Structural distortion below the N\'eel temperature in spinel GeCo$_2$O$_4$

TL;DR

This study reports a structural phase transition in GeCo$_2$O$_4$ below its Néel temperature, revealing a decoupling of structural and magnetic orderings and highlighting magnetodielectric coupling, with comparative analysis of related spinels.

Contribution

It provides the first complete crystallographic description of GeCo$_2$O$_4$'s tetragonal phase and discusses the origin of its structural distortion in magnetic spinels.

Findings

Structural transition from cubic to tetragonal at 16 K in GeCo$_2$O$_4$.

Decoupling of structural and magnetic orderings at different temperatures.

Discovery of a second antiferromagnetic transition in GeFe$_2$O$_4$.

Abstract

A structural phase transition from cubic $Fd\bar{3}m$ to tetragonal $I$4$_1$/$amd$ symmetry with $c/a >$ 1 is observed at $T_{\rm{S}}$ = 16 K in spinel GeCo$_2$O$_4$ below the N\'eel temperature $T_N$ = 21 K. Structural and magnetic ordering appear to be decoupled with the structural distortion occurring at 16 K while magnetic order occurs at 21 K as determined by magnetic susceptibility and heat capacity measurements. An elongation of CoO$_6$ octahedra is observed in the tetragonal phase of GeCo$_2$O$_4$. We present the complete crystallographic description of GeCo$_2$O$_4$ in the tetragonal $I$4$_1$/$amd$ space group and discuss the possible origin of this distortion in the context of known structural transitions in magnetic spinels. GeCo$_2$O$_4$ exhibits magnetodielectric coupling below $T_{\rm{N}}$. The related spinels GeFe$_2$O$_4$ and GeNi$_2$O$_4$ have also been examined for comparison. Structural transitions were not detected in either compound down to $T \approx$ 8 K. Magnetometry experiments reveal in GeFe$_2$O$_4$ a second antiferromagnetic transition, with $T_{\rm{N1}}$ = 7.9 K and $T_{\rm{N2}}$ = 6.2 K, that was previously unknown, and that bear a similarity to the magnetism of GeNi$_2$O$_4$.