Magnetic Structures in FeCr2S4 and FeCr2O4

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 μ_B. The magnetic intensities of this compound at 4°K are in good agreement with those based on a simple Néel model with μ_Fe=4.2 μ_B and μ_Cr=2.9 μ_B. 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.