Dielectric anomalies and spiral magnetic order in CoCr2O4

TL;DR

This study explores the complex magnetic and dielectric behaviors of CoCr2O4, revealing a transition to spiral magnetic order and associated dielectric anomalies, with implications for using dielectric spectroscopy to probe magnetic structures.

Contribution

It provides new insights into the dielectric and magnetic properties of CoCr2O4, highlighting the link between short-range spiral magnetic order and dielectric anomalies.

Findings

Long-range ferrimagnetic order develops below 94 K.

A phase transition at 25 K to spiral magnetic order.

Dielectric constant shows anomalies linked to magnetic transitions.

Abstract

We have investigated the structural, magnetic, thermodynamic, and dielectric properties of polycrystalline CoCr$_2$O$_4$, 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 $T_N$ $\approx$ 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 CoCr$_2$O$_4$, show a sharp decrease in the dielectric constant at $T_N$, 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 \textit{short-range} spiral magnetic order, and discuss these results in the context of utilizing dielectric spectroscopy to investigate non-collinear short-range magnetic structures.