Spin-orbit torque (SOT)-induced magnetization switching under small in-plane magnetic fields in as-deposited and annealed Ta/CoFeB/MgO structures is studied. For the as-deposited samples, partial SOT-induced switching behavior is observed under an in-plane field of less than 100 Oe. Conversely, for the annealed samples, an in-plane field of 10 Oe is large enough to achieve full deterministic magnetization switching. The Dzyaloshinskii-Moriya interaction at the Ta/CoFeB interface is believed to be the main reason for the discrepancy of the requisite in-plane magnetic fields for switching in the as-deposited and annealed samples. In addition, asymmetric field dependence behavior of SOT-induced magnetization switching is observed in the annealed samples. Deterministic magnetization switching in the absence of an external magnetic field is obtained in the annealed samples, which is extremely important to develop SOT-based magnetoresistive random access memory.

Co/Ni multilayers with different layer thicknesses and repetition numbers were fabricated by magnetron sputtering. The films with appropriate Co and Ni layer thicknesses show strong perpendicular magnetic anisotropy. The results of magnetic force microscopy indicate that the films show a maze domain in the demagnetization state and that the domain width decreases with increasing layer thickness and repetition number. The magnetostatic and domain wall energies of the film stack were calculated on the basis of an irregular maze domain pattern. The results suggest that the magnetostatic energy is the main reason for the variation of the domain width in Co/Ni multilayers.

Spin-orbit torques (SOTs) in Ta/CoFeB/MgO structures are studied by harmonic voltage method. The results indicate that both Slonczewski-like (HSL) and field-like (HFL) effective field are enhanced by annealing in the film stacks with various Ta thicknesses. Investigation of the crystallographic phase of the Ta layers and resistance of Hall bar devices suggest that annealing may induce a phase transformation in the Ta layers from the α to the β phase, which results in the enhanced HSL of the annealed samples. Current-induced magnetization switching experiments revealed a corresponding decrease of the switching current in the annealed samples because of their enhanced SOTs.