Spin-orbit torques emerge as a promising method for manipulating magnetic configurations of spintronic devices. Here, we show that these torques can induce a magnetization reversal via domain wall propagation which may open new ways in developing novel spintronic devices and in particular in realizing high-density multi-level magnetic memory. Our devices are bi-layer heterostructures of Ni0.8Fe0.2 on top of β-Ta patterned in the form of two or three crossing ellipses which exhibit in the crossing area shape-induced biaxial and triaxial magnetic anisotropy, respectively. We demonstrate field-free switching between discrete remanent magnetic states of the structures by spin-orbit torques induced by flowing electrical current through one of the ellipses. We note switchings induced by the coupling between the ellipses where current flowing in one ellipse triggers a reversal in a neighboring ellipse which propagates from the center outwards. Numerical tools successfully simulate the observed coupling-induced switching using experimentally extracted parameters.

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.

We present a magnetic tunnel junction (MTJ) where its two ferromagnetic layers are in the form of a single ellipse (SE) and two-crossing ellipses (TCEs). The MTJ exhibits four distinct resistance states corresponding to the four remanent states of the TCE structure. Flowing current in an underlying Ta layer generates in the adjacent TCE structure spin–orbit torques, which induce field-free switching of the four-state MTJ between all its resistance states. The demonstrated four-state MTJ is an important step toward fabricating multi-level MTJs with numerous resistance states, which could be important in various spintronics applications, such as multi-level magnetic random access or neuromorphic memory.

No abstract