Magnetic Moment Arrangements and Magnetocrystalline Deformations in Antiferromagnetic Compounds

Abstract

The neutron diffraction data of Shull et al. on the powder specimens of MnO, FeO, CoO, etc., are re-examined. It is found that except for FeO the observed Debye-Scherrer patterns may be produced by a number of magnetic moment arrangements with the same antiparallel coupling among the moments on the next nearest neighboring ions, but with different sublattice correlations and different orientations of magnetic axis (or axes). The symmetry change observed at the Néel point is interpreted as a strong effect of spontaneous anisotropy magnetostriction. As a consequence the uniaxial direction of the modified symmetry must coincide with that of the magnetic axis. Assignments of antiferromagnetic arrangements for the compounds are reached. Our prescription does not agree with that of Shull et al. except in the case of FeO. The sharp readjustment of lattice parameters at the transition temperature and the orientation of spin axis in other antiferromagnetic compounds are discussed from the viewpoint of the anisotropy magnetostriction approach.