Phenomenological model of anomalous magnon softening and damping in half-metallic manganites

To describe anomalous zone-boundary softening and damping of magnons in manganites we present a phenomenological two-fluid model containing ferromagnetic Fermi-liquid and non-Fermi-liquid components associated with the itinerant and core electrons of Mn. The observed discontinuous increase of magnon damping is explained by the intersection of magnon dispersions with the electron-hole Stoner continuum arising due to the breakdown of the half-metallic ground state of manganites supported by the experiments and analysis of zero-point effects. Coupling of the Fermi-liquid and non-Fermi-liquid fluids yields conventional long wavelength magnons damped due to their interaction with longitudinal spin fluctuations.