Field-free approaches for deterministic spin–orbit torque switching of the perpendicular magnet

Abstract

All-electrical driven magnetization switching attracts much attention in next-generation spintronic memory and logic devices, particularly in magnetic random-access memory (MRAM) based on the spin-orbit torque (SOT), i.e., SOT-MRAM, due to its advantages of low power consumption, fast write/read speed, and improved endurance, etc. For conventional SOT-driven switching of the magnet with perpendicular magnetic anisotropy (PMA), an external assisted magnetic field is needed to break the inversion symmetry of the magnet, which not only induces the additional power consumption but also makes the circuit more complicated. Over the last decade, significant effort has been devoted to field-free magnetization manipulation by using SOT. In this review, we introduce the basic concepts of SOT. After that, we mainly focus on several approaches to realize the field-free deterministic SOT switching of the perpendicular magnet. The mechanisms mainly include mirror symmetry breaking, chiral symmetry breaking, exchange bias, and interlayer exchange coupling. Furthermore, we show the recent progress on the study of SOT with unconventional origin and symmetry. The final section is devoted to the industrial-level approach for potential applications of field-free SOT switching in SOT-MRAM technology.

Exchange Bias Between van der Waals Materials: Tilted Magnetic States and Field‐Free Spin–Orbit‐Torque Switching

“…In general, to achieve deterministic switching of a magnetic layer with PMA using SOT from a high-symmetry material like Pt requires an external symmetrybreaking field, [42] which can be accomplished, for example, with an applied magnetic field (B ext ) in the sample plane parallel to the direction of applied current, [43,44] in-plane exchange bias from an adjacent antiferromagnet or ferromagnet layer, [41,[45][46][47] or other effective fields. [48] We perform pulsed-current measurements with different fixed B ext , and after each pulse we measure the Hall voltage near zero current. Before each pulse-current switching sequence, we initialize the magnetization of the FGT with an out-of-plane field just above the coercivity.…”

Wedge-shaped HfO2 buffer layer-induced field-free spin-orbit torque switching of HfO2/Pt/Co structure

“…This inconsistency excludes the possibility of spin reflection-induced out-of-plane spin current-induced field-free switching. The field-free switching is not induced either by uneven current density due to shunting effects, [30] as experiments demonstrate high resistance in HfO 2 and negligible influence on Hall resistance. We attribute the field-free switching in the system to the anisotropy gradient induced by the wedged-shaped HfO 2 buffer layer.…”

Spin-orbit torque switching of magnetic tunnel junctions for memory applications

“…Accommodating a magnetic field source to the SOT-MRAM cell represents a problem for downscaling and device density. Substitution of a permanent magnet by integration-friendly alternatives is, therefore, a key objective in SOT switching applications, which has resulted in a broad variety of conceptually different approaches to magnetic field-free switching [2,133,134].…”