Notes on the Vollhardt "invariant" and phase transition in the helical itinerant magnet MnSi

TL;DR

This paper discusses how magnetic field influences smeared second-order phase transitions in MnSi, challenging the interpretation of the Vollhardt crossing point and proposing a model for the transition from planar ferromagnetic to helical order.

Contribution

It provides a new perspective on the nature of phase transitions in MnSi, emphasizing the smeared transition and its relation to magnetic field effects, and clarifies the interpretation of crossing points.

Findings

Heat capacity and elastic modulus indicate smeared second-order transition.

Magnetic field spreads out the phase transition, affecting crossing points.

Transition from planar ferromagnetic to helical order at lower temperatures.

Abstract

In this paper we argue that rounded "hills" or "valleys" demonstrated by the heat capacity, thermal expansion coefficient, and elastic module are indications of a smeared second order phase transition, which is flattened and spread out by the application of a magnetic field. As a result, some of the curves which display a temperature dependence of the corresponding quantities cross almost at a single point. Thus, the Vollhardt crossing point should not be identified with any specific energy scale. The smeared phase transition in MnSi preceding the helical first order transition most probably corresponds to the planar ferromagnetic ordering, with a small or negligible correlation between planes. At lower temperatures, the system of ferromagnetic planes becomes correlated, acquiring a helical twist.