Field evolution of the magnetic phase transition in the helical magnet MnSi inferred from ultrasound studies

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 up 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 disk shape of the sample. The magnetic phase transition in MnSi in zero magnetic field is signified by a quasidiscontinuity in the ${c}_{11}$ elastic constant, which varies significantly with magnetic field. It is notable that the region where the ${c}_{11}$ discontinuity almost vanishes closely corresponds to the extent of skyrmion phase along the magnetic to paramagnetic transition. This implies that the ${c}_{11}$ elastic constant is almost continuous through the transition from the skyrmion to paramagnetic phases. A recovery of the discontinuity of ${c}_{11}$ and enhanced sound absorption occur at the crossing of the phase transition line and the line of minima in ${c}_{11}$. The powerful fluctuations at the minima of ${c}_{11}$ make the mentioned crossing point similar to a critical end point, where a second order phase transition meets a first order one.