= RKKY interaction
{c}
A local moment induces conduction-electron spin polarization, which mediates a second-order coupling to another moment. The static electronic spin susceptibility fixes the spatial interaction. For a three-dimensional spherical Fermi surface its leading large-distance form oscillates as $\cos(2k_FR)/R^3$, up to the overall sign convention and prefactor. The sign alternates with separation. Other band structures and dimensions change this form. <Indirect Exchange Coupling of Nuclear Magnetic Moments by Conduction Electrons by M. A. Ruderman and Charles Kittel>[1954, original conduction-electron-mediated magnetic coupling]{ref}
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