Linear Enhancement of Spin-Orbit Torques and Absence of Bulk Rashba-Type Spin Splitting in Perpendicularly Magnetized [Pt/Co/W]n Superlattices

TL;DR

This paper reports on [Pt/Co/W]n superlattices with giant spin-orbit torques, strong perpendicular magnetic anisotropy, and linear enhancement with layer repetition, clarifying the origin of SOTs and demonstrating potential for spintronic memory applications.

Contribution

It introduces a new superlattice structure with enhanced SOTs and PMA, and clarifies that the SOTs originate mainly from the spin Hall effect, not bulk Rashba splitting.

Findings

SOTs increase linearly with layer number n.

Giant dampinglike SOT reaches 225% at n=12.

Deterministic switching achieved at low current density.

Abstract

The development of magnetic heterostructures with strong spin-orbit torques (SOTs), low impedance, strong perpendicular magnetic anisotropy (PMA), and good integration compatibility at the same time is central for high-performance spintronic memory and computing applications. Here, we report the development of the symmetry-broken spin-orbit superlattice [Pt/Co/W]n that can be sputtered-deposited on commercial oxidized silicon substrates and have giant SOTs, strong uniaxial PMA of 9.2 Merg/cm3. The dampinglike and fieldlike SOTs of the [Pt/Co/W]n superlattices exhibit a linear increase with the repeat number n and reach the giant values of 225% and -33% (two orders of magnitude greater than that in clean-limit Pt) at n = 12, respectively. The dampinglike SOT is also of the opposite sign and much greater in magnitude than the fieldlike SOT, regardless of the number of n. These results clarify that the spin current that generates SOTs in the [Pt/Co/W]n superlattices arises predominantly from the spin Hall effect rather than bulk Rashba-type spin splitting, providing a unified understanding of the SOTs in the superlattices. We also demonstrate deterministic switching in thicker-than-50-nm PMA [Pt/Co/W]12 superlattices at a low current density. This work establishes the [Pt/Co/W]n superlattice as a compelling material candidate for ultra-fast, low-power, long-retention nonvolatile spintronic memory and computing technologies.