Probing the Magnetic Cations in Multiferroic Magnetoelectric Thin Films

Magnetoelectric multiferroic materials show considerable potential for use in low-power computer memory and storage devices due to their coupled ferromagnetism and ferroelectricity, meaning that one could use an electric field to induce ferromagnetic domain switching. The opposite is also true, whereby a magnetic field can also cause ferroelectric domain switching. One such multiferroic material system is Aurivillius-phase Bi₆Ti_xFe_yMn_zO₁₈ (B6TFMO) thin films, where this coupling has been shown to exist in a single phase at room temperature. A large contribution to the room-temperature ferromagnetic ordering is the preferential partitioning of the magnetic cations (Mn and Fe) towards the center of the perovskite layers. Furthermore, even more complex polar topologies such as vortices around antiphase boundaries have been observed using atomic-scale scanning transmission electron microscopy (STEM) and polarization mapping. Understanding the mechanisms behind the cation segregation in the crystal structure and around crystallographic defects is of great importance if one aims to exploit the magnetoelectric ordering in these thin films for the manufacture of low-power, multi-state memory devices.<br/>While atomic-scale STEM alongside techniques like Electron Energy Loss Spectroscopy (EELS) and Energy Dispersive X-Ray (EDX) spectroscopy have been invaluable characterization tools in analysing this material system, they only provide a 2D-projected view for what really are 3D features, especially in the case of complex topologies such as polar vortices. Hence, Atom Probe Tomography (APT) will be used due to its unique 3D chemical and structural analysis capabilities and a correlated workflow will be presented. For the same sample, structural information and specimen shape measurements from STEM will be correlated with APT analysis, as well as aid in the correct choice of atom probe reconstruction parameters. This combined dataset will provide invaluable insight about the mechanisms behind the fascinating room-temperature multiferroicity in these thin film material systems.<br/>With the aim of better understanding the cation segregation mechanisms, we will show a controlled experimental method whereby magnetic Mn cations are implanted on Bi₆Ti₃Fe₂O₁₈ and their segregation will be observed before and after annealing, and compared to B6TFMO in which Mn was incorporated during growth from chemical solution deposition.