magnons

Magnons are the quanta of spin waves in magnetically ordered solids. They represent collective precession of electron spins around their ordered direction and are bosonic quasiparticles that quantify spin deviations from long-range order.

In the quantum Heisenberg model, linear spin wave theory yields the magnon dispersion. For a simple ferromagnet with nearest-neighbor exchange J, the frequency is omega(k) = 2JS [1 − gamma_k], which at long wavelengths reduces to omega ~ D k^2, where D is the spin stiffness. In antiferromagnets there are two magnon branches. Magnons either increase or decrease the net spin along the magnetization depending on their mode.

Experimentally, magnons are observed with inelastic neutron scattering, Brillouin light scattering, Raman spectroscopy, and ferromagnetic resonance. They contribute to thermodynamics (for example, the low-temperature specific heat in 3D scales as T^{3/2}) and support spin transport with low dissipation. Magnonics aims to use spin waves as information carriers in next-generation devices.

Magnons occur in many materials, notably insulating ferromagnets such as yttrium iron garnet (YIG). They obey Bose-Einstein statistics, and under suitable pumping can form a magnon Bose-Einstein condensate in some systems.