Abstract
The results of studying the spin dynamics of a classical Fe65Ni35 invar alloy are presented and analyzed. The investigations are performed via small-angle polarized-neutron scattering in the oblique geometry of a magnetic field at various temperatures (T < T C). This approach is based on the analysis of left-right asymmetry in the magnetic scattering of polarized neutrons. The asymmetry effect arises when the magnetization direction of a sample is inclined with respect to the wave vector of the incident beam. The spin-wave scattering is concentrated within a range bounded by the cutoff angle θc determined by the magnetic field: θ c 2 (H) = θ 0 2 −(gμB H)θ0/E, where θ0=ℏ212Dmn , H is the external magnetic field, E is the initial neutron energy, D is the spin-wave stiffness constant, and m n is the neutron mass. The scattering is blurred by spinwave damping in the vicinity of the cutoff angle. The spin-wave stiffness constant can be obtained from a comparison of the asymmetric contribution to scattering and a model function. The temperature dependence D = D(T) is well defined by the expression D = D 0 |τ| x , where τ=1−TTC , x = 0.47 ± 0.01, D 0 = 137 ± 3 meVÅ2, and τ > 0.1 in the entire temperature range. The given method enables us to construct the temperature dependence of the spin-wave stiffness constant with a high accuracy and a small step.
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