Electrical transport, magnetism, and magnetoresistance in La0.7Sr0.3(Mn1-xCox)O-3

The crystal structure (rhombohedral) is not changed on the replacement of M n by Co. The lattice parameters a and c decrease with the increasing Co con centration. In this paper we have systematically investigated the effect of Co doping on the Mn site in the La0.7Sr0.3MnO3 and find that Co substituti on leads to a drastic suppression of ferromagnetic long-range order and met allic state. However, we observed substantial enhancement of MR ratio over the entire temperature studied for x = 0.1 and a large increase of one at t he low temperature for the compositions of x = 0.2 and 0.3. The largest MR ratio as high as approximate to 78% is observed at 15 K in x = 0.2 and 0.3. As x is further increased toward 1, conduction and ferromagnetism strongly recover but the MR ratio decreases obviously. The end member with x = 1.0 has metallic and cluster spin-glass ferromagnetic behavior (T < T-C) and a smaller value of MR (approximate to 2%-8%) in T < 300 K with a maximum MR r atio of 8% at around T-C (= 238 K).