Magnetic cluster structures in an iron–nickel invar alloy

Small-angle scattering from an Fe₆₅Ni₃₅ single-crystal has been measured with the magnetic field applied parallel and perpendicular to the scattering vector κ. Scattering cross sections due to longitudinal and transverse fluctuations have been calculated. The transverse cross section consists mainly of spin-wave scattering and has a long tail up to κ = 0.2 Å⁻¹. This suggests that there are spin-wave modes heavily damped by magnetic inhomogeneities. The longitudinal cross section can be described by a Guinier approximation (dσ/dΩ)₀ exp (− κ²R²_g/3) for the range 0.05 < κ < 0.2 Å⁻¹. The forward-scattering cross section (dσ/dΩ)₀ and the radius of gyration R_g obtained from experiment are compared with values calculated from Kachi's model. In this model the magnetization is zero for every cluster of 60 atoms with a concentration of nickel below 29%. The comparison shows that the observed value for R_g (= 7.7 Å) is about twice as large as the calculated one. The observed forward scattering (dσ/dΩ)₀ has slightly different values for different crystal orientations and is on the average 75% of the calculated value. The agreement is remarkable with regard to the crudeness of the model.