Magnetic coupling in the insulating and metallic ferromagnetic La_1-xCa_xMnO₃

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) La_1-xCa_xMnO₃ (LCMO) as a function of hole doping x (0.2, 0.25, and 0.30) and temperature, above and below the Curie temperature T_C. While the spin-diffusion coefficients A(T) and T_C's increase smoothly with doping concentration x, the spin-stiffness constant D(T) 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 D(T) as T → T_C and nondiverging spin-correlation lengths at T_C. These results present a serious challenge to the understanding of these materials using models such as Heisenberg ferromagnetism, double exchange, or modified double exchange.