Highly nonlinear magnetoelectric effect in antiferromagnetic Co4Ta2O9 single crystals

TL;DR

This paper reports a highly nonlinear magnetoelectric effect in single crystals of Co4Ta2O9, revealing complex magnetic-field-dependent ferroelectric polarization behavior distinct from related compounds.

Contribution

It demonstrates the discovery of a nonlinear magnetoelectric response in Co4Ta2O9, highlighting the role of inequivalent Co2+ sublattices in generating opposite-signed polarization.

Findings

Nonlinear polarization behavior under magnetic fields.

Ferroelectricity emerges along the [110] direction.

Magnetic field induces polarization crossing zero and increasing to 60-80 μC/m².

Abstract

Strongly correlated materials with multiple order parameters provide unique insights into the fundamental interactions in condensed matter systems and present opportunities for innovative technological applications. A class of antiferromagnetic honeycomb lattices compounds, A4B2O9 (A = Co, Fe, Mn; B = Nb, Ta), have been explored owing to the occurrence of linear magnetoelectricity. We observe a highly nonlinear magnetoelectric effect on single crystals of Co4Ta2O9 (CTO), distinctive from the linear behavior in the isostructural Co4Nb2O9. Ferroelectricity emerges primarily along the [110] direction under magnetic fields, with the onset of antiferromagnetic order at TN = 20.5 K. For in-plane magnetic field, a spin-flop occurs at HC ~ 0.3 T, above which the ferroelectric polarization gradually becomes negative and reaches a broad minimum. Upon increasing magnetic field further, the polarization crosses zero and increases continuously to ~60 uC/m2 at 9 T. In contrast, the polarization for a magnetic field perpendicular to the hexagonal plane increases monotonously and reaches ~80 uC/m2 at 9 T. This observation of a strongly nonlinear magnetoelectricity suggests that two types of inequivalent Co2+ sublattices generate magnetic field-dependent ferroelectric polarization with opposite signs. These results motivate fundamental and applied research on the intriguing magnetoelectric characteristics of these honeycomb lattice materials.