Non-collinear long-range magnetic ordering in HgCr2S4

TL;DR

This study reveals a complex non-collinear long-range magnetic order in HgCr2S4, characterized by incommensurate spiral magnetic structure below 22K, with implications for its electric polarization properties.

Contribution

The paper provides the first detailed neutron diffraction analysis of HgCr2S4's magnetic structure, identifying a non-collinear spiral order and its symmetry, which impacts understanding of magnetoelectric coupling.

Findings

Long-range incommensurate magnetic order appears below 22K.

Magnetic structure forms a spiral of ferromagnetic layers along c-axis.

Ordered magnetic phase has symmetry incompatible with ferroelectricity.

Abstract

The low-temperature magnetic structure of \HG has been studied by high-resolution powder neutron diffraction. Long-range incommensurate magnetic order sets in at T$_N\sim$22K with propagation vector \textbf{k}=(0,0,$\sim$0.18). On cooling below T$_N$, the propagation vector increases and saturates at the commensurate value \textbf{k}=(0,0,0.25). The magnetic structure below T$_N$ consists of ferromagnetic layers in the \textit{ab}-plane stacked in a spiral arrangement along the \textit{c}-axis. Symmetry analysis using corepresentations theory reveals a point group symmetry in the ordered magnetic phase of 422 (D$_4$), which is incompatible with macroscopic ferroelectricity. This finding indicates that the spontaneous electric polarization observed experimentally cannot be coupled to the magnetic order parameter.