  In this paper, we investigate spin entanglement in the $XXZ$ model defined on
a $d$-dimensional bipartite lattice. The concurrence, a measure of the
entanglement between two spins, is analyzed. We prove rigorously that the
ground state concurrence reaches maximum at the isotropic point. For
dimensionality $d \ge 2$, the concurrence develops a cusp at the isotropic
point and we attribute it to the existence of magnetic long-range order.
