Magnetic-field dependence of low-energy magnons, anisotropic heat conduction, and spontaneous relaxation of magnetic domains in the cubic helimagnet ZnCr2Se4

TL;DR

This study investigates the low-temperature magnetic and thermal properties of ZnCr2Se4, revealing complex spin-wave behavior, anisotropic heat conduction, and magnetic domain relaxation phenomena related to its helimagnetic order.

Contribution

It provides detailed insights into the magnetic excitations, heat transport anisotropy, and domain dynamics in ZnCr2Se4, highlighting the effects of magnetic field and quantum criticality.

Findings

Spin-wave spectrum evolves nonmonotonically across the quantum-critical point.

A tiny spin gap vanishes at the critical point, restoring cubic symmetry.

Thermal conductivity is highly anisotropic and sensitive to magnetic fields.

Abstract

Anisotropic low-temperature properties of the cubic spinel helimagnet ZnCr2Se4 in the single-domain spin-spiral state are investigated by a combination of neutron scattering, thermal conductivity, ultrasound velocity, and dilatometry measurements. In an applied magnetic field, neutron spectroscopy shows a complex and nonmonotonic evolution of the spin-wave spectrum across the quantum-critical point that separates the spin-spiral phase from the field-polarized ferromagnetic phase at high fields. A tiny spin gap of the pseudo-Goldstone magnon mode, observed at wave vectors that are structurally equivalent but orthogonal to the propagation vector of the spin helix, vanishes at this quantum critical point, restoring the cubic symmetry in the magnetic subsystem. The anisotropy imposed by the spin helix has only a minor influence on the lattice structure and sound velocity but has a much stronger effect on the heat conductivities measured parallel and perpendicular to the magnetic propagation vector. The thermal transport is anisotropic at T < 2 K, highly sensitive to an external magnetic field, and likely results directly from magnonic heat conduction. We also report long-time thermal relaxation phenomena, revealed by capacitive dilatometry, which are due to magnetic domain motion related to the destruction of the single-domain magnetic state, initially stabilized in the sample by the application and removal of magnetic field. Our results can be generalized to a broad class of helimagnetic materials in which a discrete lattice symmetry is spontaneously broken by the magnetic order.