We consider the quantum ferromagnetic Heisenberg model in three dimensions, for all spins $S= 1/2$. We rigorously prove the validity of the spin-wave approximation for the excitation spectrum, at the level of the first non-trivial contribution to the free energy at low temperatures. The proof combines a bosonic representation of the model introduced by Holstein and Primakoff with probabilistic estimates, localization bounds and functional inequalities. Joint work with Michele Correggi and Alessandro Giuliani