Dispersion Relation in Amorphous Ferromagnets

Dispersion of spin waves in the amorphous ferromagnetic alloy Fe 48 Ni 34 P 18 can be described within the model of a ferromagnet with random anisotropy: ϵ ( q ) = Aq 2 + g μ B H + δω( q ), where δω( q ) is an additional term linear in | q |. The method of small-angle scattering of polarized neutrons is used to prove the importance of the additional term δω( q ) in dispersion. The measurements are carried out for different values of the external magnetic field H and neutron wavelength λ. The scattering map of neutrons represents a circle centered at the point q = 0. The stiffness A of spin waves is derived directly from the λ-dependence of the radius of this circle. The spin-wave stiffness A of the amorphous alloy weakly decreases from 140 to 110 meV Å 2 as temperature increases from 50 to 300 K. The field dependence of the radius demonstrates the presence of an additional term δω( q ) in the form of an energy gap that is almost independent of field and temperature. The value of the additional term is Δ = 0.015 ± 0.002 meV.