Localized Magnons (Theory)

摘要: The Heisenberg Hamiltonian for impure ferro — or antiferromagnets is of the class originally discussed by Lifshitz(1) in his work on properties energy spectra disordered systems. Within simple (non-interacting) spin wave approximation, applicable low temperature limit, problem closely analogous to that phonons lattices, which general theory was first applied. One expects (and finds) appropriate impurity parameters true localized modes at energies outside pure crystal band and virtual (or “resonances”), with finite lifetimes, within band. However, even this picture, there no analog most easily treated lattice a single isotopic impurity. magnitude enters dynamically exchange interaction “force constant” associated magnetic must therefore be changed along its “mass.” In general, substitutional may differ from atom it replaces constants coupling neighbors as well magnitude.