the pairs of the atom. Ising model is one of the simplest approximations of such pairwise interaction. Here the two atoms of a pair are at . Their interaction is determined by their distance vector . In order to simplify the calculation, it is often assumed that interaction happens between neighboring atoms only and is a constant. The effect of such interaction is often approximated as a mean field and, in our case, the Weiss field. Modification of Curie's law due to Weiss field The Curie-Weiss law is an adapted version of Curie's law, which for a paramagnetic material may be written in SI units as follows, assuming : Here μ0 is the permeability of free space; M the magnetization (magnetic moment per unit volume), is the magnetic field, and C the material-specific Curie constant: where is Boltzmann's constant, the number of magnetic atoms (or molecules) per unit volume, the Landé g-factor, the Bohr magneton, the angular momentum quantum number. For the Curie-Weiss Law the total magnetic field is where is the Weiss molecular field constant and then which can be rearranged to get which is the Curie-Weiss Law where the Curie temperature is See also Curie's law Paramagnetism Pierre Curie Pierre-Ernest Weiss Exchange interaction Notes References http://theory.tifr.res.in/~sgupta/courses/qm2013/hand5.pdf External links Magnetism: Models and Mechanisms in E. Pavarini, E. Koch, and U. Schollwöck: Emergent Phenomena in Correlated Matter, Jülich 2013, Magnetic ordering Pierre Curie