Structural, magnetic and transport properties ofLa0.8Sr0.2MnO3/xNiO composites

Abstract

The effect of NiO doping on the structure, magnetic and magnetotransport properties ofLa0.8Sr0.2MnO3(LSMO)/xNiO has been investigated.Two types of LSMO/xNiO composites were prepared by different processes using the solid-state reactionmethod. In the first type, the LSMO powders are mixed with small particle size NiO(SAx 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 grainboundaries and grain surfaces. The presence of NiO at the grain boundaries increases thenumber of disordered states at the surface of the grains and therefore the low-temperatureresistivity increases very quickly with the increase of NiO doping level. Resultsalso 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 of0≤x≤3 and decreases at ahigh doping level of x≥5. The spin-dependent tunnelling and scattering at the interfaces ofthe grain boundaries are responsible for the increasing of LFMR atx≤3, while reduction of theLFMR at x≥5 originatesfrom substitution of Ni2+ for the Mn3+ ions, which introduces magnetic dilution into the samples. Also it can be found that, asT<230 K, the MR value ofthe SB3 sample is largerthan that of the SA3 sample due to the presence of a larger amount of NiO at the grain boundaries andsurfaces.