ELECTRICAL-TRANSPORT, MAGNETISM, AND MAGNETORESISTANCE IN FERROMAGNETIC OXIDES WITH MIXED EXCHANGE INTERACTIONS - A STUDY OF THE LA0.7CA0.3MN1-XCOXO3 SYSTEM

In this paper we report the results of an extensive investigation of t he La0.7Ca0.3Mn1-xCoxO3 system. Substitution of Mn by Co dilutes the d ouble-exchange (DE) mechanism and changes the long range ferromagnetic order of La0.7Ca0.3MnO3 to a cluster glass-type ferromagnetic (FM) or der similar to that observed in La0.7Ca0.3CoO3. This happens even for the lowest Co substitution of x = 0.05 and persists over the entire co mposition range studied (0.05 less than or equal to x less than or equ al to 0.5). The Co substitution also destroys the metallic state and t he resistivity increases by orders of magnitude even with a very small extent of Co substitution. The charge localization due to Co substitu tion is likely to have its origin in polaronic lattice distortion. The Co substitution also suppresses the colossal magnetoresistance (CMR) of the pure manganate (x = 0) over the entire temperature and composit ion range and it becomes very small for x greater than or equal to 0.2 . We conclude that the DE interaction and the resulting metallic state is very ''fragile'' and hence even a small amount of Co substitution can destroy the FM order, the metallic state, and the CMR.