In ferromagnets, small-angle spin deviations propagate as spin waves (magnons). The Heisenberg exchange interaction gives a quadratic dispersion ω(k) = ω₀ + Dk² where D is the exchange stiffness (spin-wave stiffness constant). Magnons are bosonic quasiparticles with density of states ∝ ω^(1/2), governing low-temperature heat capacity and magnetization corrections.
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