Phase transitions and off-stoichiometric effects of vanadium spinel oxide CoV₂O₄

We have investigated magnetic properties and phase transitions in polycrystalline and single crystal samples of the spinel compound CoV2O4 by magnetization, specific heat, and x-ray diffraction measurements. In addition to the ferrimagnetic transition, another phase transition is clearly observed at T2 = 47 K in the specific heat measurement which has not been observed in single crystals to date. The magnetization and specific heat measurements for the polycrystalline samples of the Co1+xV2−xO4 system showed that the phase transition at T2 is suppressed as x increases, however, it remains for x ≤ 0.15. From the result, the behavior of magnetization and specific heat in our single crystal is found to be consistent with that in the polycrystalline sample for Co1.1V1.9O4. This suggests that the fundamental physical properties of the Co1+xV2−xO4 system are determined by the concentration x and independent with whether the sample is polycrystalline or single crystal. These results reveal that the phase transition at T2 is intrinsic in CoV2O4 and exists even in the single crystal with less excess Co. The detailed magnetic properties of the single crystal showing the transition at T2 have also been investigated.