Superfluid spin transport through antiferromagnetic insulators

Abstract

A theoretical proposal for realizing and detecting spin supercurrent in an isotropic antiferromagnetic insulator is reported. Superfluid spin transport is achieved by inserting the antiferromagnet between two metallic reservoirs and establishing a spin accumulation in one reservoir such that a spin bias is applied across the magnet. We consider a class of bipartite antiferromagnets with Néel ground states, and temperatures well below the ordering temperature, where spin transport is mediated essentially by the condensate. Landau-Lifshitz and magnetocircuit theories are used to directly relate spin current in different parts of the heterostructure to the spin-mixing conductances characterizing the antiferromagnet|metal interfaces and the antiferromagnet bulk damping parameters, quantities all obtainable from experiments. We study the efficiency of spin angular-momentum transfer at an antiferromagnet|metal interface by developing a microscopic scattering theory for the interface and extracting the spin-mixing conductance for a simple model. Within the model, a quantitative comparison between the spin-mixing conductances obtained for the antiferromagnet|metal and ferromagnet|metal interfaces is made.

Nonlocal Magnetoresistance Mediated by Spin Superfluidity

“…An exciting frontier explores how analogs of conventional superfluidity, as observed in liquid 4 He, can be obtained in magnetic systems, and how dissipationless spin currents can be realized and detected in such systems [2]. Conventional superfluidity is characterized by a rigid U(1) order parameter, a single quantum-mechanical wave function describing a macroscopic number of constituent particles.…”

“…Here, dissipationless spin current (polarized out of plane) is triggered by a collective reorientation, i.e., a planar spiraling texture of the magnetic order. Theoretical proposals for realizing and detecting such superfluid spin transport have been put forth for (ferro-and antiferro-) magnetic insulators [3][4][5] and multiferroic materials [6]. …”

“…Specifically, recall that for a ferromagnetic insulator, n corresponds to the direction of the local spin density, and the U(1) easy plane can generally be defined by the shape anisotropy [3]. For an isotropic antiferromagnet, n is the direction of the local Néel order, and the U(1) plane is defined to be normal to a uniform external magnetic field [4]. For the axially symmetric magnetic state, the spin density polarized along the z axis is a hydrodynamic quantity that is approximately conserved.…”

“…This dynamics can be quenched by such anisotropies which tend to pin the texture. While this pinning may be overcome by injecting large enough spin currents [2], achieving such large spin currents does pose experimental challenges [4].…”

“…Superfluid spin transport may be detected using a twoterminal spin transport setup, in which two spin-orbit coupled metals are attached on opposite ends of the magnetic insulator and direct (inverse) spin Hall effect facilitates spin injection (detection) [3,4]. While the two-terminal spin conductance so obtained can reveal a smoking-gun signature of spin superfluidity-with Gilbert damping spin supercurrent decays algebraically over space while spin current carried by incoherent thermal magnons decays exponentially over the spin diffusion length-such probe requires a study of spin transmission through various sample sizes.…”