Impact of Magnetic Ion Substitution on the Crystal Structure of Multiferroic Aurivillius Phases

The five-layered (m = 5) Bi6Ti2.99Fe1.46Mn0.55O18 Aurivillius material is a rare example of a single-phase room temperature ferroelectric-ferrimagnetic multiferroic that shows promise for energy-efficient memory devices. Its ferrimagnetism is thought to derive from the natural partitioning of magnetic ions to the central perovskite layer, engendered by chemically-driven lattice strains, together with ferromagnetic coupling via superexchange mechanisms. Motivated by the expectation of an enhancement in magnetization with increased magnetic ion content, this study examines systematic B-site substitutions with the aim of increasing (from the current level of 40%) the proportion of magnetic ions within the structure. The solubility limits of magnetic cations in this structure and their influence on the superlattice layering are investigated. Studies of Aurivillius phase films on c-sapphire with composition Bi6TixFeyMnzO18 (B6TFMO; x = 2.3 to 3.2, y = 1.2 to 2.0, z = 0.3 to 0.9) demonstrated that above ca. 46% of B-site magnetic cations, the m = 5 structure first rearranges into a mixed-phase material based on m = 5 and six-layered (m = 6) structures and eventually evolves into an m = 6 phase with 54% magnetic cations at the B-site. It is postulated that increasing the number of perovskite layers by forming the m = 6 structure facilitates the accommodation of additional magnetic cations at a lower average manganese oxidation state (+3.3) compared with an equivalent m = 5 stoichiometry (+4.0). While the minor out-of-plane ferroelectric response decreases as expected with increasing structural reorganization towards the m = 6 phase, the predominant in-plane piezoresponse remains unaffected by magnetic cation substitution. This work shows that higher-layered Aurivillius homologues can be synthesized using aliovalent substitution, without requiring epitaxial growth or kinetically constrained methods.