Abstract
A two-band model for the magnetism is used to calculate the stiffness constant in ferromagnetic systems. One band is narrow and degenerated, representing the "quasi-localized" electrons. The second one is a wide band containing very few itinerant electrons. The transverse dynamical susceptibility is calculated within the random-phase approximation (RPA). The value of the stiffness constant of the acoustical spin wave is essential to satisfy the criterion of ferromagnetic stability at T = 0K. From the poles of the susceptibility two spin-wave branches are derived: one acoustic and one optic. It is shown that taking into account the interatomic exchange coupling between electrons of the narrow bands is necessary for the spin-wave stiffness constant to assume the experimental value.
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