Acoustic study for dynamical molecular-spin state without undergoing magnetic phase transition in spin-frustrated ZnFe\(_2\)O\(_4\)

Abstract

Ultrasound velocity measurements were performed on a single crystal of spin-frustrated ferrite spinel ZnFe\(_2\)O\(_4\) from 300 K down to 2 K. In this cubic crystal, all the symmetrically-independent elastic moduli exhibit softening with a characteristic minimum with decreasing temperature below \(\sim\)100 K. This elastic anomaly suggests a coupling between dynamical lattice deformations and molecular-spin excitations. In contrast, the elastic anomalies, normally driven by the magnetostructural phase transition and its precursor, are absent in ZnFe\(_2\)O\(_4\), suggesting that the spin-lattice coupling cannot play a role in relieving frustration within this compound. The present study infers that, for ZnFe\(_2\)O\(_4\), the dynamical molecular-spin state evolves at low temperatures without undergoing precursor spin-lattice fluctuations and spin-lattice ordering. It is expected that ZnFe\(_2\)O\(_4\) provides the unique dynamical spin-lattice liquid-like system, where not only the spin molecules but also the cubic lattice fluctuate spatially and temporally.