Electrical transport, magnetism, and magnetoresistance in La0.7Sr0.3(Mn1-xCox)O3

The crystal structure (rhombohedral) is not changed on the replacement of Mn by Co. The lattice parameters a and c decrease with the increasing Co concentration. In this paper we have systematically investigated the effect of Co doping on the Mn site in the La_0.7Sr_0.3MnO₃ and find that Co substitution leads to a drastic suppression of ferromagnetic long-range order and metallic 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 the low temperature for the compositions of x=0.2 and 0.3. The largest MR ratio as high as ≈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 (≈2–8%) in T<300 K with a maximum MR ratio of 8% at around T_C (=238 K). Received: 11 September 1998 / Accepted: 19 November 1998 / Published online: 17 March 1999