Heisenberg Exchange Theory Quantum Approach

When two particles are far apart, their spins do not interact and it makes no difference if their spins are parallel or antiparallel. When they are brought close together and the wavefunctions overlap, there will be a strong repulsion if their spins are parallel due to Pauli exclusion. Flence the electron spins are antiparallel in the ground state of the Fl2 molecule. There is however, an intermediate distance where the exchange interactions between the two particles favor the parallel spins. This exchange energy can be calculated 

Bethe-Slater type curve in which the Curie temperature rather than the exchange energy is plotted against the ratio of atomic radius to the radius of the d-electron shell (f-electron shell in the case of Dy). Negative temperatures for Cr and Mn correspond to their Neel temperatures. (Data taken from Kittel, 7th edn., 19%). 

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