La-139 spectrum and spin-lattice relaxation measurements of La2/3Ca1/3MnO3in the paramagnetic state

This paper reports La-139 NMR measurements of a powder sample of the coloss al magnetoresistance compound La2/3Ca1/3MnO3(T-c = 268 K) performed in the paramagnetic state (292-575 K) and in high magnetic fields (2.00-9.40 T). A nalysis of the spectrum measured at 575 K establishes that the spectrum is a standard powder pattern broadened to a significant degree by a variation in lattice distortions around lanthanum nuclear Sites. At lower temperature s, but still above T-c, the spectrum shifts and broadens. Both the shift an d broadening exhibit Curie-Weiss behavior, indicating that the shift measur es the polarization of the electron spin system, and the broadening reflect s a distribution of magnetic susceptibilities. This distribution may result from variations of local susceptibility in the bulk of the sample or from differences in demagnetizing factors among powder grains. Close inspection of the spectrum indicates that the lattice distortions do not change as the temperature is lowered. Spectral diffusion measurements suggest that the t emperature dependence of the spectrum shape does not result from the freezi ng out of the motion of magnetic polarons. Variations in the nuclear spin-l attice relaxation across the spectrum indicate that magnetic fluctuations, not lattice vibrations, dominate nuclear relaxation. Nuclear spin-lattice r elaxation therefore measures electron spin dynamics in this system. The mag netic held dependence of the spin-lattice relaxation indicates that the ele ctron spin-spin correlation function adopts simple single exponential behav ior with a slow field-independent correlation time of 10(-8) s near T-c. Th e spin-spin correlation function changes form at higher temperatures and ca n be described by introducing a held dependence to the correlation time and to the magnitude of the fluctuating field. Even at the highest temperature s, the correlation time remains slow, on the order of 10(-9) s. The spin-la ttice relaxation therefore indicates the prescence of extremely slow dynami cs above T-c. [S0163-1829(99)05713-6].