Topological features of the electronic structure and phase diagram of the chiral ferromagnet MnSi

It is shown that the reason for the appearance of the complex pattern of chiral spin short-range order observed in the helical ferromagnet MnSi is a topological electronic transition (TET). TET occurs under conditions of thermodynamic instability of ferromagnetism, when the mode-mode parameter in the Ginzburg–Landau functional becomes negative, and the chemical potential falls into the energy region of Berry curvature. It was found that the topological features of the electronic structure lead to the appearance of skyrmion lattices phases and fluctuations of left-chiral spin helices. In the paramagnetic region, a phase of fluctuations of left- and right-handed spin spirals appears. The emergence of a thermodynamically stable non-chiral paramagnetic phase is accompanied by a shift in the chemical potential beyond the energy region of Berry curvature and an abrupt disappearance of local magnetization (delayed magnetic phase transition). The constructed $h$–$T$ phase diagram ($h$ is the magnetic field strength, $T$ is the temperature) is consistent with experiment.