Magnetic Behavior of an $S=\frac{1}{2}$ Amorphous Antiferromagnet

The magnetic behavior of bound shallow donors in CdS, a model $S=\frac{1}{2}$ amorphous antiferromagnet, has been measured by a Faraday rotation technique down to $T\sim 50$ mK. The susceptibility, $\chi$, and $\frac{d\chi }{\mathrm{dT}}$ as well increase monotonically with decreasing $T$ with no sign of an ordering transition even though the median exchange interaction of a spin is ∼7 K. The results agree with a hydrogeniclike exchange model which is solved in terms of exactly diagonalized weakly interacting clusters.