A new effective field theory for the anisotropic Heisenberg ferromagnet

A new type of effective field theory (NEFT) is developed for the quantum spin system with anisotropic exchange interaction. The calculation is based on a two-atom cluster approximation, and effects from the rest spins are treated by the operator expansion technique assuming Ising character for surrounding spins. The Curie temperature Tc and spontaneous magnetization m are calculated for all Ising-, Heisenberg- and XY-like spin systems, and discussed comparing with results by other traditional pair spin theories. It is practically confirmed that Tc of the NEFT correctly reflects details of the geomtry of the lattice more than coordination number, and that it gives no anti-Curie point even for the XY-like spin system.