A Raman study of the temperature and magnetic field dependence of electronic and lattice properties in MnSi

Abstract

The temperature and magnetic field dependence of lattice and carrier excitations in MnSi is studied in detail using inelastic light scattering. The pure symmetry components of the electronic response are derived from the polarization dependent spectra. The $E$ and $T_2$ responses agree by and large with longitudinal and optical transport data. However, an anomaly is observed right above the magnetic ordering temperature $T_{\rm C}=29\,{\rm K}$ that is associated with the fluctuations that drive the transition into the helimagnetic phase first order. The $T_1$ spectra, reflecting mostly chiral spin excitations, have a temperature dependence similar to that of the $E$ and $T_2$ symmetries. The response in the fully symmetric $A$ representation has a considerably weaker temperature dependence than that in the other symmetries. All nine Raman active phonon lines can be resolved at low temperature. The positions and line widths of the strongest four lines in $E$ and $T_2$ symmetry are analyzed in the temperature range $4<T<310\,{\rm K}$. Above $50\,{\rm K}$, the temperature dependence is found to be conventional and given by anharmonic phonon decay and the lattice expansion. Distinct anomalies are observed in the range of the helimagnetic transition and in the ordered phase. Applying a magnetic field of $4\,{\rm T}$, well above the critical field, removes all anomalies and restores a conventional behavior highlighting the relationship between the anomalies and magnetism. The anomaly directly above $T_{\rm C}$ in the fluctuation range goes along with an anomaly in the thermal expansion. While the lattice constant changes continuously and has only a kink at $T_{\rm C}$, all optical phonons soften abruptly suggesting a direct microscopic coupling between spin order and optical phonons rather than a reaction to magnetostriction effects.