Sublattice Magnetization and Lattice Distortions in MnO and NiO

Abstract

If in the Heisenberg interaction J (S̄_a·S̄_b), J is separation dependent and the lattice is distortable, then an extended system of arbitrary angular momentum yields more long‐range order at a given temperature than the molecular field predicts for the ``clamped crystal.'' Both MnO and NiO show such departures from molecular field prediction and earlier estimates based on isotropic deformations could not account for the observed behavior. Anisotropic deformations have not previously been considered in this regard. The form of these anisotropic distortions is a contraction along the [111] crystallographic direction perpendicular to the sublattice magnetization. In NiO this trigonal deformation corresponds to a change in angle of the original cubic cell from 90° to 90°06′. This distortion accounts for only a small part of the sublattice magnetization's deviation from molecular field prediction. For MnO, on the other hand, neutron diffraction data indicate that this angle is as large as 90°26′, a distortion that would account for about a third of the observed estimate. We have made a direct x‐ray diffraction measurement of this deformation and fully confirm this value. The x‐ray data supply additional evidence that the disordering transition in MnO is thermodynamically of first order.