Vollhardt invariant and phase transition in the helical itinerant magnet MnSi

In this Rapid Communication we argue that rounded ``hills'' or ``valleys'' demonstrated by the heat capacity, thermal expansion coefficient, and elastic moduli 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 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.