Spin Waves are collective excitations in the ordered phase of ferromagnetic materials. As spin waves have very low-energies, they are easily created as thermal fluctuations. In two landmark papers in 1956 Dyson pointed out that, nevertheless, the contribution of spin waves to the thermo-dynamic quantities is surprisingly small. I am going to present a simplified version of Dyson's argument, and show that it extends to other formulations of spin-wave theory.
Joint work with Ian Jauslin.