Low-Temperature Behavior of the Heisenberg Ferromagnet

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Abstract
The mechanics of spin deviations is formulated in a simple manner, which maintains the true spin kinematics and does not involve the introduction of any artificial interactions. A virial expansion for the thermodynamics, based on this mechanics, clearly distinguishes kinematical and dynamical effects. At low temperatures, low-density kinematical effects are easily proved exponentially small. The second virial coefficient is computed unambiguously. Dyson's low-temperature free energy is straightforwardly rederived, and an upper bound is estimated on the validity of the associated low-density boson picture.