Crossover from incommensurate to commensurate magnetic orderings in CoCr₂O₄

The conical spin order of multiferroic CoCr₂O₄ has been studied by a neutron diffraction to investigate its magnetic phase transitions at temperatures below 40K. Magnetic order of a spiral spin component with an incommensurate propagation vector of (0.63, 0.63, 0) was observed at 26K, while at 14.5K, the incommensurate conical spin order showed a transition into the fixed commensurate propagation vector of (2/3,2/3,0). In addition, two satellite peaks with propagation vectors of (0.035, 0, 0) and (0, 0.035, 0) from the commensurate vector were observed. The widths of these peaks indicate a long-range magnetic order. This new magnetic configuration below 14.5K may lead to a new model of multiferroic behavior differing from the well-known spin-current model for magnetic ferroelectricity.