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

TL;DR

This study reveals that ZnFe₂O₄ exhibits a dynamical molecular-spin state with lattice fluctuations at low temperatures, without undergoing magnetic phase transitions or spin-lattice ordering, indicating a unique spin-lattice liquid-like behavior.

Contribution

It provides the first evidence of a dynamical spin-lattice liquid-like state in ZnFe₂O₄, showing spin and lattice fluctuations without phase transition or ordering.

Findings

Elastic anomalies suggest coupling between lattice and spin excitations.

Absence of magnetostructural transition indicates no spin-lattice ordering.

Dynamical spin state persists without precursor fluctuations.

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.