Ferromagnetism, glassiness, and metastability in a dilute dipolar-coupled magnet

We have measured the ac magnetic susceptibility of the model dilute dipolar-coupled Ising system $\mathrm{Li}{\mathrm{Ho}}_x{\mathrm{Y}}_{1-x}{\mathrm{F}}_4$. The $x=0.46\mathrm{}$ material displays an ordinary ferromagnetic transition, while the $x=0.045\mathrm{} \mathrm{and} 0.167$ samples are two very different magnetic glasses. Thermal relaxation times are more than five times longer for $x=0.167\mathrm{}$ than for $x=0.045\mathrm{}$. In addition, the more concentrated glass shows history dependence and metastability upon field cooling.