Thermodynamic perturbation theory for dipolar superparamagnets - art. no. 174416

Thermodynamic perturbation theory is employed to derive analytical expressi ons for the equilibrium linear susceptibility and specific heat of lattices of anisotropic classical spins weakly coupled by the dipole-dipole interac tion. The calculation is carried out to the second order in the coupling co nstant over the temperature, while the single-spin anisotropy is treated ex actly. The temperature range of applicability of the results is, for weak a nisotropy (A/k(B)T much less than1), similar to that of ordinary high-tempe rature expansions. but for moderately and strongly anisotropic spins (A/k(B )T greater than or similar to1) it can extend down to the temperatures wher e the superparamagnetic blocking takes place (A/k(B)T similar to 25), provi ded only that the interaction strength is weak enough. Besides, taking exac tly the anisotropy into account, the results describe as particular cases t he effects of the interactions on isotropic (A=0) as well as strongly aniso tropic (\A \ --> infinity) systems (discrete orientation model and plane ro tators).