The coexistence and competition of low-field magnetoresistance and colossal magnetoresistance in polycrystalline La_0.49Sr_0.51(Mn_1-xNb_x)O₃

The magnetic and transport properties of polycrystalline La_0.49Sr_0.51(Mn_1-xNb_x)O₃ have been studied by the measurement of dc magnetization and magnetoresistance (MR). With the increase of Nb doping, the ferromagnetism evolves from the inferior position in the x = 0 sample to the dominant position in the x = 0.05 and 0.15 samples, and then to the inferior position again in the x = 0.25 sample. Therefore, there are not enough ferromagnetic domains to form percolative conduction paths in the x = 0 and 0.25 samples, as a result, no obvious MR has been found under a 4500 Oe magnetic field in the two samples. But the intrinsic colossal MR (CMR) effect and the extrinsic low-field MR (LFMR) effect have been observed coexisting in the samples of x = 0.05 and 0.15 under 4500 Oe. On the other hand, the anisotropic MR has been observed in the x = 0.05 and 0.15 samples when a 4500 Oe field is respectively applied perpendicular and parallel to the plane of the pellet-shape samples. The anisotropic MR is attributed to the existence of LFMR.