Transport and magnetic properties in perovskite compound La0.7Sr0.3Mn1-xCoxOy

Magnetic and transport properties of the series La0.7Sr0.3Mn1-xCoxOy (x = 0 , 0.01, 0.03, 0.05, 0.1, 0.3, 0.5, 1.0) have been investigated. It was foun d that with increasing Co content, the Curie temperature T-C decreases, whi le the spin glass transition temperature T-g decreases first then increases . As for the sample with x = 0.3, the relation of T-g and the merging point T-m (for the zero-field-cooled and field-cooled magnetization curves start ing to merge) with the applied magnetic field H follows a relation T-j/T-C = 1 - alpha H-beta(T-j = T-m, T-g), where alpha and B are constants. The re sistivity increases drastically with Co doping and a semiconductor-like beh aviour is displayed in the sample x = 0.3. These facts strongly suggest tha t the double-exchange interaction in the perovskite materials is very fragi le since a small Co content can change the magnetic and relevant transport properties of the system.