THE 1/D EXPANSION FOR LOW-DIMENSIONAL CLASSICAL MAGNETS

The physical characteristics of two-dimensional classical ferro- and antiferromagnets have been calculated in the whole temperature range by an analytical approach based on the expansion in powers of 1/D, where D is the number of spin components. This approach works rather well since it yields exact results for antiferromagnetic susceptibility and specific heat at T=0 already in the first order in 1/D and it is consistent with HTSE at high temperatures. For the quantities singular at T = 0, such as ferromagnetic susceptibility and correlation length, the 1/D expansion supports their general-D functional form in the low-temperature range obtained by Fukugita and Oyanagi. The critical index eta calculated in the first order in 1/D proves to be temperature dependent: eta = 2theta/(piD) (theta = T/T(c)(MFT), T(c)(MFT) = J0/D, J0 is the zero Fourier component of the exchange interaction).