Electron paramagnetic resonance probing of the spin fluctuation transition in the conical spiral phase of MnSi

In the spiral conical phase of manganese monosilicide, MnSi, the EPR spectra are studied at a frequency of 9.3 GHz in the temperature range of 5-30 K for an external magnetic field oriented along the crystallographic direction [100]. It is found that in the temperature range T < T* = 25 K, located 4 K below the paramagnetic phase-conical phase transition temperature T-C = 29 K, a complex EPR spectrum consisting of several lines appears. A procedure is proposed for decomposing experimental spectra into components, which makes it possible to find for each spectral line the temperature dependences of the resonance magnetic field, line width, and integrated intensity. At T-f similar to 16 K, one of the main lines in the EPR spectrum with the highest integrated intensity demonstrates a maximum in the temperature dependence of the line width W(T), corresponding to a change in W by a factor of similar to 2.8. We argue that this unusual behavior appears as a consequence of a spin fluctuation transition in the conical spiral magnetic phase with long range magnetic order.