Local-band theory of itinerant ferromagnetism. II. Spin waves

The short-wavelength magnons which are observed by neutron scattering above as well as below the Curie temperature TC are studied by the methods of the local-band theory, described in the preceding paper. We find a temperature shift of the spin-wave constant D, of a type previously proposed, but which we estimate to be numerically small. A novel downward shift mechanism gives a lowering -2D|→a|ₐᵥ², where |→a|ₐᵥ² is proportional to the magnetic energy. |→a|ₐᵥ² saturates shortly above TC at about 5 ×{} 10^-2 ^-2 in both iron and nickel. The form of this term is strictly correct only for q²|→a|ₐᵥ². However, it accounts for the greater part of the observed magnon energy shift, both in temperature dependence and shape of the dispersion curve. Finally, we find a new magnon-width mechanism Γq proportional to |→a|ₐᵥ²q which seems capable of accounting for the observed width.