High-magnetic-field investigation of CoCr₂O₄

We report on magnetic, optical, and thermodynamic properties of multiferroic CoCr₂O₄ in magnetic fields up to 14 T. We have found indications of a new phase transition at T* = 5 − 6 K. The phase between T* and the lock-in transition at 15 K is characterized by magnetic irreversibility. At higher fields the irreversibility increases. Heatcapacity measurements confirm the transition at T*, and also show the irreversible behaviour. We construct a field-temperature phase diagram of CoCr₂O₄. Below the ferrimagnetic transition (94 K), the low-frequency (terahertz) optical response is dominated by a magnetic exchange mode, which shows an anomalous temperature dependence and a softening at the structural transition (26 K).