  Magnon eigenmodes in easy-plane antiferromagnetic insulators are linearly
polarized and are not expected to carry any net spin angular momentum.
Motivated by recent nonlocal spin transport experiments in the easy-plane phase
of hematite, we perform a series of micromagnetic simulations in a nonlocal
geometry at finite temperatures. We show that by tuning an external magnetic
field, we can control the magnon eigenmodes and the polarization of the spin
transport signal in these systems. We argue that a coherent beating oscillation
between two orthogonal linearly polarized magnon eigenmodes is the mechanism
responsible for finite spin transport in easy-plane antiferromagnetic
insulators. The sign of the detected spin signal is also naturally explained by
the proposed coherent beating mechanism. Our finding opens a path for on-demand
control of the spin signal in a large class of easy-plane antiferromagnetic
insulators.
