Search for the Magnetic Monopole at a Magnetoelectric Surface

Q N Meier,T Nozaki,P Borisov,M Fiebig,A Suter,P Schoenherr,I E Dzyaloshinskii,M Sahashi,H Luetkens,T Prokscha,M Lilienblum,N A Spaldin,M Fechner,Z Salman

doi:10.1103/physrevx.9.011011

Abstract

We show, by solving Maxwell's equations, that an electric charge on the surface of a slab of a linear magnetoelectric material generates an image magnetic monopole below the surface provided that the magnetoelectric has a diagonal component in its magnetoelectric response. The image monopole, in turn, generates an ideal monopolar magnetic field outside of the slab. Using realistic values of the electric- and magnetic- field susceptibilties, we calculate the magnitude of the effect for the prototypical magnetoelectric material Cr$_2$O$_3$. We use low energy muon spin rotation to measure the strength of the magnetic field generated by charged muons as a function of their distance from the surface of a Cr$_2$O$_3$ films, and show that the results are consistent with the existence of the monopole. We discuss other possible routes to detecting the monopolar field, and show that, while the predicted monopolar field generated by Cr$_2$O$_3$ is above the detection limit for standard magnetic force microscopy, detection of the field using this technique is prevented by surface charging effects.

Highlights

The elusiveness of magnetic monopoles, which are expected in classical electrodynamics because of the duality symmetry between electricity and magnetism, has intrigued physicists for centuries
We show that conventional linear magnetoelectric materials, of which Cr2O3 is the prototype [12,13], can generate an external monopolar magnetic field when an electric charge is placed above any flat sample surface
We derived the form of the electric and magnetic fields that are induced by an electric charge above a surface of a semi-infinite slab of magnetoelectric material

Summary

Introduction

The elusiveness of magnetic monopoles, which are expected in classical electrodynamics because of the duality symmetry between electricity and magnetism, has intrigued physicists for centuries. Their relevance was emphasized by Dirac, who introduced a description allowing monopoles to remain consistent with the known zero divergence of magnetic fields and showed that their existence would explain the observed quantization of electric charge in the Universe [1]. While the existence of true magnetic monopoles has not yet been verified, a number of condensed-matter systems have been shown to provide intriguing analogues.

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