Spin dynamics of the itinerant helimagnet MnSi studied by positive muon spin relaxation.

Abstract

The local magnetic fields and spin dynamics of the itinerant helimagnet MnSi (${\mathit{T}}_{\mathit{c}}$\ensuremath{\simeq}29.5 K) have been studied experimentally using positive muon spin rotation and relaxation (${\mathrm{\ensuremath{\mu}}}^{+}$SR) methods. In the ordered phase (T${\mathit{T}}_{\mathit{c}}$), zero-field \ensuremath{\mu}SR was used to measure the hyperfine fields at the muon sites as well as the muon spin-lattice relaxation time ${\mathit{T}}_{1}^{\mathrm{\ensuremath{\mu}}}$. Two magnetically inequivalent interstitial ${\mathrm{\ensuremath{\mu}}}^{+}$ sites were found with hyperfine coupling constants ${\mathit{A}}_{\mathrm{hf}}^{(1)}$=-3.94 kOe/${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ and ${\mathit{A}}_{\mathrm{hf}}^{(2)}$=-6.94 kOe/${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$, respectively. In the paramagnetic phase (T>${\mathit{T}}_{\mathit{c}}$), the muon--nuclear-spin double-relaxation technique was used to simultaneously but independently determine the spin-lattice relaxation time ${\mathit{T}}_{1}^{\mathrm{Mn}}$ of $^{55}\mathrm{Mn}$ spins and that of positive muons (${\mathit{T}}_{1}^{\mathrm{\ensuremath{\mu}}}$) over a wide temperature range (${\mathit{T}}_{\mathit{c}}$T\ensuremath{\le}150 K). The temperature dependence of ${\mathit{T}}_{1}^{\mathrm{Mn}}$ and ${\mathit{T}}_{1}^{\mathrm{\ensuremath{\mu}}}$ in both phases shows systematic deviations from the predictions of self-consistent renormalization theory.