L.J. Zhang

Electrical transport behavior of La0.67Ca0.33MnO3/Fe3O4 composites

Abstract The influence of Fe 3 O 4 contents on the electrical transport properties (resistivity and ac susceptibility) of a series of composite samples of La 0.67 Ca 0.33 MnO 3 /Fe 3 O 4 is studied. Results show that the Fe 3 O 4 phase not only shifts the intrinsic insulator–metal (I–M) transition temperature T P1 to a lower temperature, but also causes a new I–M transition at a lower temperature T P2 ( T P2 T P1 ). On the basis of an analysis by scanning electron microscopy and X-ray diffraction, we suggest that the decrease of the I–M transition temperature and the formation of the new I–M transition are caused by the segregation of a new phases related to the Fe 3 O 4 at grain boundaries or surfaces of the La 0.67 Ca 0.33 MnO 3 grains.

A comparative study of low-field magnetoresistance for La2/3Ca1/3Mn1−xCuxO3 (x = 0% and 4%) synthesized at different temperatures

Polycrystalline samples of nominal La2/3Ca1/3Mn1−xCuxO3 (x = 0% and 4%) were fabricated by a sol–gel method following sintering treatments at temperature Ts 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 Ts, 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 MR0(≡ Δρ/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.

Cu powder doping effects on the structure and electro-magnetic properties of La0.7Ca0.3MnO3

The structure, electronic and magnetic properties of perovskite (1 ? x)La0.7Ca0.3MnO3(LCMO)/xCu (x is a mass percentage, x = 1%, 3%, 5%, 7%, 9%, 15%, 20%) have been studied. The nano-scale metal Cu powder was doped into the LCMO matrix. The results showed that some Cu element substituted for Mn ion in LCMO and the remainder resided in the grain boundaries in the form of a CuO phase because of the oxidation process when the Cu powder was sintered at high temperature in air. Cu existed in the form of Cu3+ ions in the samples at a low doping level (x ? 3%) and Cu2+ ions appeared when x > 3%. Because of the larger ionic radius of Cu2+ ions compared with the average Mn radius, it is likely that the Cu2+ ions in the proximity of the grain boundaries tend to separate into the boundaries. An amorphous phase of the Cu element was formed in the grain boundaries, which respond to the unusual electrical and magnetic behaviours of the heavily doped samples. A maximum MR value (~50%) around 210?K was observed in a magnetic field of 3000?Oe for the 5% doped samples. Besides the effect of Cu substituting for Mn ions in LCMO, the MR behaviour is also attributed to the properties of dopants which reside in the grain boundaries of the matrix grains.