Spin Dynamics in Invar (Fe₆₅Ni₃₅, Fe₃Pt) and Non Invar (Fe₅₀Ni₅₀) Alloys

Abstract

Spin dynamics of a disordered Invar alloy Fe₆₅Ni₃₅, an ordered Invar alloy Fe₃Pt as well as a non Invar disordered alloy Fe₅₀Ni₅₀ have been studied by neutron scattering. It has been found that both Invar alloys show common anomalous dynamical properties. Well defined magnon groups could be detected up to the Curie temperature. The spin wave stiffness constant D(T) varies with temperature as D(T)=D₀(1−CT^5⁄2). The magnon excitations, however, could explain only about a half of the temperature variation of magnetization M(T) for the Invar alloys, in spite of the fact that the integrated intensity measurements of the magnon spectra suggest that no other excitations take part in M(T). M(T) of non Invar alloy could be accounted for by the magnon excitations. The magnons in the Invar alloys dampen significantly with increasing temperature as was apparent in the linewidth Γ(q, T) obeying a relation Γ(q, T)=(Γ₀+aT^α)q² with α≤1, quite different from the Heisenberg system.