Low-energy paramagnetic spin fluctuations in the weak itinerant ferromagnet MnSi

Low-energy paramagnetic excitations in the weak itinerant ferromagnet (WIF) MnSi have been studied by neutron scattering. The observed spectrum has a Lorentzian form (ΓΓ²+ω²) and is clearly separated from excitations in the Stoner continuum. The generalized susceptibility, χ(q), has been obtained by integrating the scattering intensity over energy. It is found that χ(q) depends upon the wave vector q as χ(q)^-1=κ²(T)+q² for q≤0.125(2πa) with κ²(T)=κ₀²(T-Tc). After extrapolating these results to $q=0$, it is found that χ(q=0) follows the Curie-Weiss law, suggesting that the observed spin fluctuations correspond to the Moriya-Kawabata (MK) spin fluctuations responsible for the Curie-Weiss dependence of the static susceptibility of a WIF. The linewidth Γ is found to be proportional to qχ(q) as predicted by the MK theory, in contrast with the q²χ(q) relation expected in a Heisenberg system. These results provide the first direct experimental evidence for the existence of MK spin fluctuations in a WIF above Tc.