MAGNETIC EXCITATIONS IN VERY DILUTE AMORPHOUS FEXNI80-XB18SI2 FERROMAGNETS

The magnetization of seven amorphous ferromagnetic alloys FexNi80-xB18 Si2 with the magnetic atom concentrations x in the vicinity of the mag netic percolation threshold (x(c) congruent to 2.32 at% Fe) has been s tudied. Over a broad temperature range from 1.8 K to T-c of the respec tive alloy, the temperature dependence of the magnetization M(T) is we ll described for all alloys by using the effective density of states, N-eff(omega), of the magnetic excitations, which in the limit of low f requencies (omega much less than omega(c)) reflects the magnon density of states, whereas at high frequencies (omega much greater than omega (c)) represents the density of states of the short-wavelength (localiz ed) fractons. The values derived for the spin-wave stiffness constants , D, and the crossover frequencies, omega(c), show a power-law depende nce on the reduced concentration. Accordingly, the disappearance of th e ferromagnetism with decreasing concentration x of magnetic atoms in the studied alloy system is well described in terms of the percolation theory. (C) 1998 Elsevier Science B.V. All rights reserved.