Signature of magnetic monopole and Dirac string dynamics in spin ice

TL;DR

This paper demonstrates experimentally that magnetic monopole-like excitations in spin ice materials exhibit diffusive dynamics constrained by Dirac strings, with measurable signatures in magnetic relaxation.

Contribution

It provides the first experimental signature of monopole dynamics in spin ice, interpreting magnetic relaxation data in terms of monopole diffusion and Dirac string networks.

Findings

Magnetic relaxation in Dy2Ti2O7 reflects monopole diffusion.

Dirac string networks constrain monopole motion in spin ice.

Surface monopole density gradients are observed in open systems.

Abstract

Magnetic monopoles have eluded experimental detection since their prediction nearly a century ago by Dirac. Recently it has been shown that classical analogues of these enigmatic particles occur as excitations out of the topological ground state of a model magnetic system, dipolar spin ice. These quasi-particle excitations do not require a modification of Maxwell's equations, but they do interact via Coulombs law and are of magnetic origin. In this paper we present an experimentally measurable signature of monopole dynamics and show that magnetic relaxation measurements in the spin ice material $Dy_{2}Ti_{2}O_{7}$ can be interpreted entirely in terms of the diffusive motion of monopoles in the grand canonical ensemble, constrained by a network of "Dirac strings" filling the quasi-particle vacuum. In a magnetic field the topology of the network prevents charge flow in the steady state, but there is a monopole density gradient near the surface of an open system.