Enhancements of magnetoresistance in Nb5+-doped La0.67Sr0.33MnO3

The samples were prepared by doping Nb2O5 into the La0.67Sr0.33MnO3 powder, which is synthesized by the sol-gel method. The XRD result indicates that all samples can be indexed based on the rhombohedral structure with single phase. The resistivities of the samples are influenced remarkably by Nb5+ addition. There is a maximum resistivity p for the sample with x=0.06 (x is the molar ratio of the Nb ions to LSMO), which is higher than that for LSMO by five orders of magnitude. It is due to the enhancement of spin dependent and independent scattering and tunneling effects on the interfaces of grain boundaries and inside the grains. Enhancements of the low-field magnetoresistance (LFMR) and high-field magnetoresistance (HFMR) were observed. The maximum MR ratios at 77 K with H=1T and H=0.0T are 42% and 25% for the 0.07 molar ratio doped sample, which are 2 times and 1.7 times as large as that for LSMO, respectively. A MR effect up to 7% was also found for the sample with x=0.03 at room temperature. The spin dependent tunneling and scattering at the interfaces of grain boundaries are responsible for the LFMR while the HFMR originates from a noncollinear spin structure in the surface layer.