On the asymmetry of the magnonic Hanle effect in antiferromagnets

In the magnonic Hanle effect, which has been discovered only recently [Wimmer et al., PRL 125, 247204 (2020)], a spin current is injected into an insulating antiferromagnet (hematite) and tuned by means of an applied magnetic field. We present a theoretical framework to describe the spin transport in terms of low-energy waves of the antiferromagnetic Néel order --- the classical counter part of magnons. Intriguingly, these waves come with two different polarizations which we describe, in analogy to optics, using the Stokes vector on the Poincaré sphere. We find that the polarization changes periodically with a frequency that is nonlinear in the magnetic field leading to an asymmetry in the measured spin signal. By modelling the energy-dependent spin injection and assuming a diffusive transport regime, we are able to reproduce the signatures found in the experiment.