Effects of Hydrostatic Pressure on the Paramagnetic Resonance Spectra of Several Iron Group Ions in Cubic Crystals

The magnetic resonance spectra of Cr³⁺, Mn²⁺, Fe³⁺, and Ni²⁺ present as substitutional impurities in MgO crystals and powders, and of Mn²⁺ in cubic ZnS have been observed as functions of hydrostatic pressure at room temperature. The results are interpreted assuming the local compressibilities to be equal to those of the pure host lattices.The measured volume dependence of the orbital contributions to the magnetic moments of the F-state ions, Cr³⁺ and Ni²⁺, are consistent with a point-charge model within the experimental error. This simple model can only crudely account for the observed magnitudes of the orbital singlet-triplet splittings, however.The pressure dependences of the cubic field splittings of the S-state ions, Mn²⁺ and Fe³⁺, are identical in MgO and correspond roughly to a fourth power law if an ionic potential is assumed. The cubic field parameter for Mn²⁺ in ZnS varies half as rapidly with volume. These results are consistent with recent calculations of Powell, Gabriel, and Johnston if it is assumed that the volume dependence of the cubic potentials in these lattices are given by the ionic model, though the relative magnitudes are not.The hyperfine structure of the manganese spectra also proves sensitive to sample volume, particularly in the sulfide. The effect may be qualitatively understood in terms of the exchange-polarization theory of the strong electron-nuclear interaction.