The sign of Weiss temperature determined from high temperature CW fit may not always reflect the  type of dominant exchange interaction. For example, in many Ce- and Yb- based systems CEF effects dictate the sign of Weiss temperature in the CW fit rather than the exchange interactions. One has to carefully look at the particular system as it cannot be generalized.

Thanks for your suggestion Ajeesh. Can you suggest me some papers on these and particularly I am talking about TMOs.

in principle, the sign of theta reflect the type of exchange interactions negative theta means positive exchange interaction ( ferromagnetic)  and the magnitude reflects the strength of this interactions