P.T. Phong

Perovskite nanoparticles synthesised by reactive milling combined with thermal processing: preparation, morphology and structure characterisation

Reactive milling and thermal processing methods were used to prepare a large variety of perovskites like ABO3 and La2NiO4. The single-phase nanoparticle of ABO3 was synthesised at different milling times in dependence on the starting oxides. The grain size of ABO3 nanoparticles in the range of 10-30 nm was conditional upon milling time. Single-phase nanoparticles of La2NiO4 were obtained by using 6 h milling combined with annealing at 750°C. A structure conversion from a layer into a single perovskite of La2NiO4 was observed as milling time increased, whereas the crystalline structure of ABO3 remained almost unchanged. Further analysis of HRTEM images confirms a core-shell structure of nanoparticles.

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

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.

Magnetic Properties of Annealed Powders Prepared By Mechanical Alloying

Fe-Co alloy powders were prepared by mechanical alloying of the elemental Fe and Co powders in air and subsequently annealed at various temperatures. Structural and magnetic characteristics of the annealed powders were studied in detail as a function of annealing temperatures by using an X-ray diffractometer (XRD), a field emission scanning electron microscope, a vibrating sample magnetometer, and a physical property measurement system. The XRD results showed an existence of the nanocrystals with sizes of 15-50 nm. The magnetic studies indicated a strong increase of magnetization and a sharp decrease of coercivity as annealing temperature increased. Both the effect of the oxidation on the magnetic properties as well as magnetization stability of the annealed samples will be discussed.

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

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.

The Role of Spin-Polarized Tunneling on Transport Properties of (1–x)La0.7Ca0.3MnO3 + xAl2O3 Nanocomposites (x = 0 ÷ 5wt%)

We have investigated the effect of artificial grain boundaries on the electromagnetic properties of (1–x)La0.7Ca0.3MnO3 + xAl2O3 nanocomposites (x x= 0 x÷ 5wt%). Based upon a spin-polarized tunneling mechanism we have proposed a phenomenological model to explain the observed electrical transport behavior over the whole temperature range (70 ÷ 300K), especially the gradual drop of metal-insulator transition temperature (Tp) as a function of increasing Al2O3 content, while the feromagnetic-paramagnetic transition temperature (TC) remains almost constant (TC x= 250K). A large low-field magneto-resistance was observed for all the composite samples and the largest magnetoresistance ratio was recorded for a composition with x x=0.01.