Structural, magnetic and transport properties of La0.8Sr0.2MnO3/xNiO composites

The effect of NiO doping on the structure, magnetic and magnetotransport properties of La0.8Sr0.2MnO3(LSMO)/xNiO has been investigated. Two types of LSMO/xNiO composites were prepared by different processes using the solid-state reaction method. In the first type, the LSMO powders are mixed with small particle size NiO (SA(x) samples) and in the second type the LSMO powders are mixed with as-powder NiO (SBx samples). These studies show that some part of the Ni goes into the perovskite lattice, substituting Mn in LSMO, and the remainder segregates as a separate phase at the grain boundaries and grain surfaces. The presence of NiO at the grain boundaries increases the number of disordered states at the surface of the grains and therefore the low-temperature resistivity increases very quickly with the increase of NiO doping level. Results also show that the NiO doping has an effect on the low- field magnetoresistance (LFMR). The value of magnetoresistance (MR) increases for a low doping level of 0 <= x <= 3 and decreases at a high doping level of x >= 5. The spin- dependent tunnelling and scattering at the interfaces of the grain boundaries are responsible for the increasing of LFMR at x <= 3, while reduction of the LFMR at x >= 5 originates from substitution of Ni2+ for the Mn3+ ions, which introduces magnetic dilution into the samples. Also it can be found that, as T < 230 K, the MR value of the SB3 sample is larger than that of the SA(3) sample due to the presence of a larger amount of NiO at the grain boundaries and surfaces.