Spin-orbit Torque Switching of Multi-state Magnetic Structures: A Novel Route towards Multi-Level Magnetic Random Access Memory

One of the main challenges facing the spintronics industry is identifying efficient and scalable switching methods of magnetic nanostructures. A promising route is using heavy metal (HM) films in proximity with ferromagnetic (FM) films – namely, HF/FM heterostructures. When current flows through such structures, the charge current flowing through the HM layer generates, due to the large spin-orbit interaction (SOI), a spin current in transverse direction to the charge current and into the FM layer. In addition, the SOI can also generate an effective magnetic field at the HM/FM interface due to the Rashba effect. The two mechanisms yield two types of spin-orbit torque (SOT) on the FM layer known as field like and anti-damping like torques. Until now, the SOT-induced magnetization switching in HM/FM heterostructures has been mainly demonstrated in structures with uni-axial anisotropy and uniform magnetization.

Here we study SOT-induced magnetization switching of multi-state magnetic structures in Ta(5 nm)/Ni_0.8Fe_0.2(2 nm)/Ti(3nm) heterostructures where the magnetization of the NiFe layer is in plane and the magnetic anisotropies are induced by shape. Three types of magnetic structures are used: an elongated ellipse exhibiting a uniaxial anisotropy parallel to its long axis, two crossing ellipses exhibiting bi-axial anisotropy where the easy axes are between the axes of the ellipses and three crossing ellipses exhibiting tri-axial anisotropy where the easy axes are along the arms of the ellipses. For the latter two structures the high order anisotropy is confined to overlap area of the ellipses. We monitor the magnetic state of our structures with the planar Hall effect and demonstrate field-free current-induced switching. In addition, we identify a dominant role of the anti-damping torque. We discuss how the results pave the way for a novel multi-level magnetic random access memory with SOT writing operation.