Spin Dynamics in the Amorphous Invar Alloy of Fe86B14

The spin waves in an amorphous Invar alloy Fe 86 B 14 11 , have been studied by inelastic neutron scattering. Well defined magnon groups were detected around (0, 0, 0) up to q =0.12 Å -1 for various temperatures below T ≤0.9 T c . The spin waves show the dispersions \( ω q(T)=E₀(T)+D(T)q²\) with D ( T ) varying with temperature according to D ( T )= D (0)(1- a T 5/2 ) with D (0)=118 ±6 (meV A 2 ). The spin wave stiffness at 0 K, D (0), thus determined is almost twice as large as the value D M (0)=68 ±2 (meV A 2 ) determined for the same sample by a magnetization measurement. The spin wave linewidth \( \) varies with wave vector q as \(=aq²\) and it shows only limited temperature dependence. All of these features are quite similar to those found in the crystalline Invar alloys as Fe 65 Ni 35 and Fe 3 Pt.