Magnetoelectric multipoles in metals

TL;DR

This paper computationally demonstrates the existence of magnetoelectric multipoles in noncentrosymmetric magnetic metals, revealing their properties and potential implications for magnetic and transport phenomena in such systems.

Contribution

It is the first to identify and analyze magnetoelectric multipoles in metallic systems using first-principles calculations, expanding understanding beyond insulators.

Findings

Magnetoelectric multipoles exist in noncentrosymmetric magnetic metals.

Surface-induced symmetry breaking generates magnetoelectric monopoles.

Distinct multipolar structures correlate with different magnetic phases and transport properties.

Abstract

We demonstrate computationally the existence of magnetoelectric multipoles, arising from the second order term in the multipole expansion of a magnetization density in a magnetic field, in noncentrosymmetric magnetic metals. While magnetoelectric multipoles have long been discussed in the context of the magnetoelectric effect in noncentrosymmetric magnetic insulators, they have not previosuly been identified in metallic systems, in which the mobile carriers screen any electrical polarization. Using first-principles density functional calculations we explore three specific systems: First, a conventional centrosymmetric magnetic metal, Fe, in which we break inversion symmetry by introducing a surface, which both generates magnetoelectric monopoles and allows a perpendicular magnetoelectric response. Next, the hypothetical cation-ordered perovskite, SrCaRu$_2$O$_6$, in which we study the interplay between the magnitude of the polar symmetry breaking and that of the magnetic dipoles and multipoles, finding that both scale proportionally to the structural distortion. Finally, we identify a hidden antiferromultipolar order in the noncentrosymmetric, antiferromagnetic metal Ca$_3$Ru$_2$O$_7$, and show that, while its competing magnetic phases have similar magnetic dipolar structures, their magnetoelectric multipolar structures are distinctly different, reflecting the strong differences in transport properties.