Thermal and Magnetic Responses of Spin Ice via SQUID Magnetometry*

The spin ice pyrochlore class of frustrated magnets exhibits quasiparticle excitations that behave like Dirac monopoles on the pyrochlore lattice. We are studying single crystals of the spin ice Dy$_2$Ti$_2$O$_7$ using dc SQUID magnetometry to look for thermal fluctuations in the monopole density. Specifically, we are seeking to observe a local excess of monopole magnetic charge induced by the simultaneous application of a temperature gradient, which should create a gradient in the monopole density, and a parallel magnetic field, which drives the flow and counterflow of oppositely-charged monopoles. This configuration should allow us to investigate the distribution, dynamics, and relaxation of the monopole-like excitations in spin ice pyrochlores.