Distant-Neighbor B-B Interactions in Cobalt Chromite

Abstract

We have recently shown that five distinct distant-neighbor B-B exchange interactions can play significant roles in determining the ground-state spin configuration in chromium spinels having nonmagnetic A-site ions. This investigation is now extended to materials with magnetic A-site ions and is further generalized to include A-A interactions. The results are used to reinterpret our experimental findings for cobalt chromite. Reasonable values for the distant-neighbor interactions lead to an eight percent decrease in the theoretical wavelength of the minimum-energy ferrimagnetic spiral, which brings it into agreement with the observed value. Furthermore, inclusion of these values in the minimization of the free energy (in the molecular-field approximation) leads to the prediction of an additional magnetic transition (where a phase angle vanishes) at about one third of Tc. The resulting kink in the computed variation of magnetization with temperature is in close agreement with experiment. Our neutron-diffraction pattern is shown to be incompatible with contradictory results recently reported for a CoCr2O4 sample produced by conventional ceramic techniques. The overall agreement between theory and experimental results obtained with our sample indicates that the precursor method of preparation yields more nearly ideal CoCr2O4.