  We investigate multipartite entanglement in rings of arbitrary spins with
antiferromagnetic interactions between nearest neighbors. In particular, we
show that the non-degenerate ground state of rings formed by an even number
($N$) of spins is $N$-partite entangled, and exchange energy can thus be used
as a multipartite-entanglement witness. We develop a general approach to
compute the energy minima corresponding to biseparable states, and provide
numerical results for a representative set of systems. Despite its global
character, exchange energy also allows a spin-selective characterization of
entanglement. In particular, in the presence of a magnetic defect, one can
derive separability criteria for each individual spin, and use exchange energy
for detecting entanglement between this and all the other spins.
