Creation and measurement of long-lived magnetic monopole currents in spin ice

The recent discovery of 'magnetricity' in spin ice raises the question of whether long-lived currents of magnetic 'monopoles' can be created and manipulated by applying magnetic fields. Here we show that they can. By applying a magnetic-field pulse to a Dy2Ti2O7 spin-ice crystal at 0.36 K, we create a relaxing magnetic current that lasts for several minutes. We measure the current by means of the electromotive force it induces in a solenoid coupled to a sensitive amplifier, and quantitatively describe it using a chemical kinetic model of point-like charges obeying the Onsager-Wien mechanism of carrier dissociation and recombination. We thus derive the microscopic parameters of monopole motion in spin ice and identify the distinct roles of free and bound magnetic charges. Our results illustrate a basic capacitor effect for magnetic charge and should pave the way for the design and realization of 'magnetronic' circuitry.