Non-Korringa nuclear relaxation in the ferromagnetic phase of the bilayered manganite La_(1.2)Sr_(1.8)Mn_2O_7

摘要

In contrast to ferromagnetic (FM) three-dimensional manganites, ~(55)Mn NMR spectra obtained for the FM phase of the colossal magnetoresistance bilayer manganite La_(1.2)Sr_(1.8)Mn_2O_7 show a broad distribution of hyperfine fields at Mn sites. The hyperfine distribution reflects variations in the electronic structure at the local level. ~(55)Mn spin-lattice relaxation rates have a surprisingly weak dependence both on temperature and on applied magnetic field. Significant departures of the relaxation rate from Korringa temperature dependence below 40 K provide evidence for nonFermi liquid behavior in this quasi-two-dimensional metal. At temperatures approaching T_c from below, in the range where colossal magnetoresistance appears, further anomalous and field-dependent behavior is found in the relaxation rate temperature dependence. The results provide evidence for changes in the electronic structure with temperature in this poorly metallic system. At low temperatures the changes are possibly linked to orbital ordering effects. In addition, statistical fluctuations in dopant concentration may play some role in inducing local variations in the electronic structure. Above 90 K the emergence of polarons is likely to be responsible for the observed decrease in the relaxation rate.