Magnetic excitations in the weak itinerant ferromagnet MnSi

Abstract

Magnetic excitations in an intermetallic compound MnSi have been studied by neutron scattering. At 5 K in a magnetic field of 10 kOe, well-defined spin-wave excitations have been observed below 2.5 meV. The dispersion relation is almost isotropic and is $h{\omega }_q\left(\mathrm{meV}\right)=0.13+52\mathrm{}{q}^2$ (${\mathrm{\AA }}^{-2\mathrm{}}$) in the [100] direction. Above 3 meV, the excitation linewidth increases substantially, suggesting that the dispersion relation merges into the Stoner continuum. The Stoner excitations, which extend over almost all of the Brillouin zone, show a broad peak on the extension of the spin-wave dispersion relation. The spin-wave excitation renormalizes with increasing temperature and collapses into critical scattering above 40 K. On the other hand, the excitation in the Stoner continuum is affected little by temperature; the excitations are qualitatively the same at $\frac{T}{T_N}=10\mathrm{}$ as at 5 K. The Stoner boundary energy decreases with increasing temperature. The results provide us with the first example of magnetic excitations in a weak itinerant ferromagnet.