)are deposited on single crystalline Gd3Ga5O12(GGG) (111) substrates using RF magnetron sputtering at room temperature, followed by annealing in air atmosphere at 800°C for 4 hours.[24,25]To avoid divergence between the magnetic properties of YIG films, all YIG films used in this experiment are prepared under the same condition in one single batch.Then, a series of Pt (3 nm)/Co1?xTbx(10 nm)/Pt(5 nm) multilayers are grown subsequently on the YIG film.The Co1?xTbxlayer is prepared by co-sputtering Co and Tb targets with adjustable power.During the multilayer deposition,a hard mask is utilized to fabricate strip structures(3 mm in length and 0.2 mm in width).Next, using both SPE and SSE,the pure spin currents can be efficiently generated in the YIG/Pt(3 nm)bilayer and detected in the adjacent Co1?xTbxlayer by ISHE.For the SPE measurement,a microwave excitation is generated by a GSG coplanar waveguide,and a static magnetic fieldHis applied in the film plane with an angle θHwith respect to theyaxis.For the SSE measurement,a longitudinal temperature gradient ?TZis established by illumination of a perpendicularly incident white laser beam,[26,27]and the dc magnetic fieldHis applied along theyaxis.The magnetic properties of YIG/Pt/Co1?xTbx/Pt films are characterized by magneto-optical Kerr effect(MOKE)measurements.All measurements are performed at room temperature.

3.Result and discussion

First,we demonstrate that the pure spin current can be efficiently generated in the YIG/Pt(3 nm)bilayer by both SPE and SSE.Figure 1(b)shows the typical spectra of spin pumping voltageVSPas a function of magnetic fieldHat θH=90°and 270°respectively.The line shape ofVSPis fully symmetric with respect to the ferromagnetic resonance position,and can be well fitted using Lorentzian functions.No obvious dispersive line shape is observed due to the absence of anisotropic magnetoresistance(AMR)in YIG.[28,29]Then,the thermovoltageVTis directly obtained from the same YIG/Pt sample under laser illumination with power up to 290 mW.As depicted in Fig.1(c),theVTsignal presents extremely narrow hysteresis,in consistent with the steep magnetization reversal of YIG and typical characteristics of ISHE.[3]These measurements clearly indicate that the spin current can be reliably generated in YIG by both SPE and SSE,injected into the adjacent Pt layer,and then efficiently detected by ISHE.

Fig.1.(a)Schematic of the YIG/Pt/Co1?xTbx/Pt structure and the measurement configuration.(b)VSP versus H spectra of YIG/Pt(3 nm)with Prf =80 mW at 4 GHz.(c) Thermal voltage VT as a function of H for YIG/Pt (3 nm) under white laser illumination of 290 mW.The inset shows the M–H loop of the YIG/Pt bilayer.(d)Representative MOKE hysteresis loops for YIG/Pt/Co1?xTbx/Pt(x=0.17,0.30).

Next, we examine the magnetic properties of the YIG/Pt/Co1?xTbx/Pt films.To avoid interference of the adjacent in-plane magnetized YIG layer, the MOKE measurements are performed under out-of-plane magnetic field to characterize the perpendicular magnetic anisotropy(PMA)of the Co1?xTbxfilms.Figure 1(d) displays typical hysteresis loops of Co0.83Tb0.17and Co0.70Tb0.30,clearly demonstrating their strong PMA.It is noted that the polarity of the Kerr loop changes with Tb concentration, due to that the Kerr signal is primarily sensitive to the variation of Co sub-lattice around the magnetization compensation composition.[30,31]

Now we explore the spin transmission properties of the YIG/Pt/Co1?xTbx/Pt structure by the SPE and SSE respectively.Figure 2(a) shows the field dependence of the spin pumping voltageVSPof YIG/Pt/Co0.70Tb0.30/Pt under 4 GHz microwave excitation of 80 mW.Note that theVSPexhibits a sin3θ dependence (Fig.2(b)), which is a typical characteristic of the ISHE as a result of the in-plane rf field.[29,32]These results definitely confirm that the spin currents, generated in YIG/Pt by SPE,can propagate through Co0.70Tb0.30/Pt and indeed be detected by the ISHE.In addition, as shown in the inset of Fig.2(b), both the linear dependence ofVSPon the rf powerPrf,[5]and the dispersion relationship between the frequencyfand the resonance fieldHreswhich can be well fitted by the Kittle equation,[33]reconfirm the effectiveness of the Co–Tb detection layer.In contrast, theVSPsignal of the control sample of Si/Pt/Co0.70Tb0.30/Pt is totally invisible under the same measurement geometry, eliminating the possibility that the observedVSPof YIG/Pt/Co0.70Tb0.30/Pt originates from the CoTb layer itself.For the SSE experiment,the thermal voltageVTfor YIG/Pt/Co0.70Tb0.30/Pt is measured with the magnetic field applied in the film plane under a laser power of 290 mW.Figure 2(c) shows the magnetic field dependence ofVTof YIG/Pt/Co0.70Tb0.30/Pt,and the magnitude ofVTis proportional to the illuminating laser powerPLaser(see Fig.2(d)),in well agreement with the SSE features in the longitudinal configuration.[3,36]Similarly, the absence ofVTof the control sample Si/Pt/Co0.70Tb0.30/Pt indicates that the spin current solely comes from YIG rather than Co0.70Tb0.30layer(see Fig.2(c)).Hence, both the SPE and SSE measurements clearly demonstrate that the spin current generated in YIG/Pt can be efficiently injected to the adjacent Co0.70Tb0.30layer and detected through ISHE.

Fig.2.(a)The VSP versus H of YIG/Pt/Co0.70Tb0.30/Pt and Si/Pt/Co0.70Tb0.30/Pt structures at θH =90°.(b)Angular dependence of VSP,the red curve is the best fit to VSP.The insets show the microwave power dependence of VSP and the Hres dependence of frequency f.(c) Field dependence of thermal voltage VT in YIG/Pt/Co0.70Tb0.30/Pt and Si/Pt/Co0.70Tb0.30/Pt samples.(d)Laser power dependence of VT.

To evaluate the impact of magnetic structure on spincharge conversion,we examine the spin current transport properties of YIG/Pt/Co0.70Tb0.30/Pt with the Co–Tb layer set to different magnetization states.By applying DC (±4000 Oe)and AC fields normal to the film plane,the Co0.70Tb0.30film is switched to saturated and demagnetized states,whereMCoTb=Ms,MCoTb=?MsandMCoTb=0,respectively.As depicted in Fig.3,there are no discernible differences in eitherVTandVSPsignals obtained between different magnetized states.Note that the weak in-plane field used to magnetize YIG in both SPE and SSE measurements will not alter the magnetization states of Co0.70Tb0.30due to its strong PMA.Therefore, the magnetic structure of the Co0.70Tb0.30layer hardly affects the S-C conversion efficiency in our experiment.In addition, the similar results have been obtained in other Co–Tb films with different composition(data not shown).According to Davidson’s perspectives,[17]when spin currents flow alongzaxis into magnetic layer m where spin polarization σ⊥m,a magnetically related voltage signal would arise due to the ITSHE,satisfying the relationVITSHE∝[m×(σ×m)]× ?z.It is easily seen thatVITSHEshould remain the same while m reverses its direction.For in-plane magnetized films,the direct experimental evidence has been reported in several YIG-based ferromagnetic heterostructures,[13,14,16]in good agreement with the theoretical prediction.For perpendicularly magnetized structure,such as CoTb in the present experiment,VITSHEis an even function of mCoTb.Thus, theVITSHEremains unchanged forMCoTb=MsandMCoTb=?Ms.[20]On the other hand, in the perpendicularly demagnetized state,the CoTb layer consists of multi-domains with each domain magnetized randomly alongzdirection,MCoTb= 0, hence the sum of contributions toVITSHEfrom each domain should make no difference with theVITSHEwhetherMCoTb=MsorMCoTb=?Ms.Therefore,our measurements clearly reveal that the spin transport efficiency is independent of magnetization states of the perpendicular magnetized Co1?xTbxfilms,which is consistent with the theoretical model of ITSHE.

Fig.3.(a) Field dependences of VSP and (b)VT of YIG/Pt/Co0.70Tb0.30/Pt at different magnetization states.

Next, we perform both SPE and SSE measurements on YIG/Pt/Co1?xTbx/Pt structures with various Tb concentrations so as to investigate its impact on the spin transmission efficiency.To exclude the impact of the total resistance (R) of the sample onVISHE,the SPE and SSE signals have been normalized byRand plotted as a function of Tb composition,as shown in Fig.4.It is clearly seen that bothVRSPandVRTin the high Tb concentration region outweigh those in low Tb region, obviously indicating that the S-C conversion efficiency is significantly affected by the Tb impurity,and the higher Tb concentration directly leads to large spin–charge conversion efficiency.It is well accepted that the spin-dependent Hall effects(ISHE, SHE and AHE)share common microscopic origins:the intrinsic mechanism which is based on band structure of perfect crystals, and the impurity-related extrinsic mechanism including skew scattering and side-jump.[34,35]Due to the strong spin–orbit interaction of Tb, the extrinsic impurity scattering is expected to increase with increasing the Tb concentration, accordingly resulting in enhanced spin–charge conversion.[36–38]On the other hand,owing to the amorphous nature of the Co1?xTbxalloy films,the missing periodic order of crystal structure severely hampers the expressing of intrinsic contribution in the forms of crystal momentum.[39]Therefore, we cannot assess the intrinsic contribution of Co1?xTbxalloys in the present experiments,which needs to be explored in future study with optimized material design.

Fig.4.Tb concentration dependences of normalized spin pumping voltage and thermal voltage.

4.Conclusion

We experimentally study the spin transport efficiency of YIG/Pt/Co1?xTbx/Pt structures.By the SPE and SEE measurements, we demonstrate experimentally that the ITSHE is irrelevant to the magnetization states in the perpendicular magnetized system.Moreover, the spin transport is significantly affected by varying the Tb concentration of the Co1?xTbxalloys,revealing the dominant role of extrinsic impurity scattering caused by Tb impurity.

Acknowledgements

Project supported by the National Key Basic Research Project of China (Grant No.2016YFA0300600), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No.XDB33020300), and the National Natural Science Foundation of China (Grant Nos.11604375 and 11874416).