Dispersion Relation in Amorphous Ferromagnets

Abstract

Dispersion of spin waves in the amorphous ferromagnetic alloy Fe₄₈Ni₃₄P₁₈ can be described within the model of a ferromagnet with random anisotropy: \(\epsilon \)(q) = Aq² + gμ_BH + 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 Å² 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.