Spin dynamics of the amorphous Invar alloy Fe0.86B0.14

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Abstract
High‐resolution neutron scattering studies have been made of the long wavelength spin excitations in a ribbon sample of amorphous Fe0.86B0.14, which exhibits Invar properties. Spin waves were observed in the wave vector range 0.05 Å−1≤q≤0.12 Å−1 at temperatures between 300 K (0.54 Tc) and 500 K (0.90 Tc). The spin wave energies are well described by a dispersion relation E=Dq2+Δ. The small energy gap Δ of ≊0.04 meV is attributed primarily to the dipole‐dipole interaction. The stiffness constant renormalizes with temperature as D=D(0)[1.0−0.86 (T/Tc) 5/2] in the range of temperatures under study, with D(0)=132 meV Å2. This value of D(0) is approximately twice as large as that calculated from the T3/2 coefficient of the magnetization, a discrepancy common to many Invar materials. Plots of the intrinsic linewidths against q4 and T2 reveal that the data are consistent with the Γ∝q4[Tln(kT/E)]2 dependence predicted for a Heisenberg ferromagnet. There are no anomalies in the spin‐wave lifetimes at long wavelengths which appear to relate to the Invar effect seen in the FexB1−x system.