Spin-Wave Energies and Exchange Parameters in Iron-Nickel Alloys

The energies of long‐wavelength spin waves in iron‐nickel alloys have been measured as a function of composition by a neutron small‐angle‐scattering technique. The behavior of the composition dependence can be explained in terms of a simple model invoking long‐range interaction functions $\mathcal{J}(r)$ between the three different types of atom pair: Fe–Fe; Fe–Ni; and Ni–Ni. There is evidence that on the bcc lattice the iron‐nickel interaction $\mathcal{J}(r)$ changes sign from +ve to ‐ve as a function of distance. Between Fe–Fe pairs on a fcc lattice, the ``exchange integral'' as normally defined is −8±2 MeV, consistent with other indications that fcc iron is antiferromagnetic.