Current-driven spin-orbit-torque phenomena in perpendicularly magnetized heavy metal|ferromagnet|insulator multilayer structures

In this thesis, we have systematically studied on the current-driven Spin-Orbit-Torque (SOT) phenomena in perpendicularly magnetized heavy-metal|ferromagnet|insulator multilayer structures to optimize the free-part of the magnetic tunnel junction. Firstly, we worked on the effect of heavy-metal layer thickness on the structural, magnetic, transport and spintronics properties in such a structure. A non-monotonic behavior in the Damping-Like (DL)-SOT is found as a function of the thickness of the heavy-metal layer. It is observed that the sign of the DL-SOT changes with increasing the thickness of the Hf layer in the multilayer structure. Secondly, we worked on the effect of oxide layer on magnetic, transport and spintronics properties in Hf|CoFeB|MgO and Hf|CoFeB|TaOx multilayer structures. Both DL and Field-Like (FL)-torques are found to be significantly stronger in MgO-capped structures than those in TaOx-capped structures. The difference in FL and DL SOTs in different structures may be attributed to the different Rashba-like Hamiltonian, arising from the difference in the electric potential profiles across the oxide|ferromagnet interfaces in the two cases. Finally, we worked on the engineering of structural asymmetry in multilayer structures to switch the magnetization without external field. The studied structures exhibit broken in-plane inversion symmetry due to a wedged CoFeB layer, resulting in a field-like spin-orbit torque, which can be quantified by a perpendicular effective magnetic field. In particular, while the current-driven deterministic perpendicular magnetic switching was observed at zero magnetic field in Hf|CoFeB|MgO, an external magnetic field is necessary to switch the CoFeB layer deterministically in Hf|CoFeB|TaOx. Based on the experimental results, the SOT magnitude in Hf|CoFeB|MgO was found to be almost 13× larger than that in Hf|CoFeTaOx. The CoFeB thickness dependence of the magnetic switching behavior, and the resulting effective magnetic field by in-plane currents are also investigated in this work. These findings could potentially be used to engineer SOT devices with enhanced current-driven switching efficiency.