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^{139}La NMR measurements of La_{2/3}Ca_{1/3}MnO_3 performed in theparamagnetic state and high magnetic fields are reported. Analysis of the hightemperature spectrum measured establishes that the spectrum it is a standardpowder pattern broadened by a variation in lattice distortions. At lowertemperatures, the spectrum shifts and broadens. Both the shift and broadeningexhibit Curie-Weiss behavior, indicating that the shift measures thepolarization of the electron spin system, and the broadening reflects adistribution of magnetic susceptibilities. This distribution may result fromvariations of local susceptibility in the bulk of the sample or fromdifferences in demagnetizing factors among powder grains. The spectrum alsoindicates that the lattice distortions do not change as the temperature lowers.Spectral diffusion measurements suggest that the temperature dependence of thespectrum shape does not result from freezing out of motion of magneticpolarons. Variation of the nuclear spin-lattice relaxation across the spectrumindicate that magnetic fluctuations, not lattice vibrations, dominate nuclearrelaxation. Nuclear spin-lattice relaxation therefore measures electron spindynamics in this system. The magnetic field dependence of the spin-latticerelaxation indicates that the electron spin-spin correlation function adoptssimple single exponential behavior with a slow field-independent correlationtime of 10^{-8} seconds near Tc. The spin-spin correlation function changesform at higher temperatures, but remains slow. The spin-lattice relaxationtherefore indicates the presence of extremely slow dynamics above Tc.Comment: 10 pages, 11 figures, to be published in Physical Review

physical review. b, condensed matter

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139Laspectrum and spin-lattice relaxation measurements ofLa2/3