Structure and Magnetism in the Bond Frustrated Spinel, ZnCr2Se4

TL;DR

This study investigates the crystal and magnetic structures of ZnCr2Se4, revealing a spin-lattice distortion, helical magnetic order, local spin correlations at high temperatures, and a gapless magnetic excitation mode.

Contribution

The paper provides a comprehensive analysis combining diffraction, muSR, and neutron scattering to elucidate the complex magnetic and structural behavior of ZnCr2Se4, highlighting the interplay of frustration and spin-lattice coupling.

Findings

Spin-lattice distortion from cubic to tetragonal below 21 K.

Helical magnetic order with a magnetic moment of 3.04 μB.

Presence of local spin correlations above the magnetic transition.

Abstract

The crystal and magnetic structures of stoichiometric ZnCr2Se4 have been investigated using synchrotron X-ray and neutron powder diffraction, muon spin relaxation (muSR) and inelastic neutron scattering. Synchrotron X-ray diffraction shows a spin-lattice distortion from the cubic spinel to a tetragonal I41/amd lattice below TN = 21 K, where powder neutron diffraction confirms the formation of a helical magnetic structure with magnetic moment of 3.04(3) {\mu}B at 1.5 K; close to that expected for high-spin Cr3+. MuSR measurements show prominent local spin correlations that are established at temperatures considerably higher (< 100 K) than the onset of long range magnetic order. The stretched exponential nature of the relaxation in the local spin correlation regime suggests a wide distribution of depolarizing fields. Below TN, unusually fast (> 100 {\mu}s-1) muon relaxation rates are suggestive of rapid site hopping of the muons in static field. Inelastic neutron scattering measurements show a gapless mode at an incommensurate propagation vector of k = (0 0 0.4648(2)) in the low temperature magnetic ordered phase that extends to 0.8 meV. The dispersion is modelled by a two parameter Hamiltonian, containing ferromagnetic nearest neighbor and antiferromagnetic next nearest neighbor interactions with a Jnnn/Jnn = -0.337.