Elastic Instabilities within Antiferromagnetically Ordered Phase in the Orbitally-Frustrated Spinel GeCo$_2$O$_4$

TL;DR

This study uncovers elastic instabilities in GeCo$_2$O$_4$ caused by phonon-spin interactions and orbital frustration, revealing magnetic-field-induced orbital transitions and novel phenomena within its antiferromagnetic phase.

Contribution

It provides new insights into the elastic and magnetic properties of orbitally-frustrated antiferromagnets, highlighting the role of orbital frustration and phonon-spin coupling.

Findings

Elastic anomalies due to phonon-spin coupling in GeCo$_2$O$_4$

Magnetic-field-induced orbital order transitions

Evidence of geometrical orbital frustration

Abstract

Ultrasound velocity measurements of the orbitally-frustrated GeCo$_2$O$_4$ reveal unusual elastic instabilities due to the phonon-spin coupling within the antiferromagnetic phase. Shear moduli exhibit anomalies arising from the coupling to short-range ferromagnetic excitations. Diplike anomalies in the magnetic-field dependence of elastic moduli reveal magnetic-field-induced orbital order-order transitions. These results strongly suggest the presence of geometrical orbital frustration which causes novel orbital phenomena within the antiferromagnetic phase.