Magnetic Structures in FeCr2S4 and FeCr2O4

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Abstract
The magnetic structures in FeCr2S4 and FeCr2O4 have been investigated by the powder neutron diffraction technique. From room temperature diffraction patterns, it is established that both compounds are normal spinels with sulfur and oxygen parameters u=0.384 and 0.386, respectively. The FeCr2S4 specimen exhibits a Néel temperature of 180°K, with σ0=1.6 μB. The magnetic intensities of this compound at 4°K are in good agreement with those based on a simple Néel model with μFe=4.2 μB and μCr=2.9 μB. The magnetic properties of FeCr2O4 are, on the other hand, more complex. The moment σ0 is considerably lower than the value expected from the Néel model. Moreover, the crystal becomes tetragonal with c/a<1 around 135°K. The magnetic reflections at 4°K show satellite peaks characteristic of a cone spiral structure described in the theory of Menyuk, Dwight, Lyons, and Kaplan for tetragonal spinels. However, a quantitative comparison of observed and calculated intensities indicates that some modifications of the simple cone model are necessary. The satellite reflections disappear at 35°K, and the spin arrangement is collinear between this temperature and the Néel temperature at 80°K.