Non-Landau Damping of Magnetic Excitations in Systems with Localized and Itinerant Electrons

We discuss the form of the damping of magnetic excitations in a metal near a ferromagnetic instability. The paramagnon theory predicts that the damping term should have the form gamma(q, Omega) proportional to Omega/Gamma(q), with Gamma(q) proportional to q (the Landau damping). However, the experiments on uranium metallic compounds UGe2 and UCoGe showed that Gamma(q) is essentially independent of q. A nonzero gamma(q = 0, Omega) is impossible in systems with one type of carrier (either localized or itinerant) because it would violate the spin conservation. It has been conjectured recently that a near-constant Gamma(q) in UGe2 and UCoGe may be due to the presence of both localized and itinerant electrons in these materials, with ferromagnetism involving predominantly localized spins. We present the microscopic analysis of the damping of near-critical localized excitations due to interaction with itinerant carriers. We show explicitly how the presence of two types of electrons breaks the cancellation between the contributions to Gamma(0) from the self-energy and vertex correction insertions into the spin polarization bubble. We compare our theory with the available experimental data.