Generation of Nonreciprocity of Gapless Spin Waves by Chirality Injection

TL;DR

This paper theoretically demonstrates that nonreciprocal, gapless spin waves can be generated in nonchiral magnets through nonlocal spin current injection, enabling electrically controllable magnetochiral effects.

Contribution

It introduces a novel mechanism for spin-wave nonreciprocity in nonchiral magnets via nonlocal spin injection, expanding potential applications in spintronics.

Findings

Nonreciprocal spin waves can occur without intrinsic bulk chirality.

Injected spin current induces a non-equilibrium chiral spin texture.

The nonreciprocity is proportional to the injected spin current.

Abstract

In chiral magnets with intrinsic inversion symmetry breaking, it has been known that two spin waves moving in opposite directions can propagate at different velocities, exhibiting a phenomenon called magnetochiral nonreciprocity which allows for realizations of certain spin logic devices such as a spin-wave diode. Here, we theoretically demonstrate that the spin-wave nonreciprocity can occur without intrinsic bulk chirality in certain magnets including easy-cone ferromagnets and easy-cone antiferromagnetic. Specifically, we show that nonlocal injection of a spin current from proximate normal metals to easy-cone magnets engenders a non-equilibrium chiral spin texture, on top of which spin waves exhibit nonreciprocity proportional to the injected spin current. One notable feature of the discovered nonreciprocal spin waves is its gapless nature, which can lead to a large thermal rectification effect at sufficiently low temperatures. We envision that nonlocal electric injection of chirality into otherwise nonchiral magnets may serve as a versatile route to realize electrically controllable magnetochiral phenomena in a wide class of materials.