= Hund's coupling
{c}
{title2=$H_H=-J_H\mathbf S_1\cdot\mathbf S_2,\quad J_H>0$}
An effective local exchange coupling can favor parallel <electron spins> in different orbitals on the same atom. The displayed sign convention represents that preference. It underlies the high-spin tendency in <Hund's rules> and the alignment of itinerant and core spins used in <double exchange>. The coupling is an effective description of intra-atomic Coulomb and exchange energies, rather than a separate classical magnetic force.
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