Ferromagnetism, glasssiness, and metastabilily in a dilute dipolar-coupled magnet.

We have measured the a.c. magnetic susceptibility of the model dilute dipolar-coupled Ising system LiHo(x)Y(1-x)F(4). The x = 0.46 material displays an ordinary feromagnetic 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 dependence and metastability upon field-cooling.