Abstract
Magnetic and transport properties of the series La 0.7 Sr 0.3 Mn 1-x Co x O y (x = 0, 0.01, 0.03, 0.05, 0.1, 0.3, 0.5, 1.0) have been investigated. It was found that with increasing Co content, the Curie temperature T C decreases, while 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 starting to merge) with the applied magnetic field H follows a relation T j /T C = 1 - αH β (T j = T m , T g ), where α and β are constants. The resistivity increases drastically with Co doping and a semiconductor-like behaviour is displayed in the sample x = 0.3. These facts strongly suggest that the double-exchange interaction in the perovskite materials is very fragile since a small Co content can change the magnetic and relevant transport properties of the system. © 1999 IOP Publishing Ltd.