  We examine the entanglement of general mixed states of a two qubit Heisenberg
XYZ chain in the presence of a magnetic field, and its detection by means of
different criteria. Both the exact separability conditions and the weaker
conditions implied by the disorder and the von Neumann entropic criteria are
analyzed. The ensuing limit temperatures for entanglement in thermal states of
different XYZ models are then examined and compared with the limit temperature
of the symmetry-breaking solution in a mean field type approximation. The
latter, though generally lower, can also be higher than the exact limit
temperature for entanglement in certain cases, indicating that
symmetry-breaking does not necessarily entail entanglement. The reentry of
entanglement for increasing temperatures is also discussed.
