Ferromagnetic resonance in amorphous alloys

The frequency and temperature dependences of the magnetic properties of amorphous transition metal-metalloid alloys are studied by means of ferromagnetic resonance (FMR). In the series (TM x Ni 1− x ) 75P 16B 6Al 3, TM = {Fe, Co}, it is found that, while the frequency dependence of the FMR linewidth, Γ, in all these materials is well characterized by the usual Landau-Lifshitz-Gilbert relaxation parameter, there is also a frequency independent contribution to Γ. This is attributed to a static magnetic non-uniformity. In addition, the FMR linewidths increase at low T (with decreasing T) in the alloys with a low spin concentration, x. This effect is linked to the development of a dynamic magnetic inhomogeneity in these alloys at low T and is thought to be related to the onset of a spin glass state (in zero field) at low temperatures. Finally, the T dependence of the magnetization shows that the magnetic excitations in the high x alloys are spin waves. However, in the more dilute alloys the spin wave description is no longer valid. A simple model of clusters of strongly coupled spins (“buoys”) embedded in a “sea” of weakly coupled spins is suggested to qualitatively account for all of the data.