C.Q. Tang

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.

Grain boundaries and low-field transport properties in colossal magnetoresistance materials

The grain-boundary structure and the temperature dependence of resistivity were investigated for (1-x)LCMO + xYSZ, where LCMO and YSZ represent La2/3Ca1/3MnO3 and yttria-stabilized zirconia, respectively. It is shown that the YSZ doped samples for x 2%. The metal-insulator transition temperature (Tp) decreases for x 2% as x is increased. LFMR increases with x for x 2%. The experimental observations are discussed on the basis of scanning electron microscopy (SEM) analysis, which reveals YSZ appearing at the grain boundaries of LCMO for x 2%.

Electrical transport and low-field magnetoresistance in the series of mixed polycrystals (1-m)La2/3Ca1/3MnO3 + mLa2/3Sr1/3MnO3

The perovskites of La2/3Ca1/3MnO3 (LCMO) and La2/3Sr1/3MnO3 (LSMO) were synthesized by the sol-gel method. The two perovskites were mixed into polycrystalline materials, (1-m)LCMO + mLSMO, where m is the relative weight ratio of LSMO. The compound with m~0.3-0.5 has the lowest semiconductor-metal transition temperature Tp and the largest low-T resistivity. Moving away from this ratio, Tp increases while the low-T resistivity decreases with increasing or decreasing m. Measurements of the magnetoresistance (MR) show that the mixed polycrystals exhibit a sizable MR effect in low magnetic fields which increases upon cooling and reaches its largest value for T?0?K. It is also interesting to observe the almost T-independent MR effect over a wide temperature window below 300?K for the mixed materials with smaller m.

Monte Carlo simulation of colossal magnetoresistance in doped manganese perovskites

Resistance as a function of temperature and applied magnetic field for doped manganese perovskites is simulated on the basis of a random-resistor-network model. The network, consisting of ferromagnetic metallic particles with number density p and paramagnetic insulating particles with number density 1-p, is generated through the Monte Carlo method. Approximating p by the reduced magnetization (m) determined from the mean-field theory, we show that the simulation can yield the main features of the colossal magnetoresistance in doped manganese perovskites. Comparisons between simulated and experimental data are also presented for (La1-xYx)2/3Ca1/3MnO3 (x = 0.2). The excellent agreement between the simulations and experiments gives strong support to the present approach.

The Defects, Grain Size and Electrical Property in the Non-stoichiometric BaTiO3 Ceramics

The defects, grain size and room-temperature resistivity (RTR) in the La-doped BaTiO3 ceramics with different the barium to titanium ratios ([Ba]/[Ti]) have been investigated by using the positron annihilation, scanning electron microscopy (SEM) and resistance measurement. The results show that the excess of titanium would induce an increase of Ba vacancies, larger grain size and smaller RTR in the samples, and the excess of barium would induce formation of Ti vacancies, smaller grain size and larger RTR in the samples. The variation of RTR is explained by the variations of the grain size and defects in different formulated [Ba]/[Ti] ratios samples.

Effect of A-site deficiency on electrical transport properties of YSZ doped manganites

Abstract The electrical transport properties of the composite samples: A-site stoichiometry (1−x)La2/3Ca1/3MnO3+xYSZ and A-site deficiency (1−x)(La2/3Ca1/3)0.95MnO3+xYSZ with different YSZ percentage x were investigated, respectively. The similar metal–insulator transition at different temperature TP was observed in the two types of composite samples: With increasing of the YSZ percentage, the zero-field resistivity of the composite samples increases and TP shifts to the low temperature for the range of x 2%. For the same YSZ percentage x, TP of (1−x)La2/3Ca1/3MnO3+xYSZ is lower than that of (1−x)(La2/3Ca1/3)0.95MnO3+xYSZ. The different effects between the two type composites were also observed in the nonlinear current–voltage (I–V) behaviors. The results suggest that A-site deficiency and YSZ dopant can be used to adjust purposely the metal–insulator transition temperature and resistivity of composites.

Phase Separation and Transport Behavior in La0.5Ca0.5–xBaxMnO3

We show that for La 0.5 Ca 0.5-x Ba x MnO 3 , the sample is single phase with an orthorhombic structure for x x d . The observed phase separation is a chemically driven inhomogeneity caused by the large size-mismatch (σ 2 ) which occurs as σ 2 > 1.42 x 10 -4 nm 2 . In the single-phase region, increasing x to >0.14 causes a transition to metallic state, and the magnetoresistance near the transition shows behavior similar to that in double-exchange manganite oxides. In the two-phase region, although the compound also shows an insulator-metal transition, both transport and magnetoresistance show behavior completely different from that in the single-phase region. Some anomalous transport and magnetoresistance behaviors are also revealed at low temperatures. A possible phase diagram is presented as a function of T and x and the experimental findings are discussed according to different ground states.

Studies on thermal fatigue in 3Cr2W8V die steel by positron lifetime and hardness measurements

Abstract Studies on thermal fatigue in 3Cr2W8V die steel have been carried out by using positron lifetime and microhardness measurements. It is found that the mean positron lifetime and the microhardness exhibit periodic up-and-down variations with an increase in the number of fatigue cycles. The experiments indicate that in the process of thermal fatigue there are two opposing effects on variation of defects: microdeformation and dynamic recovery and recrystallization.