Anisotropic electrical and magnetic properties in grain-oriented Bi₄Ti₃O₁₂-La_0.5Sr_0.5MnO₃

Aurivillius compounds have many fascinating properties such as ferroelectricity, magnetism, dielectricity, and piezoelectricity. Their structures and properties can be tuned flexibly, thus they have broad applications in FeRAM, spintronics, photocatalysts, capacitors, etc. We synthesized layer-inserted Aurivillius phase semiconducting Bi₄Ti₃O₁₂-La_0.5Sr_0.5MnO₃ (BIT-LSMO) nanoparticles by the hydrothermal method and subsequently obtained highly grain-oriented ceramics (Lotgering factor LF = 98.69% for muffle calcined samples and 99.87% for hot-pressed samples) by calcination. Significant electrical anisotropy at room temperature (the resistivity magnitude of the out-of-plane direction is about an order larger than that of the in-plane direction) and magnetic anisotropy at low temperature in the oriented ceramics were observed. Through the grain boundary conductivity estimation with the "brick layer" mode, we concluded that the anisotropy originates from the anisotropy within grain interiors for the Aurivillius layered structure rather than the contribution of grain boundary density difference. The transport path is "blocked" to some extent along the c-direction since hole hopping through the Mn⁴⁺-O^2--Mn³⁺ double exchange effect is the main conducting mechanism in our samples. The oxygen vacancies and element valence states of samples using different synthesis processes were investigated. The oxygen vacancies increase and the lattice shrinks during the sintering process due to the volatilization of bismuth element. The valence state of Ti is less than but near to +4, and the valence state of Mn is about +3.4. The electrical and magnetic anisotropies in BIT-LSMO provide an additional freedom in functional applications for layered complex oxides.