1 D EXPANSION FOR 2-DIMENSIONAL ANTIFERROMAGNETIC D-COMPONENT VECTOR MODEL - MAGNETIZATION AND SUSCEPTIBILITY IN MAGNETIC-FIELD/

The thermodynamics of low-dimensional classical magnets described by t he d-component vector model is considered with the help of the 1/D exp ansion in the whole temperature region and for nonzero magnetic fields . For the Heisenberg model (D=3) the theory reproduces with a good acc uracy the temperature dependence of the antiferromagnetic susceptibili ty chi(AF) with the maximum at T similar to J(0) and describes the sin gular behavior of chi(AF) at T, H-->0: lim(T-->0)lim(H-->0)chi(AF)=(1 -1/D)/(2J(0)) and lim(H-->0)lim(T-0)chi(AF) = 1/(2J(0)).