Spin-spin interaction
= Spin-spin interaction
{title2=$H_{ij}\propto\mathbf S_i\cdot\mathbf S_j$}
A <spin-spin interaction> couples two <spin angular momenta>. In an isotropic model it is proportional to their <dot product>, whose expectation in a total-pair-spin state follows from $2\mathbf S_i\cdot\mathbf S_j=S_{ij}^2-S_i^2-S_j^2$. Magnetic interactions generate examples in atoms and effective colour-magnetic interactions in <hadron> models.