Novel Aurivillius Bi4Ti3−2xNbxFexO12 phases with increasing magnetic-cation fraction until percolation: a novel approach for room temperature multiferroism

Abstract

Aurivillius oxides with general formula (Bi₂O₂)(A_m−1B_mO_3m+1) are being extensively investigated for room-temperature multiferroism and magnetoelectric coupling. The chemical design strategy behind current investigations is the incorporation of magnetically active BiMO₃ units (M: Fe³⁺, Mn³⁺, Co³⁺…) to the pseudoperovskite layer of known ferroelectrics like Bi₄Ti₃O₁₂, increasing m. The percolation of magnetic cations at the B-site sublattice is required for magnetic ordering and thus, phases with m ≥ 5 are searched. Alternatively, one can try to directly substitute magnetic species for Ti⁴⁺ in the perovskite slab, without introducing additional oxygen octahedra. We report here the mechanosynthesis of Aurivillius Bi₄Ti_3−2xNb_xFe_xO₁₂ phases with increasing x values up to 1. A maximum magnetic fraction of 1/3, surpassing the threshold for percolation, was reached. Preliminary structural analysis indicated a continuous solid solution, though hints of structural changes between x = 0.25 and 0.5 were found. Ceramic processing was accomplished by spark plasma sintering of the mechanosynthesized phases, including those with high-x ones with reduced thermal stability. This has enabled us to carry out full electrical characterization and to demonstrate ferroelectricity for all phases up to x = 1. Magnetic measurements were also carried out, and weak ferromagnetism was found for x = 1. Therefore, Bi₄TiNbFeO₁₂ is proposed to be a novel room-temperature multiferroic.