On the topological features of the helical phase transition in MnSi

Many decades of study have revealed very unusual properties of the helical phase transition in MnSi. This situation is briefly described and illustrated in the present paper. One peculiarity is that the phase transition point in MnSi is accompanied by shoulders or additional broad maxima or minima of different thermodynamic and kinetic quantities on the high temperature side of the transition, which in some cases look similar to a property of 2D systems. The whole situation can be tentatively described as a phase transformation of the helical phase of MnSi to the paramagnetic state, which occurs in two steps, as a first order phase transition and then by a breakdown of topological objects such as spin vortices.