Interface-modulated Spin Transport In Ferromagnetic/Non-magnetic Metallic Thin Films Heterostructures

In nowadays,CMOS-based memory and logic devices are facing significant challenges.Compared charge-based semiconductor devices with volatile and slow flash memory,spintronic devices have attracted wide attention for their advantages of high density,fast access,high power saving,reusability,and uncontrollability.Although the spin-orbit coupling provides the basic feasibility for current-driven magnetization,there remain chanllenges for highly efficient manipulation of spin transport.The present research fields are mainly divided into two parts:one is to improve the generation efficiencies of spin current by using strong spin Hall effect heavy metals or topological materials with spin-momentum locking;the other is to reduce the energy consumption for driving single magnetic storage units by using ferrimagnetic,antiferromagnetic materials,or magnetic materials with low damping factor,or chiral domain walls and skyrmions.Although both of these have made significant progress,with deep research,spin transports at the interface have been found to play a crucial role in spintronic devices,such as the interfacial spin memory loss due to interfacial lattice or spin conductance mismatch.Meanwhile,the thermal stability of magnetic material and the current control magnetization are directly related to the interface spin-orbit coupling.In this paper,we systematically investigated the effect of interface on spin transports.The main results are as follows:(1)We have studied the influence of compressive strain on the magnetic properties,dynamic properties,and interfacial spin transport in Pt/Y3Fe5O12(YIG)bilayers.At first,the epitaxial YIG films were grown on[111]-oriented Gd3Ga5O12(GGG)substrates and(Gd2.6Ca0.4)(Ga4.1Mg0.25Zr0.65)O12(SGGG)substrates by the pulsed laser deposition technique,respectively.Through X-ray,atomic force microscopy,ferromagnetic resonance,and polarized neutron reflectometry measurements,we have found that the YIG films grown on SGGG substrates had perpendicular magnetic anisotropy with an ultralow Gilbert damping constant and high quality.Then,the Pt/YIG Hall bar devices were fabricated to investigate the temperature-dependent longitudinal and transverse magnetoresistance.We have found that the transport behavior of Pt/YIG/SGGG films reveals an enhancement of spin-mixing conductance(SMC)and a large unusual anomalous Hall effect(UAHE)as compared with films Pt/YIG/GGG.Although the UAHE in Pt/YIG/SGGG films shows different characteristics with varying YIG thickness,they are all ascribed to the possible noncollinear magnetic order at the Pt/YIG interfaces induced by epitaxial strain.(2)We have studied the geometric size dependence of spin-mixing conductance at the Pt/YIG interface.The SMC is an essential parameter for spintronics devices,which represents the spin transparency of a metal/ferromagnet interface.In this work,we have investigated the influence of boundaries on local spin transports properties in Pt/YIG films by reducing lateral size.Through the angle-dependent longitudinal magnetoresistance,transverse magnetoresistance,and spin-torque ferromagnetic resonance measurements,we have found a negligible fluctuation of the imaginary part of SMC and effective magnetization with varying the geometric size of devices.In contrast,an obvious fluctuation of spin Hall magnetoresistance,Gilbert damping coefficient,and effective spin Hall angle with restricting the size of YIG films has been found at room temperature.Our results show that the SMC dramatically depends on the boundary effect from Ar+-ion milling.(3)We have studied the influence of the nonequilibrium proximity effect on spin-orbit torque(SOT)in Pt/Co/Pt/YIG multilayers.The Pt/Co/Pt trilayers were fabricated to on YIG/GGG films and traditional SiO2 substrate,respectively.The critical switching current in Pt/Co/Pt/YIG is almost half of that in Pt/Co/Pt/SiO2.Through harmonic measurements,we have demonstrated the enhancement of the effective spin Hall angle in Pt/Co/Pt/YIG.The increased efficiency of SOT is ascribed to the nonequilibrium proximity effect at the Pt/YIG interface,which suppresses the spin current reflection and enhances the effective spin accumulation at the Co/Pt interface.(4)We have explored the physical mechanisms of robustSOT in ferromagnetic multilayers with a weak bulk spin Hall effect.For our experiments,a series of magnetic films(YIG,Fe,Co,CoFeB)/non-magnetic metals(Ru,Pt,Cu)heterostructures were fabricated.We have investigated the SOT in Ru/Fe heterostructures with weak perpendicular magnetic anisotropy.Although the bulk spin Hall effect of the Ru layer is smaller than that of Pt,Ta,or W,robust SOTinduced magnetization switching can also be realized.Through nonadiabatic harmonic measurements,the effective spin Hall angle in Ru/Fe/HfO2 heterostructures is calculated to be-0.045.We have explored the mechanisms of the spin current generation,which could stem from the Ru/Fe interfaces other than the weak bulk SHE of Ru.