MAGNETIC AND TRANSPORT-PROPERTIES OF THE SYSTEM LA1-XSRXCOO3-DELTA (0-LESS-THAN-X-LESS-THAN-OR-EQUAL-TO-0.50)

Magnetic and transport measurements on the system La1-xSrx CoO3-delta (0 < x less than or equal to 0.50, delta = 0 for 0 less than or equal to x < 0.30, and delta (x) less than or equal to 0.06 for 0.30 less than or equal to x less than or equal to 0.50) have led to a phase diagram for this system. Upon Sr doping, the material segregates into hole-rich, metallic ferromagnetic regions and a hole-poor matrix similar to LaCoO3. The Co ions of the ferromagnetic phase are in intermediate-spin configurations; the hole-poor matrix experiences a thermally induced low-spin to high-spin transition. For low Sr doping (x < 0.20), the hole-rich regions are isolated from one another and show superparamagnetic behavior below T-c approximate to 240 K; long-range magnetic order via frustrated intercluster interactions occurs below a superparamagnetic freezing temperature T-g that increases with x. At x = 0.20, the coupling between the intermediate-spin, metallic ferromagnetic clusters within an interpenetrating matrix reaches a magnetic percolation threshold that orders the clusters ferromagnetically below T-c approximate to 250 K. However, the electrical conduction in the ferromagnetic regions is modulated by the matrix, which has a spin state that varies with temperature. This causes a reentrant semiconductive behavior in the samples with 0.20 less than or equal to x less than or equal to 0.25. Metallic ferromagnetic behavior is found for 0.30 less than or equal to x less than or equal to 0.50 even though the presence of an interpenetrating hole-poor matrix persists to x = 0.50. A ferromagnetic fraction less than unity leads to peculiar magnetic properties that have been attributed in the literature to a magnetic ''cluster glass.'' (C) 1995 Academic Press, Inc.