Why is the $d$-Wave spin splitting in CuF$_2$ bulk-like?

TL;DR

This paper explains why CuF$_2$ exhibits bulk $d$-wave spin splitting by linking it to antipolar displacements of F ions, structural distortions, and magnetic octupole components, contrasting with other transition-metal difluorides.

Contribution

It reveals that structural distortions in CuF$_2$ induce magnetic octupoles that alter the spin splitting pattern from planar to bulk-like, a mechanism not present in other family members.

Findings

Antipolar F displacements cause bulk $d$-wave splitting in CuF$_2$

Structural distortions introduce magnetic octupoles affecting spin splitting

External pressure can control the spin splitting pattern

Abstract

With the advent of nonrelativistic spin splitting in collinear compensated antiferromagnets, several candidate materials have also been proposed, among which the family of transition-metal difluorides stands out as a prominent example. Within this family, most members exhibit planar $d$-wave spin splitting, whereas CuF$_2$ shows bulk $d$-wave splitting with an explicit $k_z$ dependence. In this work, we show that this transition from planar to bulk $d$-wave splitting in CuF$_2$ is primarily driven by the antipolar displacements of the F ions, which are absent in the tetragonal rutile structure of the other family members. Our calculations reveal that these additional structural distortions introduce an extra plane of anisotropic magnetization density, giving rise to an additional totally symmetric component of the magnetic octupole tensor. The $k$-space representation of this octupole component, consequently, dictates an additional direction of spin splitting, thereby transforming the $d$-wave spin splitting pattern from planar to bulk-like. We further analyze the effect of spin-orbit coupling on the magnetic octupoles and the resulting spin splitting in the band structure. Our work highlights the possibility of controlling the pattern of nonrelativistic spin splitting through structural modifications, for example, via the application of external pressure.