Observation of elastic anomalies driven by coexisting dynamical spin Jahn-Teller effect and dynamical molecular spin state in paramagnetic phase of the frustrated MgCr$_2$O$_4$$

TL;DR

Ultrasound measurements on MgCr2O4 reveal elastic anomalies in the paramagnetic phase caused by geometrical frustration, indicating coexistence of dynamical spin Jahn-Teller effects and molecular spin states.

Contribution

This study provides the first experimental evidence of elastic anomalies linked to coexisting dynamical spin Jahn-Teller effects and molecular spin states in a frustrated magnet.

Findings

Huge Curie-type softening of shear modulus suggests spin Jahn-Teller precursor.

Nonmonotonic temperature dependence of $C_{44}$ indicates coupling to dynamical molecular spins.

Elastic anomalies imply coexistence of magnetostructural order and dynamical spin states.

Abstract

Ultrasound velocity measurements of magnesium chromite spinel MgCr$_2$O$_4$ reveal elastic anomalies in the paramagnetic phase that are characterized as due to geometrical frustration. The temperature dependence of the tetragonal shear modulus $(C_{11}-C_{12})/2$ exhibits huge Curie-type softening, which should be the precursor to spin Jahn-Teller distortion in the antiferromagnetic phase. The trigonal shear modulus $C_{44}$ exhibits nonmonotonic temperature dependence with a characteristic minimum at $\sim$50 K, indicating a coupling of the lattice to dynamical molecular spin state. These results strongly suggest the coexistence of dynamical spin Jahn-Teller effect and dynamical molecular spin state in the paramagnetic phase, which is compatible with the coexistence of magnetostructural order and dynamical molecular spin state in the antiferromagnetic phase.