Effective interatomic interactions and lattice mechanical properties of the NiO crystal for both paramagnetic and antiferromagnetic phases

Abstract

The antiferromagnetic oxide crystals behave rather unusually in respect to many physical properties, and the various attempts to understand them in terms of models which satisfactorily describe the simple ionic solids show conspicuous discrepancies between theory and experiment. One of the major difficulties common to all the existing calculations is that it is not possible to give a consistent description of the elastic properties and the dispersion of phonons of such a crystal in the antiferromagnetic phase within the framework of a single model. Moreover, so far, the magnetic and the mechanical properties of these crystals have been studied, one independent of the other. Apart from this difficulty, a simultaneous description of the dielectric properties and the phonon dispersion with the existing models also presents several problems. In a previous work we have suggested a model which is based on a microscopic analysis of the energy expression of an assembly of ions. Assuming a specific form of the superexchange interaction that is suggested by various experimental and theoretical investigations and incorporating the same in the above model, we have been able to reduce significantly the discrepancies mentioned above for the case of NiO crystal. In addition to this, it has also been possible to present a unified description of the magnetic and the mechanical properties of this crystal with a single model. The specific properties that we have attempted to correlate with a single set of model parameters are: the cohesive and the stability properties, the elastic and the dielectric properties, the dispersion relation of phonons and magnons, and the sublattice magnetization properties. We also indicate the rough order of magnitude of the different interactions implied in the model and the relations that they bear to the unusual features of this solid. Some of the limitations of the present model and the directions of further refinement are also discussed.