Changes in the electronic structure and spin dynamics across the metal-insulator transition in La1-xSrxCoO3

The magnetoelectronic properties of La1-xSrxCoO3, which include giant magnetoresistance, are strongly dependent on the level of hole doping. The system evolves, with increasing x, from a spin glass insulator to a metallic ferromagnet with a metal-insulator (MI) transition at x(C) similar to 0.18. Nanoscale phase separation occurs in the insulating phase and persists, to some extent, into the just-metallic phase. The present experiments at 4.2 K have used La-139 nuclear magnetic resonance to investigate the transition from hopping dynamics for x < x(C) to Korringa-like ferromagnetic metal behavior for x > x(C). A marked decrease in the spin-lattice relaxation rate is found in the vicinity of x(C) as the MI transition is crossed. This behavior is accounted for in terms of the evolution of the electronic structure and dynamics with cluster size.