See the Math Magnetic Gateway

Consider a spin, s [the polarizer], located at the origin. It induces a magnetization, , where (r, t) is the non-local magnetic susceptibility. The action of this magnetization on a second spin s', the analyzer, brings about an induced magnetic interaction between the quasiparticles whose Fourier transform in wave vector and frequency space, . If now the system is not far from antiferromagnetic order, the wave vector-dependent dynamic susceptibility in the vicinity of the ordering wave vector, Q, can become very large, as can the resulting quasiparticle attraction.