# Is spin superfluidity possible in YIG films?

## TL;DR

This paper critically analyzes the possibility of spin superfluidity in YIG films, revising the theoretical spectrum and stability conditions, and concludes that observed claims are not justified despite theoretical feasibility.

## Contribution

It provides a detailed theoretical analysis of spin superfluidity in YIG films, revising the spin-wave spectrum and stability criteria, and challenges previous experimental claims.

## Key findings

- Spin superfluidity in YIG films is theoretically possible.
- The published experimental claim of observing spin superfluidity is not justified.
- Revised the spin-wave spectrum and non-linear corrections in YIG films.

## Abstract

Recently it was suggested that stationary spin supercurrents (spin superfluidity) are possible in the magnon condensate observed in yttrium-iron-garnet (YIG) magnetic films under strong external pumping. Here we analyze this suggestion. From topology of the equilibrium order parameter in YIG one must not expect energetic barriers making spin supercurrents metastable. However some small barriers of dynamical origin are possible nevertheless. The critical phase gradient (analog of the Landau critical velocity in superfluids) is proportional to intensity of the coherent spin wave (number of condensed magnons). The conclusion is that although spin superfluidity in YIG films is possible in principle, the published claim of its observation is not justified. The analysis revealed that the widely accepted spin-wave spectrum in YIG films with magnetostatic and exchange interaction required revision. This led to revision of non-linear corrections, which determine stability of the magnon condensate with and without spin supercurrents.