Field dependence of spin dynamics in the itinerant ferromagnet MnSi

Inelastic neutron scattering has been used to study the field dependence of low-energy magnetic excitations of the weak itinerant ferromagnet MnSi. At 5 K, the gap in the spin-wave dispersion relation increases linearly with applied magnetic field, with a slope that is consistent with a $g$ factor of 2, and the spin-wave stiffness is independent of field. At 45 K, which is 15 K above the zero-field $T_c$, a 70-kOe field restores spin-wave behavior with a gap that is nearly equal to that at 5 K and with a stiffness about half as great as that observed at low temperature. The Stoner boundary seems to be shifted by the field, especially above $T_c$.