Extension of Curie-Weiss law

Generally, the exchange-interaction on a magnetic ion is expressed as H->m=λM->, here λ is a temperature independent constant and M-> is the magnetization. However in some paramagnetic materials the exchange-interaction between the electrons in different magnetic ions depends on temperature. Actually, the spin-fluctuation of magnetic ions appears in some paramagnetic materials, and it depends on temperature. In this Letter, the effect of the spin-fluctuation on the effective field is considered by introducing a temperature dependent coefficient λ, and λ is expressed as λ=α+βT. Then, the Curie-Weiss law can be extended. The correctness of the extension is examined by comparing the calculated values of the magnetic susceptibility of CeF3 crystals with the measured ones