Study On Current-Induced Torques And Magnetization Switching In Magnetic Heterostructures With Perpendicular Anisotropy

Spintronic devices have attracted widespread attention in the field of integrated devices for information storage,sensing,and storage due to their non-volatile,high-speed read and write,and durability characteristics.In the latest spin orbit torque（SOT）magnetic random access memory（MRAM）,the current induced torque effect is utilized to drive the magnetization reversal of the ferromagnetic layer to achieve information writing.Although SOT devices have advantages such as fast write speed,separation of read and write,and minimal damage to the device,they still face issues such as how to achieve current driven magnetization switching under fieldless conditions and how to further reduce the critical current density required for switching,as the former can improve device miniaturization and integration,while the latter can effectively reduce device power consumption.Based on the ferromagnetic/non-magnetic/ferromagnetic three-layer structure with perpendicular magnetic anisotropy（PMA）,this paper focuses on the above two key scientific problems,and studies the spin-torque effect caused by current in the system and the current-driven magnetization switching under the condition of no field.Furthermore,on the basis of using the electron spin Angular momentum,we used the electron orbital Angular momentum to carry out research on how to use the orbital current effect to reduce the critical current density of current driven magnetization reversal.The main innovative results are as follows:1,Co（PMA）/Ti/Co Pt magnetic heterostructures were prepared,aiming to utilize the significant current induced SOT effect in Co Pt monolayers and the spin orbit precession（SOP）effect on the interface to achieve current induced magnetization reversal under fieldless conditions.The measurement results of the magnetoresistive and hall terminal resistances by the second harmonic method show that in this system,except for the ordinary transverse spin polarization（（?）_y）current,there is also spin polarization along the current direction（（?）_x）,The spin current proves the existence of SOP effect on the interface;However,in the measurement of Anomalous Hall effect（AHE）loop shift,the spin current of vertical polarization（（?）_z）was not observed,and the current-induced magnetization switching in this system under the condition of no field was not achieved,which may be due to the weak SOP effect at the Ti/Co Pt interface.Or it is caused by the nonlinearity between the shift of the loop line and the applied current during AHE curve measurement.2,In the optimized Co/Ti/Tb-Co（PMA）three-layer structure,it is found that the switching polarity of the current-driven Tb-Co layer depends on the component of the Co magnetic moment of the in-plane magnetization along the current direction,thus achieving the current-induced deterministic magnetization switching under the field-free condition.The Damping-like（DL）-SOT effective field in the system is measured by the second harmonic and AHE loop shift method.The results show that there are spin currents（（?）_y）and（（?）_z）in the system,and the former may originate from the Co layer or the Tb-Co/Ti and Ti/Co interfaces.While the latter can be explained by the SOP effect on the Co/Ti interface.3,It is confirmed that the current-induced torques have different signs in the double-layer structure composed of different ferromagnetic layers and Cr,which is an obvious feature of the orbital current generating torque effect（orbital torque）,which is attributed to the different orbit-spin conversion efficiency in the ferromagnetic layer.On this basis,we study the variation of current-induced torque with the thickness of Tb insertion layer and Cr layer in Pt/Co（PMA）/Tb/Cr structure.It is found that changing the thickness of both Tb insertion layer and Cr layer can effectively regulate the current-induced torque,and the maximum efficiency is reached when the thickness of Tb is 4 nm.Furthermore,the dependence of DL torque efficiency on Cr thickness is fitted using the orbital flow diffusion model,and the effective orbital Hall Angle of Cr is-0.18±0.01.The measurement of current-induced magnetization switching shows that the critical flip current density can be effectively reduced by using the orbital current generated by the Cr layer in the system.4,Large DL torque efficiencies and efficient current-induced magnetization switching in Ti/Tb-Co/Cr samples were found.Second harmonic measurements show that the current-induced torque is dependent on the composition of the Tb-Co layer,which is different from the phenomenon in ordinary heavy metal/ferromagnetic structures.A simple orbital current diffusion model is used to fit the dependence of DL moment on Cr thickness,and an effective orbital Hall Angle of-0.57±0.02 is obtained for Ti/Tb_0.85Co_0.15/Cr samples.However,only very low efficiencies were observed in Ti/Co/Cr and Ti/Tb-Co/Si N control samples.These results indicate that the use of rare-earth-3d ferromagnetic alloys as a magnetic layer can effectively improve the absorption of orbital current and further increase the efficiency of current-induced torque.