Toroidal order in a partially disordered state on a layered triangular lattice: implication to UNi4B

TL;DR

This study theoretically explores toroidal order and magnetoelectric effects in a layered triangular lattice, revealing a vortex-lattice magnetic state with potential relevance to the uranium compound UNi4B.

Contribution

It introduces a model with site-dependent antisymmetric spin-orbit coupling that explains the observed magnetic order and magnetoelectric effects in UNi4B.

Findings

Identification of a vortex-lattice magnetic order in the model

Demonstration of toroidal order leading to magnetoelectric effects

Relevance of antisymmetric spin-orbit coupling to experimental observations

Abstract

A partial disorder on a layered triangular lattice is theoretically investigated from a viewpoint of toroidal ordering and magnetoelectric effects. We consider an extended periodic Anderson model including a site-dependent antisymmetric spin-orbit coupling between conduction and localized electrons. We show that, by the mean-field approximation, the model exhibits a coplanar vortex-lattice-type magnetic order as observed in a hexagonal uranium compound UNi4B, in the parameter region with intermediate hybridization and electron correlation. This peculiar state accommodates a toroidal order, which leads to the linear magnetoelectric effect. We discuss the implications of our results to UNi4B, focusing on the possible source of the site-dependent antisymmetric spin-orbit coupling.