Charge compensation and structural adaptation to accommodate increased magnetic cation content in 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-ferromagnetic multiferroic that could ideally be suited to future energy-efficient memory devices. This study examines the effect of B-site substitution with the aim of increasing the proportion of magnetic ions within the structure and consequently increasing the saturation magnetisation. Four series of Aurivillius phase films with a target composition of Bi6TixFeyMnzO18 (B6TFMO; x = 2.3 to 3.2, y = 1.2 to 2.0, z = 0.3 to 0.9) were fabricated by chemical solution deposition. Substitution of Ti4+ by Fe3+ and Mn3+ necessitates charge compensation mechanisms and requires accommodation of differing ionic radii. While valence changes of Mn3+ to Mn4+ can act to compensate charge, XRD and TEM analysis is used here to demonstrate that above a threshold of 8 % nominal Mn4+, the m = 5 structure can no longer accommodate the smaller Mn4+ ion and it rearranges into a mixed-phase material based on m = 5 and six-layered (m = 6) inter-growths. 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). This work provides valuable insight into the design and development of versatile multiferroic phases by describing how the B-site magnetic cation content can be increased to 54 % in m = 6 structures, compared to a solubility limit of 46 % in m = 5 structures.