Showing papers by "Pham Thanh Phong published in 2013"

Structural, magnetic, infrared and Raman studies of La 0.8 Sr x Ca 0.2- x MnO 3 (0 ≤ x ≤ 0.2)

Abstract: We report the results of magnetic, X-ray powder diffraction, infrared and temperature dependent Raman spectra of the La0.8Sr x Ca0.2-x MnO3 (0 ≤ x ≤ 0.2) manganite. The structure refinement using the Rietveld method indicates that the partial substitution of strontium for calcium (for x ≥ 0.15) modifies the orthorhombic structure of the CaMnO3 perovskite towards a rhombohedral phase. Magnetic measurement confirms the increase in the Curie temperature from 180 K for La0.8Ca0.2MnO3 to 307 K for La0.8Sr0.2MnO3, respectively. It is argued that the substitution with the larger Sr2+ ion strengthens the double-exchange interaction and gives rise to the observed increase of transition temperatures. All manganites show two IR active vibrational modes around 400 and 600 cm−1. Moreover, when x ≤ 0.1, the absorption band around 400 cm−1 splits into two peaks. In addition, we have analyzed the frequencies and widths of the observed Raman modes as a function of temperature for all samples with various Sr content. The mode splitting is attributed to both magnetic ordering and large orthorhombic distortion in doped rhombohedral manganites.

Electrical transport and temperature coefficient of resistance in polycrystalline La0.7−xAgxCa0.3MnO3 pellets: Analysis in terms of a phase coexistence transport model and phase separation model

Abstract: The temperature dependent resistivity and temperature coefficient of resistance of Ag doped La0.7−xAgxCa0.3MnO3 polycrystalline pellets (x=0, 0.05, 0.10, 0.15, and 0.20) are investigated. Ag substitution enhances the conductivity of this system. The Curie temperature also increases from 260 to 283 K with increasing Ag content. Using phase-coexistence transport model and phase separation model, we calculated the resistivity as a function of temperature and the temperature coefficient of resistivity (TCR) behavior. Comparing the calculated maximum TCR, we found that it is related to activation energy, transition temperature, and disorder in doped manganites. The relationship between the proposed TCR behavior and the transport parameters can suggest conditions improving TCRmax of doped manganites for the use of the bolometric infrared detectors.

Electrical transport study of La0.7Ca0.3MnO3 using phase - coexistence transport model

Abstract: In this study, we report the electric transport of La0.7Ca0.3MnO3 prepared by various methods. Based on the fact that the metallic ferromagnetic and insulating paramagnetic clusters simultaneously exist at transition temperature, we applied phase-coexistence transport model to calculate the temperature dependence of the resistivity. This model can explain quantitatively the electrical conductivity of La0.7Ca0.3MnO3 for the whole temperature range studied.

Low-field magnetoresistance in La0.7Sr0.3MnO3/BaTiO3 composites

Abstract: A manganite composite series of (1 − x)La0.7Sr0.3MnO3/xBaTiO3 (x = 0, 0.06, 0.12, and 0.18) has been fabricated by solid-state reaction combined with a high-energy mechanical milling method. Experimental results revealed that the insulator–metal transition temperature was shifted towards lower temperatures, and resistivity increases with increasing BaTiO3 content in (1 − x)La0.7Sr0.3MnO3/xBaTiO3. Meanwhile, the ferromagnetic–paramagnetic transition temperature was almost unchanged. The increase in magnetoresistance was observed in the all composites at whole measurement temperatures under an applied magnetic field of 3 kOe. Here, temperature dependences of magnetoresistance display a Curie–Weiss law-like behavior. The nature of this phenomenon is explained in detail.

Ferromagnetic insulating and spin glass properties of La0.7Sr0.3MnO3–TiO2 composites

Abstract: In this study, the effect of TiO2 doping on the electro-magnetic properties of (1−x)La0.7Sr0.3MnO3+xTiO2 (with 0%≤x≤6%, in wt%) composites has been investigated. X-ray diffraction observations show the evidence of reaction between the La0.7Sr0.3MnO3 and TiO2 grains. Also the results show that by increasing TiO2 doping levels, the metal insulator transition temperatures decrease and the system becomes an insulator. Furthermore, the paramagnetic–ferromagnetic transition temperature also decreases as TiO2 content increases. The spin glass state exists in the composites with x=5% and 6%. The dynamic properties of the magnetic properties are investigated by temperature dependence of dc magnetization and frequency dependence of ac susceptibilities. A dynamic scaling analysis of ac susceptibility data using conventional critical slowing down indicates that a finite spin-glass phase-transition temperature (Tg) and a dynamic exponent (zv) change from 103 K and 9.30 correspondingly for 0.95La0.7Sr0.3MnO3/0.05TiO2 to 133 K and 9.10, for 0.94La0.7Sr0.3MnO3/0.06TiO2. In addition, MR in low temperature of these samples decline simultaneously with occur spin-glass behavior. These results are interpreted as the dilution of Mn3+/Mn4+ interactions by the random Ti substitution on the Mn site considerably reduces the ferromagnetic double exchange interaction within the manganese lattice.

Superparamagnetic behaviour and deviation from Bloch T3/2 law of La0.7Ca0.3MnO3 nanoparticles

Abstract: In this study, we investigated superparamagnetism behaviour and saturation magnetisation of single–phase, nanocrystalline, granular La0.7Ca0.3MnO3 (LCMO) samples synthesised by reactive milling method. The superparamagnetic behaviour of as–milled samples with various milling times was studied in the temperature range from 200 K to 300 K. The saturation magnetisation of the samples increased from 14.7 to 36.8 emu/g with higher values corresponding to shorter milling time. The observed un–overlapping of the scaled M(Hext, T)/MS versus Hext/T plots and the Curie–Weiss like behaviour of the dc–susceptibility at high temperatures provide evidence that the nanoparticles are interacting superparamagnetic ensembles. The magnetisation curves of interacting nanoparticles were well described by the argument–corrected Langevin function. The saturation magnetisation versus temperature showed a Te dependence with e deviated from 3/2 (value for the Bloch law), which increased with decreasing of the milling time.