Thermal expansion and magneto-volume studies of the itinerant helical magnet MnSi

TL;DR

This study investigates the thermal and magnetic properties of MnSi, revealing that its magnetic phase transition remains first order at all conditions, with heterophase fluctuations smoothing the transition rather than a fluctuation-induced change.

Contribution

It challenges the fluctuation-induced first order transition model for MnSi, proposing heterophase fluctuations as the smoothing mechanism across all temperatures and fields.

Findings

Discontinuity in length decreases with magnetic field and then abruptly vanishes.

Thermal expansivity peaks diminish under magnetic fields.

No evidence of a tricritical point was observed.

Abstract

Thermal expansion and forced magnetostriction of MnSi were measured as a function of temperature down to 5 K and magnetic field to 3 T. The small length (volume) discontinuity at the magnetic phase transition in MnSi decreases with application of magnetic field to a value $\Delta L/L \sim 10^{-7}$, and then suddenly the discontinuity seemingly jumps to zero. Thermal expansivity peaks strongly deteriorate with magnetic fields. No specific features identifying a tricritical point were observed. We propose that the Frenkel concept of heterophase fluctuations may be relevant in the current case. Therefore, we suggest that the magnetic phase transition in MnSi always remains first order at any temperature and magnetic field, but the transition is progressively smoothed by heterophase fluctuations. These results question the applicability of a model of a fluctuation-induced first order phase transition for MnSi. Probably a model of coupling of an order parameter with other degrees of freedom is more appropriate.