Unconventional magnetostructural transition in CoCr2O4 at high magnetic fields

TL;DR

This study reveals a high-symmetry magnetostructural phase transition in CoCr2O4 under high magnetic fields, characterized by ultrasound anomalies and a metastable state with partial magnetic order, highlighting strong spin-lattice coupling.

Contribution

It uncovers a novel high-symmetry phase in CoCr2O4 induced by high magnetic fields, with distinct ultrasound and magnetization signatures, expanding understanding of magnetostructural transitions.

Findings

Ultrasound anomalies mark phase transitions at specific temperatures and fields.

A metastable high-symmetry phase with partial magnetic order is identified.

Strong spin-lattice coupling influences the observed transitions.

Abstract

The magnetic-field and temperature dependencies of ultrasound propagation and magnetization of single-crystalline CoCr2O4 have been studied in static and pulsed magnetic fields up to 14 T and 62 T, respectively. Distinct anomalies with significant changes in the sound velocity and attenuation are found in this spinel compound at the onset of long-range incommensurate spiral-spin order at T_s = 27 K and at the transition from the incommensurate to the commensurate state at T_l = 14 K, evidencing strong spin-lattice coupling. While the magnetization evolves gradually with field, steplike increments in the ultrasound clearly signal a transition into a new magneto-structural state between 6.2 and 16.5 K and at high magnetic fields. We argue that this is a high-symmetry phase with only the longitudinal component of the magnetization being ordered, while the transverse helical component remains disordered. This phase is metastable in an extended H-T phase space.