Ferrimagnetic spiral configurations in cobalt chromite

Abstract

Cobalt chromite, CoCr2O4, is a cubic spinel which is ferrimagnetic at low temperatures with a Curie point Tc ≈ 97 °K. The room temperature powder diffraction pattern on this material, corrected for temperature effects, shows that it is a normal spinel with an oxygen parameter equal to 0.38707 :f: .00005. At 4.2 °K there are, in addition to the magnetic contributions to the fundamental spinel peaks, a large number of magnetic 11 satellite peaks. All of the additional peaks can be indexed on the basis of the ferrimagnetic spiral model proposed for normal cubic spinels by Lyons, Kaplan, Dwight and Menyuk, in which the spiraling components of the spins are defined by a single k vector along a face diagonal. The experimental magnitude of |k| is approximately 5 % greater than theoretically predicted. The neutron diffraction pattern predicted on the basis of the spiral model is completely determined upon fixing the exchange ratio JBB/J AB and the cone axis direction. Taking JBB /J AB = 1. 5 on the basis of magnetization measurements previously reported, it is found that the intensities of the various peaks are in excellent agreement with the intensities predicted by the spiral model with the cone axis along an [001] direction. The ferrimagnetic spiral configuration is known to become unstable relative to small deviations for JBB SB /JAB S A > 0.98 (i.e. JBB/JAB > 0.98 in CoCr2O4). However, our results indicate that the true configuration closely approximates that predicted by the model over a range of exchange interaction ratios which extends well beyond the onset of local instability. Upon increasing the temperature above 4,2 °K, the satellite peaks disappeared between 25 °K and 35 °K, apparently degenerating into a broad plateau observed at 50 °K and 77 °K. Despite this disappearance, a self-consistent fit to neither the magnetic contributions to the fundamental peaks nor the observed thermal variation of the magnetization can be obtained from a collinear model. However, the predicted values based on a molecular field treatment of the spiral model are in good agreement with these measurements. These results further corroborate the validity of the spiral model for CoCr2O4, and indicate that the molecular field approximation accurately describes the axial component throughout its ferrimagnetic range, but not the azimuthal component.