%e have measured the ac magnetic susceptibility of the model dilute dipolar-coupled Ising system LiHo„Y& -„F4. The x 0.46 material displays an ordinary ferromagnetic transition, while the x 0.045 and 0.167 samples are two very different magnetic glasses. Thermal relaxation times are more than five times longer for x 0.167 than for x ~0.045. In addition, the more concentrated glass shows history...
