Neutron scattering from paramagnetic RbMnF3

The energy distribution of neutrons scattered from polycrystalline RbMnF 3 at room temperature ( T/T N = 2.8) has been measured using time-of-flight analysis. Evidence is obtained for the existence of a cut-off in the energy distribution at large scattering vectors. The nearest-neighbour exchange interaction 2 J 1 ( S i . S i ) is derived both by direct evaluation of the second moment of the energy distribution and from the best-fitting Gaussian. The results suggest J 1 /k = 3.3 ± 0.3 °K, in good agreement with other determinations. A general calculation is made of the effect of biquadratic exchange on the second moment. It is shown that this is unlikely to affect the above result.