Magnetic coupling in the insulating and metallic ferromagnetic (formula presented)

Low-energy spin excitations play an essential role in determining the characteristics of the phase transitions in the colossal magnetoresistant manganese-oxides (manganites). Inelastic neutron scattering has been utilized to study the spin excitations of the ferromagnetic (FM) (formula presented) (LCMO) as a function of hole doping x (0.2, 0.25, and 0.30) and temperature, above and below the Curie temperature (formula presented) While the spin-diffusion coefficients (formula presented) and (formula presented)’s increase smoothly with doping concentration x, the spin-stiffness constant (formula presented) for the insulating LCMO is 3 times smaller than that of the metallic LCMO. Furthermore, the paramagnetic-to-ferromagnetic phase transitions in LCMO manganites investigated have nonvanishing extrapolated values of (formula presented) as (formula presented) and nondiverging spin-correlation lengths at (formula presented) These results present a serious challenge to the understanding of these materials using models such as Heisenberg ferromagnetism, double exchange, or modified double exchange.