Measuring Magnetic Monopole Transport*

Dy₂Ti₂O₇ and Ho₂Ti₂O₇ are two spin-ice compounds on the pyrochlore lattice whose ground states are highly degenerate. Magnetic exitations in these compounds may be treated as effective magnetic monopoles which move freely around the lattice. Application of an alternating magnetic field allows the in-phase and out-of-phase alternating current (AC) susceptibility to be measured in a Superconducting Quantum Interference Device (SQUID) magnetometer. Recent work on Dy₂Ti₂O₇ tori^[1] and Ho₂Ti₂O₇ needles^[2] indicate that the behaviour of the magnetic monopoles may be modeled as a supercooled magnetic liquid approaching a glass transition. In this work, samples were cut into spheres to account for the demagnetizing field produced by the large magnetic moment of the rare-earth ions. The real and imaginagy AC susceptibilites are fit to a generalized Havriliak–Negami (HN) magnetic susceptibility and the relaxation rate is found. A divergence in the miscropic relaxation rate is found in Dy₂Ti₂O₇ indicating this compound exhibits the characteristics of a supercooled magnetic liquid. [1] Kassner et al., PNAS 112.28 (2015), [2] Quilliam et al., PRB 83, 094424 (2011)