A study of Mn-site substitution by Ni2þ and the effect of
annealing time in polycrystalline bulk La0.7Sr0.3MnO3 in
composition range (0x0.1) is presented. This study
involves structural, electrical, magnetoresistance, and thermoelectric
power (TEP) measurements. X-ray diffraction
was used to examine the structure for the as-prepared and
annealed samples, and it shows that there is no change in
structure with annealing time. Electrical resistivity and
magnetoresistance are strongly affected by annealing
process, where there is a dramatic increase in resistivity
with annealing time and an enhancement in magnetoresistance
in different temperature ranges. Moreover, conduction
mechanisms above and below transition temperature (Tms)
are discussed, since, variable-range hopping and smallpolaron
hopping models are applied above Tms, in addition to
some experimental relations below it. In addition, the TEP
sign changes from positive to negative with doping and
annealing.