A two-terminal planar memory device controlled by spin-orbit torques

Abstract

Spin-orbit torques (SOTs) have emerged as a versatile tool to manipulate the magnetization in magnetic heterostructures [1]. They typically occur in normal metal/ferromagnetic (NM/FM) bilayers where the bulk spin Hall and interfacial spin galvanic effects convert the injected charge current into pure spin currents [2]. The elimination of an FM polarizer to generate spin currents, in contrast to the conventional spin-transfer torque schemes, offers great flexibility in device design, architecture, and functionality [3]. One such device is the three-terminal magnetic tunnel junction (MTJ), where the magnetization of the free FM layer (i.e., the magnetic state of the device) is controlled by a planar current injection generating SOTs, and the magnetization state is probed by a vertical current injection through the oxide barrier via tunnel magnetoresistance [4]. Three-terminal MTJs are considered for scalable, low-power, and high speed magnetic random-access memory applications [5]. The development of three-terminal MTJs for memory applications is already at an advanced stage [6]. Additionally, SOTs allow for the realization of even simpler memory devices, which could simplify the MRAM production process and diversify the circuit design, material spectrum, and related physical phenomena.In this work, we report a new two-terminal device where the magnetic state is controlled and probed by currents sent through the same planar path. This simple device is made of a hard FM (TbCo) and a soft FM (Co) layer, each possessing perpendicular magnetic anisotropy, separated by a Pt layer that acts as a SOT generator. Current injection in the presence of a static in-plane field switches the magnetization of Co between up and down states as in the standard SOT switching scheme. However, unlike in the typical SOT devices, the magnetization state is not probed by the Hall effect but rather by the longitudinal resistance, which has two distinct levels for parallel and antiparallel orientations of TbCo and Co. This magnetoresistive phenomenon stems from the current-in-plane giant magnetoresistance, albeit much smaller in amplitude (<0.1%). This device offers a new pathway to a highly scalable, all-electrical, two-terminal memory that can be produced with low fabrication efforts. **