Nonlinear magnetic responses at the phase boundaries around helimagnetic and skyrmion lattice phases in MnSi: Evaluation of robustness of noncollinear spin texture

The phase diagram of a cubic chiral magnet MnSi with multiple Dzyaloshinskii-Moriya (DM) vectors as a function of temperature $T$ and dc magnetic field ${H}_{\mathrm{dc}}$ was investigated using intensity mapping of the odd-harmonic responses of ac magnetization $({M}_{1\ensuremath{\omega}}$ and ${M}_{3\ensuremath{\omega}})$, and the responses at phase boundaries were evaluated according to a prescription [J. Phys. Soc. Jpn. 84, 104707 (2015)]. By evaluating ${M}_{3\ensuremath{\omega}}/{M}_{1\ensuremath{\omega}}$ appearing at phase boundaries, the robustness of noncollinear spin texture in both the helimagnetic (HM) and the skyrmion lattice (SkL) phases of MnSi was discussed. The robustness of vortices-type solitonic texture SkL in MnSi is smaller than those of both the single DM HM and chiral soliton lattice phases of a monoaxial chiral magnet ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_{2}$, and furthermore the robustness of the multiple DM HM phase in MnSi is smaller than that of its SkL. Through magnetic diagnostics over the wide $T\ensuremath{-}{H}_{\mathrm{dc}}$ range, we found a new paramagnetic (PM) region with ac magnetic hysteresis, where spin fluctuations have been observed via electrical magnetochiral effect. The anomalies observed in the previous ultrasonic attenuation measurement correspond to the peak positions of out-of-phase ${M}_{1\ensuremath{\omega}}$. The appearance of a new PM region occurs at a characteristic magnetic field, above which indeed the SkL phase appears. It has us suppose that the new PM region could be a phase with spin fluctuation like the skyrmion gas phase.