For other bosonic quasi-particles, e.g., magnons (quantized spin waves) in ferromagnets instead of the phonons (quantized sound waves), one can derive analogous results. In this case at low frequencies one has different dispersion relations of momentum and energy, e.g., E ( ν ν ) ∝ ∝ k 2 in the case of magnons, instead of E ( ν ν ) ∝ ∝ k for phonons (with k = 2 π π / λ λ ). One also has different density of states (e.g., ∫ ∫ g ( ν ν ) d ν ν ≡ ≡ N ). As a consequence, in ferromagnets one gets a magnon contribution to the heat capacity, Δ Δ C V | m a g n o n ∝ ∝ T 3 / 2 , which dominates at sufficiently low temperatures the phonon contribution, Δ Δ C V | p h o n o n ∝ ∝ T 3 . In metals, in contrast, the main low-temperature contribution to the heat capacity, ∝ ∝ T , comes from the electrons. It is fermionic, and is calculated by different methods going back to Sommerfeld 's free electron model.