Structural distortion below the Néel temperature in spinel GeCo${}_2$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 $T_{\mathrm{S}}$ = 16 K in spinel GeCo${}_2$O${}_4$ below the Néel 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 $T_{\mathrm{N}}$. 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 $T_{N1}$ = 7.9 K and $T_{N2}$ = 6.2 K, that was previously unknown, and that bear a similarity to the magnetism of GeNi${}_2$O${}_4$.

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