Direct observation of spin-flip excitations in MnSi

Using inelastic neutron scattering with polarization analysis, the magnetic excitations in the field induced ferromagnetic state of MnSi have been measured. The spin-flip cross-section displays well defined spin waves with E_q = Dq²+Δ at low-energy transfers. Above E≈3 meV, the excitations broaden significantly and their intensity becomes small. In addition, the slope of the dispersion curve becomes very steep indicating the transition from coherent magnetic excitations to single-particle excitations. The polarized beam experiments prove directly that the Stoner excitations are spin-flip. The non spin-flip excitations are only well defined at low E, whereas at higher E they are barely visible. They have a similar q-dependence as the parallel fluctuations δS∥M in other ferromagnets like Ni or EuS.