Thermodynamics of the 2D-Heisenberg classical square lattice Part III. Study of the static susceptibility behaviours

In a previous article labelled II a closed-form expression of the dynamic and static susceptibilities has been derived (J. Curely, Physics B 254 (1998) 277, previous article in this issue). In the present one, we mainly focus on the behaviours of the static susceptibility chi. In the low-temperature domain we point out the mechanisms involved in the construction of the 2D-spin arrangement; a very simple model allows one to give a good description of the chi T behaviours. For a 2D-compensated antiferromagnet, we derive a linear variation between the temperature of the maximum of the susceptibility curve T(chi(max)) and the exchange energy J/k(B). In addition, above the Neel temperature, there is a remarkable agreement between the experimental and theoretical susceptibilities, for the compounds BaMnF4 and (CH3NH3)(2)MnCl4. Finally, when T decreases from the high-temperature (HT) domain, we verify on these experimental results that the HT theoretical expansion of chi leads to a value lower than the one derived from Rushbrooke's HT series. In addition, it allows to fit all the experimental points belonging to the characteristic maximum of the susceptibility curve whereas the Padi method just permits to reach it before breaking down. (C) 1998 Elsevier Science B.V. All rights reserved.