Magnonphonon

Magnon–phonon coupling refers to the interaction between magnons, quantized spin waves in ordered magnets, and phonons, quanta of lattice vibrations. The coupling arises primarily from magnetoelastic effects: the magnetic exchange interaction depends on interatomic distances, so lattice displacements modulate exchange; spin–orbit coupling provides a channel through which spin and lattice dynamics exchange energy and angular momentum.

When magnon and phonon dispersions approach each other, they hybridize, producing mixed excitations sometimes called magnon polarons. This hybridization leads to avoided crossings in the dispersion relations, changes in magnon group velocity and lifetime, and enhanced or suppressed spin and heat transport depending on temperature and magnetic field. The magnon–phonon interaction also opens channels for magnon relaxation via phonons and influences the spin Seebeck effect and thermal conductivity in magnetic insulators.

Experimentally, magnon–phonon coupling is observed via inelastic neutron scattering and Brillouin light scattering, which reveal hybridized modes, as well as via ferromagnetic resonance and ultrafast optical probes that track coupled dynamics. Materials with strong effects include magnetic insulators such as yttrium iron garnet (YIG), ferrimagnets, and certain multiferroics; the effect is also present in other magnets but can be weaker.

Theoretical descriptions employ linear spin-wave theory with Holstein–Primakoff magnons, lattice dynamics for phonons, and a magnetoelastic Hamiltonian that couples spin and lattice degrees of freedom. Magnon–phonon interactions are a key consideration in magnonics and spin caloritronics, where they influence spin transport, thermal management, and the control of spin information by lattice excitations.