Generation of nonreciprocity in gapless spin waves by chirality injection

Abstract

It is known that in chiral magnets with intrinsic inversion symmetry breaking, 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 easy-cone ferromagnets and easy-cone antiferromagnets. Specifically, we show that nonlocal injection of a spin current from proximate normal metals to easy-cone magnets engenders a nonequilibrium chiral spin texture, on top of which spin waves exhibit nonreciprocity proportional to the injected spin current. In particular, the easy-cone ferromagnet is shown to support the spin-wave nonreciprocity without an external field, in contrast to the previously known easy-plane ferromagnetic counterpart that requires an external field, thereby providing a field-free means to manipulate the spin-wave nonreciprocity. One notable feature of the nonreciprocal spin waves is their gapless nature, which can lead to a large thermal rectification effect at sufficiently low temperatures.