Spin dynamics of the amorphous Invar alloy Fe_0.86B _0.14

High-resolution neutron scattering studies have been made of the long wavelength spin excitations in a ribbon sample of amorphous Fe _0.86B_0.14, 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 T_c) and 500 K (0.90 T_c). 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/T_c) ^5/2] in the range of temperatures under study, with D(0)=132 meV Ų. This value of D(0) is approximately twice as large as that calculated from the T^3/2 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 Fe _xB_1-x system.