Inexhaustible physics of the helical magnet MnSi: field evolution of the magnetic phase transition inferred from ultrasound studies

TL;DR

This study investigates the magnetic phase transitions in MnSi using ultrasound measurements, revealing complex behavior of elastic constants and skyrmion phases influenced by magnetic field inhomogeneity and sample shape.

Contribution

It provides new insights into the field evolution of magnetic phase transitions and skyrmion behavior in MnSi through ultrasound studies, highlighting the role of demagnetization and inhomogeneity.

Findings

Discontinuity in elastic constant c11 signifies phase transition.

Skyrmion phase corresponds to vanishing c11 discontinuity.

Magnetic field inhomogeneity affects skyrmion stability and elastic properties.

Abstract

The longitudinal and transverse ultrasound speeds and attenuation were measured in a MnSi single crystal in the temperature range of 2 - 40 K and magnetic fields to 7 Tesla. The magnetic phase diagram of MnSi in applied magnetic field appears to depend on the experimental setups, which is related to a difference in demagnetization factors arising due to the disc shape of the sample. The magnetic phase transition in MnSi in zero magnetic field is signified by a quasi discontinuity in the c11 elastic constant, which varies significantly with magnetic field. It is notable that the region where the c11 discontinuity almost vanishes closely corresponds to the extent of skyrmion phase along the magnetic to paramagnetic transition. This implies that the c11 elastic constant is almost continuous through the transition from the skyrmion to paramagnetic phases. A recovery of the discontinuity of c11 and enhanced sound absorption occur at the crossing of the phase transition line and the line of minima in c11. The powerful fluctuations at the minima of c11 make the mentioned crossing point similar to a critical end point, where a second order phase transition meets a first order one. The skyrmion domain in the case of a perpendicular setup with a smaller demagnetization factor has a reduced temperature range, which suggests that the magnetic field inhomogeneity plays an important role in the skyrmion occurrence and, hence, opens a way of skyrmion manipulation. The small anisotropy of the shear moduli in the (001) plane found in the parallel setup is most probably also caused by the magnetic field inhomogeneity, which distorts the hexagonal symmetry of the skyrmion crystal.