Dielectric anomalies and spiral magnetic order in CoCr2O4

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We have investigated the structural, magnetic, thermodynamic, and dielectric properties of polycrystalline CoCr₂O₄, 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 Tₙ ≈ 25 K with the onset of long-range spiral magnetic order. Neutron measurements confirm that while the structure remains cubic at 80 K and at 11 K; there is complex magnetic ordering by 11 K. Density functional theory supports the view of a ferrimagnetic semiconductor with magnetic interactions consistent with non-collinear ordering. Capacitance measurements on CoCr₂O₄, show a sharp decrease in the dielectric constant at Tₙ, but also an anomaly showing thermal hysteresis falling between approximately T = 50 K and T = 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 non-collinear short-range magnetic structures.