Showing papers by "Pham Thanh Phong published in 2012"

Structural, magnetic and magnetotransport behavior of La0.7SrxCa0.3−xMnO3 compounds

Abstract: A systematic investigation of the structural, magnetic and electrical properties of a series of nanocrystalline La 0.7 Sr x Ca 0.3− x MnO 3 materials, prepared by high energy ball milling method and then annealed at 900 °C has been undertaken. The analysis of the XRD data using the Win-metric software shows an increase in the unit cell volume with increasing Sr ion concentration. The La 0.7 Sr x Ca 0.3− x MnO 3 compounds undergo a structural orthorhombic-to-monoclinic transition at x =0.15. Electric and magnetic measurements show that both the Curie temperature and the insulator-to-metal transition temperature increase from 259 K and 253 K correspondingly for La 0.7 Ca 0.3 MnO 3 ( x =0) to 353 K and 282 K, respectively, for La 0.7 Sr 0.3 MnO 3 ( x =0.3). It is argued that the larger radius of Sr 2+ ion than that of Ca 2+ is the reason to strengthen the double-exchange interaction and to give rise to the observed increase of transition temperatures. Using the phenomenological equation for conductivity under a percolation approach, which depends on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the resistivity versus temperature data measured in the range of 50–320 K and found that the activation barrier decreased with the raising Sr 2+ ion concentration.

Enhanced low-field-magnetoresistance and electro-magnetic behavior of La0.7Sr0.3MnO3/BaTiO3 composites

Abstract: We report the structural, magentoresistance and electro-magnetic properties of ferromagnet–ferroelectric–type (1−x)La0.7Sr0.3MnO3/xBaTiO3 (with x=0.0%, 3.0%, 6.0%, 12%, 15.0% and 18.0%, in wt%) composites fabricated through a solid-state reaction method combined with a high energy milling method. The insulator–metal transition temperature shifts to a lower temperature and resistivity increases while the feromagnetic–paramagnetic transition temperature remains almost unchanged with the increase of BaTiO3 content. Magnetoresistance of the composites at an applied magnetic field H=3 kOe is enhanced in the wide temperature ranges with the introduction of BaTiO3, which could be explained by the enhanced spin polarized tunneling effect induced by the introduction of BaTiO3. The low-field magnetoresistance of the composite is analyzed in the light of a phenomenological model based on the spin polarized tunneling at the grain boundaries. Furthermore, the temperature dependence of resistivity for this series has been best-fitted by using the adiabatic small polaron and variable range hopping models. These models may be used to explain effect of BTO on the electronic transport properties on high temperature paramagnetic insulating region.