Aurivillius structured Bi6Ti3Fe1.5Mn0.5O18 (B6TFMO) has emerged as a rare room temperature multiferroic, exhibiting reversible magnetoelectric switching of ferroelectric domains under cycled magnetic fields. This layered oxide presents exceptional avenues for advancing data storage technologies owing to its distinctive ferroelectric and ferrimagnetic characteristics. Despite its immense potential, a comprehensive understanding of the underlying mechanisms driving multiferroic behavior remains elusive. Herein, we employ atomic resolution electron microscopy to elucidate the interplay of octahedral tilting and atomic-level structural distortions within B6TFMO, associating these phenomena with functional properties. Fundamental electronic features at varying bonding environments within this complex system are scrutinised using electron energy loss spectroscopy (EELS), revealing that the electronic nature of the Ti4+ cations within perovskite BO6 octahedra is influenced by position within the Aurivillius structure. Layer-by-layer EELS analysis shows an ascending crystal field splitting (Δ) trend from outer to central perovskite layers, with an average increase in Δ of 0.13 ± 0.06 eV. Density functional theory calculations, supported by atomic resolution polarization vector mapping of B-site cations, underscore the correlation between the evolving nature of Ti4+ cations, the extent of tetragonal distortion and ferroelectric behavior. Integrated differential phase contrast imaging unveils the position of light oxygen atoms in B6TFMO, exposing an escalating degree of octahedral tilting towards the central layers, which competes with the magnitude of BO6 tetragonal distortion. The observed octahedral tilting, influenced by B-site cation arrangement, is deemed crucial for juxtaposing magnetic cations and establishing long-range ferrimagnetic order in multiferroic B6TFMO.

Magnetoelectric (ME) multiferroic (MF) materials with intrinsic cross-coupling between electrical and magnetic order parameters are promising for next generation memory devices where external electric fields can switch the direction of magnetization leading to enhanced speed and reduced power consumption. Despite extensive research in the past few decades, only a handful of bulk ME-MF materials that allow for reversal of magnetization with an external electric field have been discovered. Furthermore, all the observed bulk systems have below room-temperature (RT) magnetism which hinders their practical applications. Using first-principles calculations guided by group-theoretical analysis, we predict a hitherto unknown polar phase of bulk A4B3O9 layered oxides where applied electric field can switch the magnetization between 180° symmetry equivalent states. Our phonon calculations reveal that the high-symmetry paraelectric phase of these layered oxides is unstable against a zone-centre polar distortion. Furthermore, a weak ferromagnetic (wFM) mode arises spontaneously in the resulting polar phase via a canting of the ground state antiferromagnetic spin ordering of the magnetic B-site cations. The polar mode couples to the wFM mode and the AFM order in an ‘improper’ manner giving rise to a non-linear ME effect where magnetization can be reversed by an electric field via the reversal of the polar mode. Previous experimental studies on these layered oxides demonstrated long-range magnetic ordering of the spins above RT, indicating the possibility of RT electric field switching of magnetization in A4B3O9 layered oxides which is further supported by our calculations of the magnetic exchange interaction parameters.

Ferroelectric thin films hold enormous potential for a wide range of device applications, such as synaptic devices based on ferroelectric tunnel junctions, non-volatile memory, and data storage. While in-plane polarized device applications are a recent development, devices fabricated from vertically polarized thin films have seen over a decade of advancements, notably due to their superior CMOS compatibility. Hence, this work explores ways to augment the vertical polarization of Aurivillius phase ferroelectric thin films. Additionally, it presents ways to boost in-plane polarization, catering to specific application requirements. The synthesis of Aurivillius phase Bi4Ti3O12 thin films was carried out using direct liquid injection chemical vapor deposition, selected for its precise control over film thickness, crucial for scalability and industrial relevance. Our method focuses on altering supersaturation levels during deposition, resulting in two distinct morphologies of phase-pure Bi4Ti3O12. A high supersaturation process, involving simultaneous precursor injection, led to characteristics resembling Frank van der Merwe two-dimensional (2-D) nucleation and growth. In contrast, a low supersaturation technique, with sequential precursor injection, facilitated the formation of growth spirals along the substrate's crystallographic orientation, as observed through X-ray diffraction, transmission electron microscopy, and atomic force microscopy. The study also examined the effects of various growth parameters on spiral growth, such as oxygen partial pressure during nucleation. Films with denser spiral growth showed a 15V reduction in the voltage necessary to vertically switch domains out-of-plane, due to disruptions to inversion symmetry along the c-axis. Conversely, films exhibiting 2D growth characteristics demonstrated a 15V decrease in the voltage required to laterally switch domains in-plane. Further investigations considered the impact of cooling pressure, substrate lattice mismatch, and the integration of magnetic cations such as iron and manganese into the Aurivillius structure, with an emphasis on assessing their effect on resulting multiferroic properties. The findings provide valuable insights into the growth of Aurivillius phase thin films and offer opportunities to tailor the ferroelectric and multiferroic properties for diverse device applications.