Weiss's constant.
One formula partially named for this man breaks down at small temperature differences in the denominator, which must then be raised to the critical exponent to give better approximations. His namesake constant, found in the numerator of that equation, can be found by inverting the slope of a graph of molar susceptibility versus temperature. At very high temperatures, his namesake constant in the equation partially named for him is replaced by (*) Weiss's constant. One law named for this man, valid for large temperatures and weak fields, states that magnetisation is proportional to magnetic field strength over temperature, or that M equals chi times H. This man's namesake "point" for iron is 1043 Kelvin and is the temperature at which ferromagnetism gives way to paramagnetism. For 10 points, identify this scientist who was married to radiation-studying Marie.
One formula partially named for this man breaks down at small temperature differences in the denominator, which must then be raised to the critical exponent to give better approximations.
Pierre Curie [accept Pierre Weiss before \"numerator\"]