Multiple lattice instabilities resolved by magnetic-field and disorder sensitivities in cubic paramagnetic phase of the orbital-degenerate frustrated spinel MgV$_2$O$_4$

TL;DR

This study uses ultrasound measurements to reveal complex lattice instabilities in MgV$_2$O$_4$, showing how magnetic fields and disorder influence orbital and spin fluctuations in its cubic paramagnetic phase.

Contribution

It uncovers two distinct elastic anomalies in MgV$_2$O$_4$'s cubic phase and demonstrates their sensitivity to magnetic fields and disorder, advancing understanding of lattice-electron interactions.

Findings

Identification of Curie-type softening in elastic moduli.

Observation of a concave temperature dependence with a minimum.

Evidence of coupling between lattice, orbital fluctuations, and spin excitations.

Abstract

Ultrasound velocity measurements of the orbital-degenerate frustrated spinel MgV$_2$O$_4$ are performed in the high-purity single crystal which exhibits successive structural and antiferromagnetic phase transitions, and in the disorder-introduced single crystal which exhibits spin-glass-like behavior. The measurements reveal that two-types of unusual temperature dependence of the elastic moduli coexist in the cubic paramagnetic phase, which are resolved by magnetic-field and disorder sensitivities: huge Curie-type softening with decreasing temperature, and concave temperature dependence with a characteristic minimum. These elastic anomalies suggest the coupling of lattice to coexisting orbital fluctuations and orbital-spin-coupled excitations.