Field-free magnetization switching is critical towards practical, integrated spin-orbit torque (SOT)-driven magnetic random-access memory with perpendicular magnetic anisotropy. Our work proposes a technique to modulate the spin reflection and spin density of states within a heavy-metal Pt through interfacing with a dielectric MgO layer. We demonstrate tunability of the effective out-of-plane spin torque acting on the ferromagnetic Co layer, enabling current-induced SOT magnetization switching without the assistance of an external magnetic field. The influence of the MgO layer thickness on effective SOT efficiency shows saturation at 4 nm, while up to 80% of field-free magnetization switching ratio is achieved with the MgO between 5 and 8 nm. We analyze and attribute the complex interaction to spin reflection at the dielectric/heavy metal interface and spin scattering within the dielectric medium due to interfacial electric fields. Further, through substituting the dielectric with Ti or Pt, we confirm that the MgO layer is indeed responsible for the observed field-free magnetization switching mechanism.

Spin–orbit torque (SOT) induced magnetization switching and SOT modulation by interfacial coupling exhibit good potential in spintronic devices. In this work, we report the enhancement of damping-like field and SOT efficiency of up to 60% and 23%, respectively, in perpendicularly magnetized Pt/Co/HfOx heterostructures over a Pt/Co system at an optimal thickness of 2 nm HfOx. The SOT improvement is primarily attributed to the interfacial oxidization of the Co layer, and the strength is tunable via voltage-induced oxygen ion migration at the Co/HfOx interface. Our measurement reveals that by controlling gate voltages, the Co oxidation can be increased, which leads to the SOT efficiency enhancement. Our work promotes the SOT enhancement and modulation by oxidation effects for energy-efficient spintronic devices.

We have shown that, by applying a DC bias during harmonic Hall measurement in Ta/Co/Pt structure with in-plane magnetic anisotropy, a net spin accumulation that results in an amplitude offset of the measured first harmonic Hall resistance in the Co layer, can be electrically quantified. A difference of the harmonic Hall resistance amplitudes when at opposite magnetisation states under a fixed DC bias was obtained, which gave direct measurements of spin accumulation up to 0.29% per kV/m of the local magnetization. The strength of both spin accumulation and damping-like efficiency was found to increase with larger Ta thicknesses, further verifying our experimental measurement that provides direct allelectrical quantification of spin accumulation.

Topological insulators demonstrate high charge-spin conversion efficiency due to their spin-momentum locking at the Dirac surface states. However, the surface states are sensitive to disruption caused by exchange coupling when interfaced with a ferromagnet. Here, we demonstrate the use of various nonmagnetic insertion layer materials, Ti, Cu, and Pt, at the Co/Bi-Sb(012) interface to preserve the topological surface state and promote spin-orbit-torque efficiency through the crystallinity control of Bi-Sb(012). For 20-nm-thick Bi-Sb, a spin Hall angle of up to 8.93 is observed with the use of a Pt insertion layer, while it is otherwise negligible for Co/Bi-Sb(012) interfaces. We further explore the enhancement of Bi-Sb(012) crystallinity with increasing Bi-Sb thickness, revealing a rapidly increasing spin-orbit-torque efficiency that gradually saturates above 30 nm. A clear correlation between spin-orbit-torque efficiency and Bi-Sb(012) crystalline size is identified using x-ray diffractometry, establishing the origin of the high spin-orbit efficiency to be the Bi-Sb(012) crystalline orientation. Our work demonstrates the spin-orbittorque origin in Bi-Sb experimentally and paves the way for the adaptation of topological insulators as a class of low-energy spin source material for spintronics applications.