Towards a microscopic theory of toroidal moments in bulk periodic crystals

TL;DR

This paper develops a theoretical framework for understanding magnetic toroidal moments in bulk periodic crystals, addressing their definition, multivaluedness, and how to calculate changes in toroidization, with applications to specific materials.

Contribution

It introduces a formal definition of toroidization in periodic systems, highlighting its multivalued nature and drawing parallels to electric polarization theory, with practical calculations for real materials.

Findings

Toroidization is multivalued under periodic boundary conditions.

Differences in toroidization are the physically meaningful quantities.

Calculated toroidization for BaNiF4, LiCoPO4, GaFeO3, and BiFeO3.

Abstract

We present a theoretical analysis of magnetic toroidal moments in periodic systems, in the limit in which the toroidal moments are caused by a time and space reversal symmetry breaking arrangement of localized magnetic dipole moments. We summarize the basic definitions for finite systems and address the question of how to generalize these definitions to the bulk periodic case. We define the toroidization as the toroidal moment per unit cell volume, and we show that periodic boundary conditions lead to a multivaluedness of the toroidization, which suggests that only differences in toroidization are meaningful observable quantities. Our analysis bears strong analogy to the modern theory of electric polarization in bulk periodic systems, but we also point out some important differences between the two cases. We then discuss the instructive example of a one-dimensional chain of magnetic moments, and we show how to properly calculate changes of the toroidization for this system. Finally, we evaluate and discuss the toroidization (in the local dipole limit) of four important example materials: BaNiF_4, LiCoPO_4, GaFeO_3, and BiFeO_3.