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 compoundLiHo_xY_(1-x)F_4 in the context of an effective Ising dipolar model. Using aMonte Carlo method we calculate the low-temperature behavior of the specificheat and linear susceptibility, and compare our results to measurements. In ourmodel the susceptibility follows a Curie-Weiss law at high temperature, chi ~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 specificheat scales linearly with dilution, C_max(T) ~ x, in disagreement with recentexperiments. Experimental studies do not reach a consensus on the functionalform of these quantities, and in particular we do not see reported scalings ofthe form chi ~ T^-0.75 and chi ~ exp(-T/T_0). Furthermore we calculate theground state magnetization as a function of dilution, and re-examine the phasediagram around the critical dilution x_c=0.24(3). We find that the spin glasssusceptibility for the Ising model does not diverge below x_c, while recentexperiments give strong evidence for a stable spin-glass phase inLiHo_0.167Y_0.833F_4.