# Partially disordered state near ferromagnetic transition in MnSi

## TL;DR

This study reveals a partially disordered chiral magnetic state in MnSi near its ferromagnetic transition, caused by spin-wave spectrum instability due to magnetic field direction and anisotropy effects.

## Contribution

It explains the emergence of a partially disordered state in MnSi through analysis of spin-wave spectrum instability influenced by magnetic field orientation and anisotropy.

## Key findings

- Negative spin-wave gap squared indicates instability in certain field directions.
- Partially disordered state occurs below the ferromagnetic transition.
- Spin-wave interactions and cubic anisotropy compete to destabilize helix.

## Abstract

The polarized neutron scattering in helimagnetic MnSi at low $T$ reveals existence of a partially disordered chiral state at ambient pressure in the magnetic field applied along $<111>$ axis below the first order transition to the non-chiral ferromagnetic state. This unexpected phenomenon is explained by the analysis of the spin-wave spectrum. We demonstrate that the square of the spin-wave gap becomes negative under magnetic field applied along $<111>$ and $<110>$ but not along the $<100>$ direction. It is a result of competition between the spin-wave interaction and cubic anisotropy. This negative sign means an instability of the spin wave spectrum for the helix and leads to a destruction of the helical order, giving rise to the partially disordered state below the first order ferromagnetic transition.