Spin superfluidity and long-range transport in thin-film ferromagnets

TL;DR

This paper investigates how long-range dipole interactions influence spin superfluidity in thin-film ferromagnets, revealing that such superfluidity exists only at certain length scales and proposing multilayer structures for its re-emergence.

Contribution

It demonstrates that dipolar interactions limit spin superfluidity to specific length scales and predicts its re-emergence in multilayer ferromagnet-normal metal systems.

Findings

Dipolar interactions destroy spin superfluidity beyond a few hundred nanometers.

Spin superfluidity can re-emerge in trilayer structures of about 1 micrometer.

Long-range spin transport persists in samples several micrometers in size.

Abstract

In ferromagnets, magnons may condense into a single quantum state. Analogous to superconductors, this quantum state may support transport without dissipation. Recent works suggest that longitudinal spin transport through a thin-film ferromagnet is an example of spin superfluidity. Although intriguing, this tantalizing picture ignores long-range dipole interactions; we demonstrate that such interactions dramatically affect spin transport. In single-film ferromagnets, "spin superfluidity" only exists at length scales (a few hundred nanometers in yttrium iron garnet) somewhat larger than the exchange length. Over longer distances, dipolar interactions destroy spin superfluidity. Nevertheless, we predict re-emergence of spin superfluidity in tri-layer ferromagnet--normal metal--ferromagnet films of $\sim 1\, \mu$m in size. Such systems also exhibit other types of long-range spin transport in samples several micrometers in size.