Search for the magnetic monopole at a magnetoelectric surface

TL;DR

This paper predicts and experimentally investigates the existence of magnetic monopoles induced by electric charges on magnetoelectric surfaces, specifically in Cr$_2$O$_3$, using theoretical modeling and muon spin rotation measurements.

Contribution

It demonstrates that electric charges on magnetoelectric surfaces can generate image magnetic monopoles and provides experimental evidence supporting this phenomenon in Cr$_2$O$_3$.

Findings

Image monopoles are generated by electric charges on magnetoelectric surfaces.

Muon spin rotation measurements are consistent with the presence of monopolar magnetic fields.

Detection of the monopolar field is feasible with magnetic force microscopy, but surface charging effects pose challenges.

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.