Reinvestigation of Magnetic Structures of CoCr2O4 and MnCr2O4 Obtained by Neutron Diffraction

Spinels CoCr2O4 and MnCr2O4 display a magnetic structure in which moments on octahedral sites split into two groups. In one of these, transverse components are antiparallel and have the fcc symmetry of the chemical cell. The transverse components of the other group together with those of the tetrahedral sites build a (3a, 3a, a) magnetic cell. Axial components are along [001] for CoCr2O4 and along [110] for MnCr2O4, and are in good agreement with magnetization measurements. Good agreement is also obtained between measured and calculated magnetic intensities. These structures can be explained in a molecular-field approximation. Short-range order is still present at low temperatures.