  We numerically investigate thermal entanglement of the spins (1/2,1) and
(1/2,1/2) in the three-mixed (1/2,1,1/2) anisotropic Heisenberg XXZ spin system
on a simple triangular cell under an inhomogeneous magnetic field. We show that
the external magnetic field induces strong plateau formation in pairwise
thermal entanglement for fixed parameters of Hamiltonian in the case of the
ferromagnetic and anti-ferromagnetic interactions. We also observe an
unexpected critical point at finite temperature in the thermal entanglement of
the spins (1/2,1) for antiferromagnetic case while the entanglement of the
spins (1/2,1) for ferromagnetic case and the entanglement of the spins
(1/2,1/2) for both ferromagnetic and antiferromagnetic cases almost decays
exponentially to zero with increasing temperature. The critical point in
entanglement of the spins (1/2,1) for antiferromagnetic case may be signature
of the quantum phase transition at finite temperature.
