Spin fluctuations and a spin-fluctuation phase transition in the magnetically ordered phase of manganese monosilicide

A manganese monosilicide (MnSi) single crystal has been studied by the high-frequency (60 GHz) electron paramagnetic resonance method. The most currently detailed temperature dependences of the linewidth and g-factor have been obtained in the temperature range of 2–40 K. They indicate that a spin fluctuation transition occurs in the spin-polarized phase of nSi, formed by magnetic polarons in a magnetic field of $B \sim 2$ T at ${T_{\text{L}}} \sim 15$ K. This transition corresponds to change in the regime of magnetic fluctuations at a temperature much lower than the transition temperature $T_c \sim 30$ K from the paramagnetic phase existing at $T>T_c$ to the magnetically ordered phase occurring at $T<T_c$. This result extends the region of applicability of the concept of spin fluctuation transitions, which were previously considered only for magnetic states without long-range magnetic order. A model for the description of the electron paramagnetic resonance parameters in the system of magnetic polarons has been discussed.