Efficient generation of out-of-plane polarized spin current in polycrystalline heavy metal devices with broken electric symmetries

TL;DR

This paper demonstrates a new, efficient method to generate out-of-plane polarized spin currents in polycrystalline heavy metals by breaking electric symmetries, enabling low-power magnetic switching without magnetic fields.

Contribution

It introduces a universal approach to produce z-spin currents in polycrystalline metals through electric symmetry breaking, compatible with semiconductor integration.

Findings

Efficient z-spin generation in polycrystalline metals via electric asymmetry.

Tunable spin-orbit torques by device geometry and material choice.

Deterministic magnetic switching without external magnetic fields.

Abstract

Spin currents of perpendicularly polarized spins (z spins) by an in-plane charge current have received blooming interest for the potential in energy-efficient spin-orbit torque switching of perpendicular magnetization in the absence of a magnetic field. However, generation of z spins is limited mainly to magnetically or crystallographically low-symmetry single crystals (such as non-colinear antiferromagnets) that are hardly compatible with the integration to semiconductor circuits. Here, we report efficient generation of z spins in sputter-deposited polycrystalline heavy metal devices via a new mechanism of broken electric symmetries in both the transverse and perpendicular directions. Both the dampinglike and fieldlike spin-orbit torques of z spins can be tuned significantly by varying the degree of the electric asymmetries via the length, width, and thickness of devices as well as by varying the type of the heavy metals. We also show that the presence of z spins enables deterministic, nearly-full, external-magnetic-field-free switching of a uniform perpendicularly magnetized FeCoB layer, the core structure of magnetic tunnel junctions, with high coercivity at a low current density. These results establish the first universal, energy-efficient, integration-friendly approach to generate z-spin current by electric asymmetry design for dense and low-power spin-torque memory and computing technologies and will stimulate investigation of z-spin currents in various polycrystalline materials.