Measurement And Study Of Spin Orbit Torque In Heavy Metal/Ferromagnet/Oxide System

Utilizing spin-orbit torque(SOT)induced magnetization switching and domain wall motion,novel SOT-based Magnetoresistive Random Access Memory(SOTMRAM)is supposed to write and read information in a fast and efficient way.To enhance the switching efficiency and the performance of next-generation spintronics devices,the mechanism behind magnetization switching and domain wall motion should be clarified.So,in this thesis,my work is focused on spin-orbit torque and relevant effects as well as their contributions to magnetization reversal and domain wall propagation in heavy metal/ferromagnet/oxide system.The research includes:(1)Slonczewski-like torque and Dzyaloshinskii-Moriya interaction(DMI)are studied in Ta/Pt/CoFeB/MgO structures.First,we fabricate a series of samples with various Pt thickness.Then,by measuring the AHE curves,Slonczewski-like torque and DMI are obtained and both of them decrease with increasing Pt thickness()in the range 0–1 nm.In consequence,through the magnetization switching curves,we find that,for the samples with higher ,their critical switching current density increase whereas the required in-plane field for deterministic switching decrease.Our results verify that a thin HM interlayer may be a suitable route to tailor the SOT and DMI at the HM/FM interface,as well as the current-induced magnetization switching in these structures.Through the Brillouin Light Scattering(BLS)experiment,we also find the sign of the DMI reverses around = 1 nm,confirming that D has the opposite sign at the Ta/CoFeB and Pt/CoFeB interfaces;but its intensity saturates at = 3 nm,suggesting that several interface monolayers may contribute to the DMI.(2)We investigate the magnetization reversal induced by spin-orbit torques in the presence of Dzyaloshinskii-Moriya interaction(DMI)in perpendicularly magnetized Ta/CoFeB/MgO structures.First,we measure the anomalous Hall effect(AHE)curves under in-plane bias field.After analyzing the in-plane field dependent spin torque efficiency from the AHE curves,an effective field value for the DMI of ~300Oe is obtained.Then we monitor the domain wall motion by Magneto-optical Kerr Microscope.Kerr imaging reveals that the current-induced reversal under small and medium in-plane field is mediated by domain nucleation at the edge of the Hall bar,followed by asymmetric domain wall propagation.However,when the in-plane field strength increases,an isotropic DW expansion is observed before reaching complete reversal.Micromagnetic simulations of the DW structure in the CoFeB layer confirm the experiment results and suggest that the DW configuration under the combined effect of the DMI and the external field should be responsible for the various DW propagation behaviors.