Stabilization of exponential number of discrete remanent states with localized spin–orbit torques

Abstract

Using bilayer films of β-Ta/Ni0.8Fe0.2, we fabricate structures consisting of two, three, and four crossing ellipses, which exhibit shape-induced bi-axial, tri-axial, and quadro-axial magnetic anisotropy in the crossing area, respectively. Structures consisting of N crossing ellipses can be stabilized in 2N remanent states by applying (and removing) an external magnetic field. However, we show that with field-free spin–orbit torques induced by flowing currents in individual ellipses, the number of remanent states grows to 2N. Furthermore, when the current flows between the edges of different ellipses, the number of remanent states jumps to 22N, including states that exhibit a π-Néel domain wall in the overlap area. The very large number of accessible remanent magnetic states that are exhibited by the relatively simple magnetic structures paves the way for intriguing spintronics applications including memory devices.

A four-state magnetic tunnel junction switchable with spin-orbit torques

“…The method presented here for achieving four resistance states per MTJ can be extrapolated to achieve a much larger number of resistance states. As shown recently, a structure consisting of N crossing ellipses may support up to 2 2N states 37 ; therefore, using in a MTJ relatively simple structures of up to N=5 would increase the number of resistance states quite considerably. We note that structures of up to N=5 were previously studied and that simulations indicate 70 resistance states of a MTJ consisting of layers of N=4 and N=5 crossing ellipses structures while considering only a small subset of accessible remanent states 36 .…”

“…Irrespective of the potential application of a multi-level MTJ, it is important that the method used for switching between the MTJ states is scalable and non-detrimental. These two requirements are met when the switching is achieved by spin-orbit torques (SOTs) which are generated by spin currents injected into a FM layer due to conversion of charge current into spin current in an adjacent heavy metal layer [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] .…”