Bethe–Slater curve

It assumes a Heisenberg model of magnetism, and explains the differences in exchange energy of transition metals as due to the ratio of the interatomic distance a to the radius r of the 3d electron shell.

[1] When the magnetically important 3d electrons of adjacent atoms are relatively close to each other, the exchange interaction,

The idea of relating exchange energy to inter-atomic distance was first proposed by John C. Slater in 1930,[2][3] and illustrated as a curve on a graph in a review by Sommerfeld and Bethe in 1933.

[4] For a pair of atoms, the exchange interaction wij (responsible for the energy E) is calculated as:[5]

The Slater curve does produce realistic results, predicting Iron, Cobalt and Nickel to be the elements with ferromagnetic ordering.