Novel Aurivillius Bi4Ti3−2xNbxFexO12phases with increasing magnetic-cation fraction until percolation: a novel approach for room temperature multiferroism

Abstract

Aurivillius oxides with general formula (Bi2O2)(Am−1BmO3m+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 BiMO3units (M: Fe3+, Mn3+, Co3+…) to the pseudoperovskite layer of known ferroelectrics like Bi4Ti3O12, increasingm. The percolation of magnetic cations at the B-site sublattice is required for magnetic ordering and thus, phases withm≥ 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 Bi4Ti3−2xNbxFexO12phases with increasingxvalues 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 betweenx= 0.25 and 0.5 were found. Ceramic processing was accomplished by spark plasma sintering of the mechanosynthesized phases, including those with high-xones with reduced thermal stability. This has enabled us to carry out full electrical characterization and to demonstrate ferroelectricity for all phases up tox= 1. Magnetic measurements were also carried out, and weak ferromagnetism was found forx= 1. Therefore, Bi4TiNbFeO12is proposed to be a novel room-temperature multiferroic.