Magnetic Charge Transport

TL;DR

This paper demonstrates the experimental measurement of magnetic charge transport in spin ice Dy2Ti2O7, confirming the existence and Coulomb interaction of magnetic monopoles and characterizing their current and elementary charge units.

Contribution

It introduces a method to directly measure magnetic charges and currents in spin ice materials, confirming theoretical predictions and expanding understanding of magnetic monopoles.

Findings

Magnetic charges exist and interact via Coulomb forces in Dy2Ti2O7.

Magnetic monopoles have a measurable current, demonstrating magnetic charge transport.

Elementary magnetic charge unit is 5 muB per Angstrom, matching theoretical predictions.

Abstract

It has recently been predicted that certain magnetic materials contain mobile magnetic charges or `monopoles'. Here we address the question of whether these magnetic charges and their associated currents (`magnetricity') can be directly measured in experiment, without recourse to any material-specific theory. By mapping the problem onto Onsager's theory of weak electrolytes, we show that this is possible, and devise an appropriate method. Then, using muon spin rotation as a convenient local probe, we apply the method to a real material: the spin ice Dy2Ti2O7. Our experimental measurements prove that magnetic charges exist in this material, interact via a Coulomb interaction, and have measurable currents. We further characterise deviations from Ohm's Law, and determine the elementary unit of magnetic charge to be 5 muB per Angstrom, which is equal to that predicted by Castelnovo, Moessner and Sondhi using the microscopic theory of spin ice. Our demonstration of magnetic charge transport has both conceptual and technological implications.