Magnetization-dependent Spin Hall Angle And Pure Spin Current Transport In Ferromagnetic Metals

Over the past 30 years,the development of spintronics can be broadly divided into two stages.The first-generation spintronic studies mainly focus on the spin-dependent transport phenomena,among which the most representative works are the giant magnetoresistance(GMR)effect and tunneling magnetoresistance(TMR)effect.The second stage is extended to the electrical manipulation of the magnetism by currentinduced spin torque.In addition to the upsurge of diluted magnetic semiconductor research,the epoch-making issues in this period are spin-transfer torque(STT)and spin-orbit torque(SOT).Spin current has been the central point in spintronics from its birth,which can be more profoundly understood whenever the spintronics has advanced to the next stage.The spin current firstly appears in the form of spin-polarized current with the accompany of electric current,which is primarily produced by the spin-related energy bands splitting in ferromagnetic metals,essentially resulting from the exchange interaction.In the recent period,spin current is mainly generated by the spin-orbit coupling of non-magnetic metals and owns 100%spin polarization for the carriers with different spins moving to opposite directions,which is also called pure spin current.There seems little correlation between ferromagnetic metals and pure spin current.However,as the latest theoretical analysis shows,if the spin-orbit coupling is introduced to the ferromagnetic metal system,combined with the spontaneous symmetry breaking caused by magnetization order parameter,a more abundant pure spin current phenomenon can be generated,which may promote the development of spintronics.Therefore,experimental demonstrations of the interplay between magnetism and pure spin current transport in ferromagnetic metals have become a key issue in future spintronics.(1)By employing Y3Fe5O12(YIG)/Cu/Ni81Fei9(Py)/Ir25Mn75(IrMn)spin valve heterostructure with the thermal inverse spin Hall effect(ISHE)of Py well separated from other thermoelectric transport and thermo Hall effects,we find that the ISHE signal amplitude in 10 nm Py increases by 80%,when changing the relative orientation of the YIG and Py magnetization from orthogonal(⊥)to collinear(‖).Moreover,the spin-diffusion length λsf and effective spin Hall angle θSHeff of Py is also spin orientation dependent and varies from λsf⊥=1.0±0.1 nm to λsf‖=2.8±0.5 nm andθSHeff(⊥)/θSHeff(‖)=1.5.Our results demonstrate magnetization orientation dependent spin relaxation and spin injection efficiency of pure spin current,revealing that exchange interaction in ferromagnetic metals strongly affects the transport of pure spin current.(2)We report the observation of the magnetic(m)inverse spin Hall effect(ISHE)in ferromagnetic Fe by spin injection with σ spin polarization from ferromagnetic insulator yttrium iron garnet(YIG).The ISHE signal(jISHE)of Fe has a sign change but keeps a comparable magnitude when σ was altered from longitudinal polarization(m ‖ σ)to transverse polarization(m ⊥ σ).Besides,jISHE was enhanced in magnitude,and even reverse the sign for longitudinal polarization spin injection via decreasing thickness.Supported by the first-principles scattering approach and considering the contributions of interfacial and bulk spin current with longitudinal(jLISHEI,jLISHEB)and transverse(jTISHEI,jTISHEB)spin polarization,ISHE in magnetic multilayers can be expressed as:jISHE=(jLISHEI+jLISHEB)cos2<m,σ>+(jTISHEI+jTTISHEB)sin2<m,σ>.Our work indicates that the interfacial spin-to-charge conversion process makes a significant contribution to the ISHE of Fe and can be tuned by magnetization and interfacial condition,shedding light on the process of pure spin current transport in ferromagnetic metals.(3)Spin Hall magnetoresistance(SMR)is studied in metallic bilayers that consist of a heavy metal(W)layer and various ferromagnetic metals(FMs)layer.We find that the amplitude of SMR changes obviously with different FMs.Then,the spin mixing conductance(gr↑↓)of three representative samples of(Co40Fe40B20,Ni81Fe1,(Ni81Fe19)xB1-x)/W are obtained by the measurement of ferromagnetic resonance-spin pumping,but the values of all also cannot account for the W thickness dependence of SMR within the framework of the conventional SMR model.Our experimental results can be well explained only when the absorption of the longitudinal spin current to the FMs layer is taken into account.These results illustrate the unique role a metallic ferromagnetic layer plays in defining spin transmission across the interface between the ferromagnetic metal and heavy metal layer.