Magnetic excitations in Fe3Ni77B18Si2 amorphous alloy

The thermal demagnetization of Fe3Ni77B18Si2 amorphous ferromagnetic alloy (T-c = 19.5 K) has been studied in the temperature range 5-30 K. The magnetization data M(T) are analysed by using the Bose-Einstein integration of the effective density of states which interpolates between the long wavelength magnon (n(omega) alpha omega(d/2-1)) and the short wavelength fracton (n(omega) alpha omega(<(d)over tilde /2-1>)) regime. The best fit yields the following parameters: the spin-wave stiffness constant D = 2.72 meV Angstrom(2), the crossover frequency omega(c) = 1.94 x 10(10) s(-1) and the fracton dimension (d) over tilde = 1.37. Therefore, our results confirm the contribution of the short wavelength excitations in the process of thermal demagnetization of our alloy and support the Alexander-Orbach conjecture that (d) over tilde = 4/3 for d greater than or equal to 2.