= Fermi contact interaction
{c}
{title2=$H_{\rm c}\propto|\psi_s(0)|^2\mathbf I\cdot\mathbf s$}
The isotropic magnetic interaction of a nuclear <magnetic dipole moment> with <electron spin> density at the <atomic nucleus>. A nonrelativistic penetrating $s$ <orbital> can have $|\psi_s(0)|^2\ne0$ and a substantial contact contribution to <hyperfine structure>. A filled paired shell cancels its net contact <spin> density in the independent-particle approximation; unpaired occupation and core polarization can restore it. Relativistic and finite nuclear-size effects modify the point-contact description.
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