Role of inhomogeneous spin currents in enabling field-free spin-orbit torque switching

Abstract

Switching of a ferromagnet in a magnetic tunnel junction (MTJ) with current-induced spin–orbit torque (SOT) has recently garnered significant attention as a promising non-volatile memory technology. Nonetheless, a fundamental challenge in SOT-MTJs with perpendicular magnetization is the symmetry constraint imposed by the SOT-induced in-plane spin polarization, which inhibits deterministic switching. One viable approach to break symmetry and enable field-free switching (FFS) is to exploit electric current gradients in SOT-MTJs. We achieve this by introducing a curvature in the SOT current path in the SOT-MTJs; we refer to these as bending structures. This curvature, in turn, generates a spin-current gradient and an inhomogeneous spin polarization in the SOT channel, thereby facilitating fully current-driven deterministic switching. Here, we vary the curvature in the SOT channel to assess the roles of the spin-current gradient and the inhomogeneous spin polarization in the deterministic switching of the bending structures. Using both experimental methods and micro-magnetic simulations, we identify that inhomogeneous spin polarization plays a major role in symmetry-breaking and dictates the switching polarity in bending structures. Furthermore, by scaling the SOT channel, we achieve an additional 48% reduction in the switching current. Lastly, we demonstrate fully electrical-driven ultrafast FFS in the bending structure with 400 ps pulses. This concept of curvature in the SOT current path is not confined to binary memory applications; it can also be explored for innovative spintronic-based computing applications.