The critical exponent differs between materials and for the mean-fieldmodel is taken as γ = 1. As temperature is inversely proportional to magnetic susceptibility
Christopher C. Eastward and westward drift oftheEarth'smagneticfield for the last three millennia (PDF). Earth and Planetary Science Letters. 2007, 254:
V.; Shepherd, S. J.; Zharkov, S. I.; Popova, E. Retraction Note: Oscillations ofthe baseline of solar magneticfield and solar irradiance on a millennial