Dielectric anomalies and spiral magnetic order in CoCr₂O₄

We have investigated the structural, magnetic, thermodynamic, and dielectric properties of polycrystalline CoCr₂O₄, an insulating spinel exhibiting both ferrimagnetic and spiral magnetic structures. Below T_c=94 K the sample develops long-range ferrimagnetic order, and we attribute a sharp phase transition at T_S≈27 K to the onset of long-range spiral magnetic order. Neutron measurements confirm that the structure remains cubic at 80 K and at 11 K; the magnetic ordering by 11 K is seen to be rather complex. Density functional theory supports the view of a ferrimagnetic semiconductor with magnetic interactions consistent with noncollinear ordering. Capacitance measurements on CoCr₂O₄ show a sharp decrease in the dielectric constant at T_S, but also an anomaly showing thermal hysteresis falling between approximately T=50 and 57 K. We tentatively attribute the appearance of this higher-temperature dielectric anomaly to the development of short-range spiral magnetic order, and discuss these results in the context of utilizing dielectric spectroscopy to investigate noncollinear short-range magnetic structures.