spinwaves

Spin waves are collective oscillations of the magnetic moments in ordered magnetic materials. They arise from the exchange and dipolar interactions that align spins and can propagate through a material as waves of spin orientation. In quantum theory, these excitations are quantized as magnons, bosonic quasiparticles. Spin waves occur in ferromagnets, antiferromagnets, ferrimagnets and in nanostructures, where they can be guided and manipulated for information transport.

Classically, spin-wave dynamics are described by the Landau-Lifshitz-Gilbert equation for the magnetization. Small deviations from a uniform ground state yield wave solutions with a dispersion relation that depends on exchange interactions, magnetic dipolar fields, anisotropy, and dimensionality. In ferromagnets, a single spin-wave branch typically describes long-wavelength excitations, while in antiferromagnets two branches can appear due to the two sublattices. The quantum description uses magnons with energy E = ħω, and their populations follow Bose–Einstein statistics. At finite temperature, magnons reduce the net magnetization and contribute to thermodynamic properties such as specific heat.

Experimental observation relies on methods like inelastic neutron scattering for bulk samples, Brillouin light scattering and ferromagnetic resonance for thin films and microstructures, and spin-polarized electron-energy-loss spectroscopy. These techniques map the dispersion, lifetime, and damping of spin waves.

Damping, characterized by the Gilbert damping parameter, determines magnon lifetimes and propagation lengths. Spin waves underpin magnonics, an area exploring information processing and communication with low power, high-frequency operation, and potential wave-based logic. They also play a role in spin and thermal transport through magnon currents.