Changes in the electronic structure and spin dynamics across the metal-insulator transition in La_(1-x)Sr_xCoO_3

摘要

The magnetoelectronic properties of La_(1-x)Sr_xCoO_3, 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 ～ 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 ~(139)La 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.