Magneto-transport properties of manganite/secondary phase inhomogeneous systems

Resumo

We have studied the artificial granularity effect on related properties in manganites, by introducing secondary phases into a ferromagnetic manganite forming an inhomogeneous manganite/secondary phase system. TiO2 insulator was introduced as a secondary phase into the ferromagnetic La0.7Ba0.3MnO3 (LBMO) manganite with ratios (x=0.02, x=0.04, x=0.06 and x=0.08) forming the LBMO/xTiO2 system. TiO2 precipitates at the grain boundaries of the LBMO increasing their resistance (R), and decreasing the conduction leading to the spin tunneling effect and the low field magnetoresistance (MR) effect, whose peak value for LBMO is enhanced from 3% to -3.3 %, -3.5 % and -3.7% with TiO2 addition for x= 0.02, 0.04 and 0.06 ratio. An improvement in magnetocaloric (MC) properties in comparison with LBMO at the same working temperature is observed. Its relative cooling power is promoted from 35 J/kg to 51 J/kg, 47 J/kg, 50 J/kg and 49 J/kg with the introduced TiO2 for x=0.02, 0.04, 0.06 and 0.08, at 1.5T applied field. Annealing is found to modify the TiO2 distribution at the grain boundaries of the LBMO leading to a change in magnetic and MC properties. LBMO/xTiO2 system was annealed at 600 °C and 800°C. Annealing at 800 °C promotes the best magnetic and MC properties. Secondary phases including Ni powder, Ni nanowires, Ag nanoparticles and Ag oxide were introduced into LBMO. The change in grain boundaries R results a secondary phase size-dependent effect, which makes related properties also size-dependent ones. An improvement in the MC properties is observed. The refrigerant capacity power value of LBMO is improved from 44 J/kg to 107 J/kg for Ni powder and to 167 J/kg for Ni nanowires introduction. MC and transport properties correlation was found. The spin order/disorder determines MC effect, MR and R at Curie temperature. The MC effect can be well scaled by R at Tc, and by MR through a K factor.