PlanetPhysics/Magnons

Magnons and magnon dispersion
Magnons are defined as collective excitations of a magnetic lattice that possesses long range magnetic order; more specifically, a single magnon excitation corresponds to the change by one unit of the magnetic moment of the lattice or system. This non-local nature of magnons is the cause of the experimentally observed dispersive behavior, that is a non-linear or non-constant relation between the transferred momentum and the transferred energy of the magnetic lattice upon its excitation. Typical values for the peak(s) of the transferred energy are on the order of 0.5 eV to 2 eV.

Two- and mutiple- magnon dispersion phenomena have been reported, and were recently employed to explain the nonlinear dispersion behavior of both crystalline and non-crystalline systems with long range ordering compared with the atomic scale.

Experimental observation of the non-linear magnon dispersion
Experimentally, magnon dispersions have been detected by resonant microwave absorption in external magnetic fields (that is, by Spin-Wave resonance excitation (SWR) and ferromagnetic resonance (FMR)) for ferromagnetic metallic glasses at ambient temperatures. Several neutron inelastic, as well as Cu $$K_\alpha$$ edge resonant inelastic X-ray scattering (RIXS), spectra were also reported for crystaline materials such as the undoped antiferromagnetic cuprates below 20 K.

Applications
Such measurements and corresponding theories are of significant interest for an improved understanding of high temperature superconductivity; upon doping ( for example with Ytrium or Lanthanum, and Barium) the long-range ordering in a antiferromagnetic lattice-- that was present in certain undoped copper oxide insulators-- becomes frustrated, thus leading to short range antiferromagnetic fluctuations, symmetry breaking and high temperature superconductivity.