A puzzling insensitivity of magnon spin diffusion to the presence of 180$^\circ$ domain walls in a ferrimagnetic insulator

TL;DR

This study shows that magnon spin diffusion in a ferrimagnetic insulator is largely unaffected by 180° domain walls, challenging existing models of magnon-domain wall interactions.

Contribution

It provides the first experimental evidence that 180° domain walls have minimal impact on magnon spin transport in ferrimagnetic insulators.

Findings

Magnon spin diffusion retains at least 95% of its signal in multi-domain states.

Spin transport is unaffected over distances five times the domain size.

Results conflict with simple magnon-domain wall interaction models.

Abstract

We present room-temperature measurements of magnon spin diffusion in epitaxial ferrimagnetic insulator MgAl$_{0.5}$Fe$_{1.5}$O$_{4}$ (MAFO) thin films near zero applied magnetic field where the sample forms a multi-domain state. Due to a weak uniaxial magnetic anisotropy, the domains are separated primarily by 180$^\circ$ domain walls. We find, surprisingly, that the presence of the domain walls has very little effect on the spin diffusion -- nonlocal spin transport signals in the multi-domain state retain at least 95% of the maximum signal strength measured for the spatially-uniform magnetic state, over distances at least five times the typical domain size. This result is in conflict with simple models of interactions between magnons and static domain walls, which predict that the spin polarization carried by the magnons reverses upon passage through a 180$^\circ$ domain wall.