Magnetically frustrated systems provide fertile ground for complex behaviour, including unconventional ground states with emergent symmetries, topological properties, and exotic excitations. A canonical example is the emergence of magnetic-charge-carrying quasiparticles in spin-ice compounds. Despite extensive work, a reliable experimental indicator of the density of these magnetic monopoles is yet to be found. Using measurements on single crystals of Ho2Ir2O7 combined with dipolar Monte Carlo simulations, a team from Warwick, Cambridge and Oxford show that the isothermal magnetoresistance is highly sensitive to the monopole density. The work also uncovers an unexpected and strong coupling between the monopoles on the holmium sublattice and the antiferromagnetically ordered iridium ions. These results pave the way towards a quantitative experimental measure of monopole density and demonstrate the ability to control antiferromagnetic domain walls using a uniform external magnetic field, a key goal in the design of next-generation spintronic devices.

• Caption to figure: Cartoon showing the effect of an applied magnetic field on the density of magnetic monopoles (circles), leading to the preferential growth of a particular antiferromagnetic domain.
• Publication: M. J. Pearce, K. Götze, A. Szabó, T. S. Sikkenk, M. R. Lees, A. T. Boothroyd, D. Prabhakaran, C. Castelnovo & P. A. Goddard, Nature Communications 13, 444 (2022).
• DOI: https://doi.org/10.1038/s41467-022-27964-y