Low-temperature properties of the dilute dipolar magnet LiHo_xY_(1-x)F_4

摘要

We analyze recent experiments on the dilute rare-earth compound LiHo_xY_(1-x)F_4 in the context of an effective Ising dipolar model. Using a Monte Carlo method we calculate the low-temperature behavior of the specific heat and linear susceptibility and compare our results to measurements. In our model the susceptibility follows a Curie-Weiss law at high temperature, X ～1/(T-T_(cw)), with a Curie-Weiss temperature that scales with dilution, T_(cw) ～x, consistent with early experiments. We also find that the peak in the specific heat scales linearly with dilution, C_(max)(T)～x, in disagreement with recent experiments. This difference could be caused by the hyperfine interaction which is not included in our calculation. Experimental studies do not reach a consensus on the functional form of the susceptibility and specific heat, and in particular, we do not see reported scalings of the form X ～ T~(-0.75) and X-exp(-T/T_0). Furthermore, we calculate the ground-state magnetization as a function of dilution and re-examine the phase diagram around the critical dilution x_c =0.24 ±0.03. We find that the spin-glass susceptibility for the Ising model does not diverge below x_c, while some recent experiments give strong evidence for a stable spin-glass phase in LiHo_(0.167)Y_(0.833)F)4.