Spin-wave dispersion relations have been measured in high-symmetry directions for metallic Gd. Analysis shows that at least five interplanar constants are required for a satisfactory fit to the data. The energy gap at $q=0$ is unmeasurably small. In the c direction the measured dispersion curve gives directly the Fourier-transformed exchange interaction J(0)-J(q). This exhibits no other extreme value except that at the origin.
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Koehler et al. (1970) studied this question.