The electrical and magnetic transport behaviour of composite samples of (1 − x)La 0.67 Ca 0.33 MnO 3 + xFe 3 O 4 with x = 0, 0.1%, 1% and 2% were studied in a temperature interval 10-300 K and for magnetic fields H = 0, 0.3, 0.5, 1 and 3 T. The temperature and magnetic field dependence of resistivity of composites show that the low Fe 3 O 4 doping levels have important effects on electrical and magnetic transport behaviour of La 0.67 Ca 0.33 MnO 3 . Especially, compared to pure La 0.67 Ca 0.33 MnO 3 , a new insulator-metal (I-M) transition was observed at a lower temperature T P2 in all Fe 3 O 4 doped La 0.67 Ca 0.33 MnO 3 composites, which may result from the existence of a new phase related to Fe 3 O 4 dopant at grain boundaries and surfaces of La 0.67 Ca 0.33 MnO 3 phases. It is of interest to note that the new I-M transition temperature, T P2 , is almost independent of the magnetic field. Meanwhile, an obvious magnetoresistance effect was observed between temperatures T P1 and T P2 . The results were discussed by considering the results of scanning electron microscopy, x-ray diffraction, dielectric relaxation and susceptibility analysis.
Polycrystalline samples of nominal La 2/3 Ca 1/3 Mn 1−x Cu x O 3 (x = 0% and 4%) were fabricated by a sol-gel method following sintering treatments at temperature T s ranging between 1000˚C and 1300˚C. Experiments indicate that doping Cu does not cause a change in crystalline structure, but strongly affects transport and magnetoresistance (MR) properties. For lower T s , when a low magnetic field of H = 0.3 T, is applied, the x = 0 samples show typical intergrain MR behaviour with a monotonic increase in MR 0 (≡ ρ/ρ(H = 0)) on cooling; while for the x = 4% samples, in addition to intergrain MR, a characteristic feature similar to colossal MR (CMR) is observed near the insulator-metal transition. The maximum MR with a value ∼80% of that for H = 0.3 T is obtained in the sample prepared at 1100˚C, which is comparable to the intrinsic CMR response usually observed in large fields of the order of several teslas.
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