C.S. Xiong

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%.

Enhanced room temperature magnetoresistance in La0.7 Ca0.2 Sr0.1 MnO3/Ag composites

The effect of added Ag on the electro-magnetic properties and enhanced room temperature magnetoresistance in (La0.7Ca0.2Sr0.1MnO3)1−x/Agx composites has been studied systematically. According to the results of x-ray diffraction, scanning electron microscopy and magnetic measurement, we can suggest that Ag segregates at the grain surface of La0.7Ca0.2Sr0.1MnO3. The results of electronic measurements show that the intrinsic insulator/metal transition temperature (TP) does not change, which is in accordance with the results for the Curie temperature (TC). It is very interesting to note that the magnetoresistance at room temperature is enhanced, which is encouraging for potential applications. These phenomena can be explained by considering that Ag, which segregated at the grain boundaries or surfaces, does not change the intrinsic structure of La0.7Ca0.2Sr0.1MnO3 and lead to a modification of grain boundaries.

A study of freezing temperature and magnetic relaxation for epitaxial La0.67Sr0.33MnO3-δ films

The variations of magnetic susceptibility and magnetic relaxation obtained by a superconducting quantum interference device magnetometer are investigated systematically for epitaxial La0.67Sr0.33MnO3-δ films grown on (001) LaAlO3 single-crystal substrate using a direct-current magnetron sputtering technique. Spin-glass-like behaviour can be observed, which is caused by the competition between ferromagnetism and antiferromagnetism, a direct consequence of the random distribution of Mn3+ and Mn4+ ions. Using the concept of spin glass and the double-potential-well model, we have calculated the relation between the applied field and the freezing temperature and the relaxation of M versus time. All the calculated results are in good agreement with the experimental values.

Phase separation and insulator-metal behaviour in highly Ba2+-doped La1-xBaxMnO3 compounds

Investigations of structural and transport features of La1-xBaxMnO3 with x = 1/3 and 2/3 are performed. The x = 2/3 sample is shown to be phase separated into a mixture of La2/3Ba1/3MnO3 and BaMnO3 with the same volume fractions, ~50%. In this two-phase system, La2/3Ba1/3MnO3 regions are connected in a percolative manner, so the electrical transport is dominated by flow along these percolative paths. Using the recently proposed random resistor network based on electronic phase separation between ferromagnetic metallic and paramagnetic insulating domains, we show that the model can yield results in quantitative agreement with the resistance versus temperature dependence measured for the x = 1/3 and 2/3 samples by using the metallic number density as a fitting parameter. This approach suggests a simple quantitative picture that can be used to explain the insulator-metal behaviour in La1-xBaxMnO3 with higher x.

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.

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.

Study on magnetic properties for La0.67Sr0.33MnO3 − δ/Pr0.7Ca0.3MnO3 − δ/La0.67Sr0.33MnO3 − δ trilayer epitaxial film

We studied systematically the magnetic properties of La0.67Sr0.33MnO3 − δ/Pr0.7Ca0.3MnO3 − δ/La0.67Sr0.33MnO3 − δ trilayer epitaxial film fabricated on (001) LaAlO3 single-crystal substrate using a direct current magnetron sputtering technique. A spin-glass-like behaviour and a magnetization relaxation phenomenon are observed from the results of the dependences of susceptibility on temperature (χ versus T curve) and magnetic relaxation on time (M versus t curve). The magnetic relaxation results are well fitted using the double-well-potential model. We also discussed the possible mechanism for the existence of spin-glass-like behaviour in the film. According to the χ versus T curves and the hysteresis loop, a magnetic anisotropy was observed in the film. The difference in electron-magnetic properties between La0.67Sr0.33MnO3 − δ single-layer epitaxial film and La0.67Sr0.33MnO3 − δ/Pr0.7Ca0.3MnO3 − δ/La0.67Sr0.33MnO3 − δ trilayer epitaxial film is also discussed.

Characteristics of La0.7Ca0.3MnO3–Cu composites fabricated by an electroless process

Ferromagnet–metal-type composites, La0.7Ca0.3MnO3(LCMO)–Cu, have been fabricated by a three-step route. We got LCMO powder using the conventional solid state method. Then, LCMO powders coated by Cu are obtained using a novel electroless plating technique, in which the contents of copper are controlled by changing the plating time. Finally these powders are pelletized and then sintered at different temperatures in argon. The powders coated by Cu are characterized by x-ray powder diffraction and scanning electron microscope. The temperature dependence of resistivity and the magnetoresistance in the applied field of 3000 Oe are measured. It is observed that the metal–insulator transition temperature (Tp) is improved above 20 K approximately up to 280 K compared with that of LCMO powder using the conventional solid state method without being coated by Cu; meanwhile, a second broad peak occurs in low temperature in the resistivity-temperature curve when sintering temperature is changed. Also an anisotropic magnetoresistance effect is observed.

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.