Unconventional ferromagnetic transition in La1-xCaxMnO3

Neutron scattering has been used to study the magnetic correlations and long wavelength spin dynamics of La1-xCaxMnO3 in the ferromagnetic regime (0 less than or equal to x < 1/2). For x = 1/3 (T-c = 250 K) where the magnetoresistance effects are largest the system behaves as an ideal isotropic ferromagnet at low T, with a gapless (<0.04 meV) dispersion relation E = D-q(2) and D-T=0 approximate to 170 meV Angstrom(2). However, an anomalous strongly field-dependent diffusive component develops above similar to 200 K and dominates the fluctuation spectrum as T --> T-c. This component is not present at lower x.