Dynamical behavior of spin clusters in La$_{1-x}$Sr$_{x}$CoO$_{3}$

Previous work has provided evidence for magnetic glassy behavior in the hole-doped cobaltite system La$_{1-x}$Sr$_{x}$CoO$_{3 }$(LSCO). Models proposed to describe the interesting and unusual magneto-transport properties of LSCO involve hole-rich clusters in a hole-poor matrix. The glassy properties, which are not well understood, have variously been interpreted in terms of spin glass and cluster glass components. The present $^{139}$La NMR spectral lineshape measurements on single crystal LSCO, that map the hyperfine field distribution in the x$-T$ plane, confirm the presence of magnetic clusters, identified as spin polarons, and provide a phase inhomogeneity diagram. NMR relaxation rates have been used to probe the dynamical behavior of the system at the nanoscale level in macroscopically insulating and metallic samples as a function of temperature in the range 4-280 K. For x less than the metal-insulator critical concentration x$_{C }$= 0.17 evidence has been obtained for two classes of glassy component with different characteristic correlation time distributions and freezing temperatures. The magnetic glass properties persist above x$_{C}$. A spin polaron model is used to explain the results.