Xia Zheng-Cai

Transport propeties of polycrystalline La2/3Ca1/3Mn1−x Cu x O3 prepared by Sol-Gel

The influence of Cu dopant (x) and sintering temperature (Ts) on the transport properties of La2/3Ca1/3Mn1−xCuxO3 series samples prepared by Sol-Gel technique was investigated. X-ray diffraction patterns show that all the samples with different Cu dopant and sintering temperatures (Ts) are of single phase without obvious lattice distortion. Experimental results indicate that the insulator-metal transition temperature is directly related to the sintering temperature and Cu dopant x. It is interesting to observe that a proper amount of Cu dopant can substantially improve magnetoresistance effects.

The Magnetic Anisotropy and Complete Phase Diagram of CuFeO2 Measured in a Pulsed High Magnetic Field up to 75T

The magnetization behavior of a CuFeO2 single crystal grown by the floating zone technique is investigated with a pulsed high magnetic field. We observe a series of field-induced multi-step-like transitions with hysteresis, of which the critical magnetic fields are temperature-dependent and show anisotropy. By using a pulsed high magnetic field up to 75 T, the magnetization behavior shows that the critical transition magnetic fields of spin-flip/flop shift to lower field regions with an increase in temperature. According to the magnetization curves, a complete magnetic phase diagram is depicted.

Electrical transport and low-field magnetoresistance in La2/3Ca1/3MnO3/YSZ composites

Electrical transport and low-field magnetoresistance (MR) are reported for (1-x)LCMO/xYSZ composites (0 2%. In particular, the composite with x≤1% shows a wide temperature window with a large constant MR below the transition. Based on microscopy analysis, a possible interpretation is presented for different observations for x 2% with emphasis on the boundary effect arising from insulating YSZ layers.

Evidence of the Griffiths Phase in Doped Manganites Studied by Electron Paramagnetic Resonance Measurements

A systematic investigation of electron paramagnetic resonance above the Curie temperature TC is performed on polycrystalline (La1−xYx)2/3Ca1/3MnO3 (x = 0, 0.15 and 0.2) samples. The results indicate anomalous paramagnetic behaviour below a temperature TG and, the anomalous extent increases with cooling from TG. Especially, the resonance linewidth ΔHpp anomalously increases with cooling for the temperature range of TC<T<TG. The experimental observation is discussed within the framework of the Griffiths theory that predicts the existence of ferromagnetic clusters above TC. We explain the observed anomalous paramagnetic behaviour to be due to the magnetic heterogeneity caused by the ferromagnetic clusters which appear in the Griffiths phase.

Correction to the Metal-Insulator Transition Temperature due to Cation Size and Strain Effects for Colossal Magnetoresistance Perovskites

A phenomenological expression of the metal-insulator transition temperature is proposed for AMnO3 manganese perovskites by taking into account the distortion of the Mn-O-Mn bond due to A-cation size and the strain-dependent effect due to performed Jahn-Teller distortions, independently. Using reasonable physical parameters, the calculated results give excellent agreement with experimental data obtained in polycrystalline samples of La2/3(Ca1-xBax)1/3MnO3, providing a strong support to this approach.

Effects of Doping on the Magnetic Properties and Frustration of Hexagonal YMn0.9A0.1O3 (A=Al, Fe, and Cu)

The doping effects on the frustration and the magnetic properties in hexagonal compounds of YMn0.9A0.1O3 (A=Al, Fe and Cu) are investigated. Experimental results indicate that both the non-magnetic and magnetic ion dopants lead to the increase of magnetic moments and the decrease of the absolute value of Curie—Weiss temperature (|θcw|). Compared with pure YMnO3, the geometrical frustration of YMn0.9A0.1O3 is greatly suppressed and the magnetic coupling in that exhibits dopant-dependent. In addition, for the doped YMn0.9A0.1O3, the antiferromagnetic transition temperature (TN) is also suppressed slightly, which shows an abnormal dilution effect and it may be ascribed to the reduction of frustration due to the chemical substitution.

Fe3O4 Segregation and Transport Properties of La0.67Ca0.33MnO3

Effect of Fe3O4 segregation at grain boundaries on the electrical transport and magnetic properties of La0.67Ca0.33MnO3 is investigated. The experimental results show that the Fe3O4 segregation not only shifts the paramagnetic–ferromagnetic transition temperature of La0.67Ca0.33MnO3 to a lower temperature region but also induces a new transition in a lower temperature region. Meanwhile, the transition processes observed in both the resistivity and magnetization curves are obviously widened. Compared to pure La0.67Ca0.33MnO3, we assume that the Fe3O4 segregation level at the grain boundaries can modify the electrical transport and magnetic properties of La0.67Ca0.33MnO3.

Unusual Transport Properties near the Insulator–Metal Transition in Colossal Magnetoresistive Manganites

It is argued that a combination of the Griffiths theory and the double-exchange mechanism predicts the random distribution of ferromagnetic metallic clusters above the Curie temperature TC in colossal magnetoresistive manganites. Along this line, we propose a model for the temperature dependence of resistivity that yields a quantitative agreement with experimental data obtained in a typical manganite La2/3Ca1/3MnO3. Our result indicates that the formation and growth of ferromagnetic metallic clusters are responsible for the observation of unusual transport properties near the insulator–metal transition.

Substantial enhancements of low-field magnetoresistance at temperatures near 77 K with doping 4% Cu at Mn sites of La2/3Ca1/3MnO3 synthesized at 1000°C

We report on investigations of magnetoresistance (MR) in polycrystalline La2/3Ca1/3Mn1-xCuxO3 (x = 0 and 4%) samples prepared by the sol-gel method followed a sintering treatment at 1000°C. For the x = 4% sample, the most remarkable MR effect appears at temperatures near 77 K and for magnetic fields lower than 1 T. For fields ~0.3 T and at temperatures ~77 K, the x = 4% sample exhibits a giant MR effect with MR0 (≡Δρ/ρ (H = 0)) between ~50 and ~70%, comparable to the so-called colossal MR response observed in the x = 0 sample for the 3 T field.

Effects of R-site compositions on the meta-magnetic behavior of Tb1 – xPrx(Fe0.4Co0.6)1.88C0.05 (x = 0, 0.8, and 1)*

We investigate the low-temperature magnetic properties of intermetallic compounds Tb1 − xPrx(Fe0.4Co0.6)1.88C0.05 (x = 0, 0.8, and 1) by detailed magnetization measurements. Obvious temperature- and field-induced irreversibilities suggest the coexistence of multiple magnetic phases. Sharp magnetization jumps across the antiferromagnetic to ferromagnetic transition are observed only in the Pr-containing samples, indicating that the behavior of the avalanche-like growth of ferromagnetic clusters is mainly related to the light lanthanide Pr ions. In addition, the time relaxation, field sweep rate, and cooling field dependence of magnetization jumps in the sample with x = 1 are consistent with those in the martensitic scenario.