Neutron Scattering by Coupled Paramagnetic Ions

The paramagnetic scattering of very low energy neutrons by MnF₂ and MnO was studied at various sample temperatures. The assumption of Van Vleck that a Gaussian distribution can be used to represent the frequency of inelastic scattering events versus energy transfer with the spin lattice is used, and the root mean square half-width of the distribution of energy transfer for MnF₂ is found to be 0.0020, 0.0026, and 0.0034 ev at 610^∘{}, 300^∘{}, and 126^∘{}K, respectively. Short-range order scattering modifies the scattering cross section for MnF₂ to the extent of 1.54π barns/steradian at 126^∘{}K, but is not observed at higher temperatures.Measurements at 300^∘{}K indicate the magnetic scattering cross section for MnO, it if were not distorted by short-range order scattering, would be about 2.54π barns/steradian lower than for MnF₂. We suggest this may be accounted for by the mechanism of super-exchange between crystal ions. Rough values of a parameter indicating the range of spin order in MnO were determined at 300^∘{} and 133^∘{}K. The amount of spin order increases with decrease in sample temperature. Above the N\'eel temperature, it is found that most of the correlation of spin direction exists only between adjacent ions on the same sublattice.