  In this paper, we use Bell inequality and nonlocality to study the bipartite
correlation in an exactly soluble two-dimensional mixed spin system. Bell
inequality turns out to be a valuable detector for phase transitions in this
model. It can detect not only the quantum phase transition, but also the
thermal phase transitions, of the system. The property of bipartite correlation
in the system is also analyzed. In the quantum anti-ferromagnetic phase, the
Bell inequality is violated thus nonlocality is present. It is interesting that
the nonlocality is enhanced by thermal fluctuation, and similar results have
not been observed in anti-ferromagnetic phase. In the ferromagnetic phase, the
quantum correlation turns out to be very novel, which cannot be captured by
entanglement or nonlocality.
