Neutron scattering has been used to study the magnetic correlations and long wavelength spin dynamics of La1−xCaxMnO3 in the ferromagnetic regime (0 ≤ x < 1 2 ). For x = 1 3 (TC = 250K) where the magnetoresistance effects are largest the system behaves as an ideal isotropic ferromagnet at low T, with a gapless (< 0.04meV ) dispersion relation E = Dq 2 and DT =0 ≈ 170 meV-Å2 . However, an anomalous strongly-field-dependent diffusive component develops above ∼ 200K and dominates the fluctuation spectrum as T → TC. This component is not present at lower x.

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