Spin Orbit Torque In Ferromagnet/Non-ferromagnet Heterojunctions

The spin orbit torques can manipulate the magnetization,drive the motion of chiral domain walls and excite high-frequency magnetization oscillation,and have some advantages,such as high efficiency,low energy consumption,structure optimization and high stability.Therefore,the spin orbit torque effect has become not only active research in spintronics,but also an important topic in the development of the next generation magnetic memory.How to improve the efficiency of spin orbit torque in spintronic devices and achieve all-electric manipulation without external magnetic field,are still the key problem of spin orbit torques.Focusing on these issues,by using ferromagnet/antiferromagnet and single crystal ferromagnet as carrier,using symmetry analysis and controlling the interface and crystal structure,our main results are summarized as follows:1.Current-induced bipolar switching of AFM interface state in a perpendicularly magnetized ferromagnetic/antiferromagnetic heterostructure were systemically studied.Based on the physical mechanism of the spin orbit torque manipulating magnetization,in heavy metal/ferromagnetic/antiferromagnetic（HM/FM/AFM）,i.e.Pt/Co/IrMn,we also consider that when the current induced spin current diffuses to the ferromagnetic/antiferromagnetic interface,the spin orbit torque will be generated on the antiferromagnetic moment of the interface.Combined with symmetry,we analyze that the spin orbit torque can realize the bipolar switching from antiparallel FM/AFM interface configuration to parallel FM/AFM interface configuration under zero magnetic field.The current induced bipolar switching without external auxiliary magnetic field originates from the symmetry breaking of the system caused by the magnetic moment pinning of adjacent ferromagnetic layer interface.However,under this symmetry breaking,the parallel configuration can only be further flipped into multilevel remanence state configuration,and the reverse process that the parallel interface configuration is flipped back to the antiparallel interface configuration is not allowed.More importantly,in Pt/Co/IrMn system we verified that magnetic field and current pulses can stably and continuously manipulate multilevel remanence state configuration.Our results will contribute to the comprehensive understanding of the spin orbit torque on the antiferromagnetic moment reversal,and can be used for the development and application of antiferromagnetic spintronic devices2.By the combing the low-symmetry point group of crystalline lattices and the FM order together,we propose and demonstrate that single-crystal ferromagnetic thin film with in-plane magnetic anisotropy can generate the Dresselhaus-like spin current（x polarized spin current）.Symmetry analysis and macro-spin model calculation show that the second harmonic Hall voltage caused by x-polarized spin current results in an antisymmetric angular dependence in xy plane,which is in contrast to the symmetric angular relationship caused by the traditional y or z polarized spin current.The anomalous angular dependence can be used as fingerprint to experimentally detect x-polarized spin current.Combined with the second harmonic Hall voltage measurement and the loop shift method,we prove that single crystal Fe_0.5Co_0.5 can generate x polarized spin current.We further prove that single crystal ferromagnetic thin film can generate the x polarized spin current by combining the low symmetry point group of the lattice with the ferromagnetic order using amorphous CoFeB,polycrystalline Co and single-crystal Fe_0.86Si_0.14 samples.In the experiment,the x polarized spin current decays exponentially with increasing the thickness of the ferromagnetic layer.By inserting an epitaxial V layer to improve the interface,the x-polarized spin current can be greatly increased by an order of magnitude.These results indicate that strong interface effect generates x polarized spin current.The research on the generation of spin current with complete polarization will help to design more effective spin orbit torques devices and improve the theory of spin orbit torques.3.The negative spin Hall magnetoresistance and spin orbit torque effect induced by interfacial spin orbit coupling in non-collinear antiferromagnetic IrMn₃/Co are systematically studied.The magnetoresistance of IrMn₃/Co in yz plane displays negative spin Hall magnetoresistance,and is independent on IrMn₃ thickness and temperature.This indicates that the interfacial charge-spin interconversion induces negative spin Hall magnetoresistance.In contrast to cos2φ angular dependence generated by z polarization,the second harmonic Hall voltage of IrMn₃/Co shows cos and cosφcos2φ angular dependence,this excludes z polarized spin current leads to the negative spin Hall magnetoresistance in IrMn₃/Co.The spin orbit torque effective field is independent of the crystal structure,temperature and thickness of IrMn₃.These results further demonstrate that the charge spin conversion in IrMn₃/Co originates from the interfacial spin orbit coupling.These results provide evidence of interfacial spin orbit coupling at the antiferromagnetic/ferromagnetic bilayer,and indicate that the interfacial spin orbit coupling effect is stronger than the bulk effect.Our results provide ideas for optimizing non-collinear antiferromagnetic spintronic devices by interface engineering.