Dielectric anomalies and spiral magnetic order in CoCr2 O4

We have investigated the structural, magnetic, thermodynamic, and dielectric properties of polycrystalline CoCr2 O4, an insulating spinel exhibiting both ferrimagnetic and spiral magnetic structures. Below Tc =94 K the sample develops long-range ferrimagnetic order, and we attribute a sharp phase transition at TS 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 CoCr2 O4 show a sharp decrease in the dielectric constant at TS, 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.