Spin-orbit torque-induced magnetic dynamics of perpendicularly magnetized CoFeB multilayer frames

Abstract

This doctoral dissertation describes an experimental study on perpendicular magnetic anisotropy (PMA) features in CoFeB-based multilayer structures and related spin-orbit torque induced magnetic dynamics. In the first step, various magnetic properties including PMA, magnetic dead layer and diffusion behavior of Ta/CoFeB/MgO multilayer frames was investigated. Increasing thickness of a CoFeB layer allows for the degradation of PMA as the portion of interfacial anisotropy gets smaller compared to that of shape anisotropy. The increase in post-annealing temperature over 300 ℃ deteriorate PMA and magnetization due to a diffusion of Ta atoms into a CoFeB layer and the interface between CoFeB/MgO. Secondly, in order to examine the clear evidence for the deterioration of PMA during thermal treatment, the influence of thickness of a Ta under layer on the degree of degradation in PMA and magnetization of a Ta/CoFeB/MgO matrix was presented. Experimental analysis suggests that the diffusion of Ta atoms into the CoFeB/MgO structures was controlled by the thickness of Ta under layer, which corresponds to the amount of diffusion sources. Thus, optimization of Ta under layer leads to proper magnetization performance with high annealing temperature stability. Thirdly, for more thermally stable CoFeB/MgO stacks, a diffusion sponge concept using TaOx layer was adopted in Ta/CoFeB/MgO. As grain boundaries can act as an efficient diffusion path, defect-rich TaOx layers under the Ta layer provide additional diffusion paths for Ta atoms, which consequentially suppress the diffusion of Ta atoms into CoFeB and MgO layers. As a result, TaOx/Ta/CoFeB/MgO configuration maintained PMA features and magnetizations up to 350 ℃ annealing process while conventional Ta/CoFeB/MgO stacks show almost zero-magnetization due to complete intermixing of CoFeB and Ta layer. Fourthly, thermally robust characteristics of W-based CoFeB/MgO multilayer was addressed. W was selected as the alternative of Ta layer due to its high cohesive force and its low formation energy of borides. Highly stable perpendicular magnetic anisotropy was achieved up to 450 °C at a specific W thickness in W/CoFeB/MgO frames. Lastly, spin-orbit torque (SOT) behaviors in Ta/CoFeB/MgO and W/CoFeB/MgO frames were systemically analyzed using in-plane direct current probing method. I address an in-plane direct current measurement approach as a generic alternative tool to identify spin orbit torque-driven effective fields in a full polar angle range without adopting the commonly used harmonic analyses.