Dipolar ferromagnets and glasses (invited)

What is the ground state and what are the dynamics of 10^23 randomly distributed Ising spins? We have attempted to answer these questions through magnetic susceptibility, calorimetric, and neutron scattering studies of the randomly diluted dipolar-coupled Ising magnet LiHo_xY_(1-x)F_4. The material is ferromagnetic for dipole concentrations at least as low as x = 0.46, with a Curie temperature obeying mean-field scaling relative to that of pure LiHoF_4. In the dilute spin limit, an x = 0.045 crystal shows very unusual glassy properties characterized by decreasing barriers to relaxation as T → 0. Its properties are consistent with a single low degeneracy ground state with a large gap for excitations. A slightly more concentrated x = 0.167 sample, however, supports a complex ground state with no appreciable gap, in accordance with prevailing theories of spin glasses. The underlying causes of such disparate behavior are discussed in terms of random clusters as probed by neutron studies of the x = 0.167 sample. In addition to tracing the evolution of the glassy and ferromagnetic states with dipole concentration, we investigate the effects of a transverse magnetic field on the Ising spin glass, LiHo_(0.167)Y_(0.833)F_4. The transverse field mixes the eigenfunctions of the ground-state Ising doublet with the otherwise inaccessible excited-state levels. We observe a rapid decrease in the characteristic relaxation times, large changes in the spectral form of the relaxation, and a depression of the spin-glass transition temperature with the addition of quantum fluctuations