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 mea sured. The spin-hip cross-section displays well defined spin waves with E-q = Dq(2) + Delta at low-energy transfers. Above E similar or equal to 3 meV , the excitations broaden significantly and their intensity becomes small. In addition, the slope of the dispersion curve becomes very steep indicatin g the transition from coherent magnetic excitations to single-particle exci tations. The polarized beam experiments prove directly that the Stoner exci tations 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 delta S\\M in other ferromagnets like Ni or EuS. (C) 1999 Elsevier Science B.V. All rights reserved.