Growth and improved magnetoelectric response of strain modified Aurivillius SrBi4.25La0.75Ti4FeO18 thin films

In this work, we grew 5-layered SrBi4.25La0.75Ti4FeO18 (SBLFT) polycrystalline thin films (80 – 330 nm thick) by the pulsed laser deposition to study their ferroelectric and magnetoelectric response. Structural/microstructural analysis confirms the formation of orthorhombic SBLFT with good crystallinity and randomly oriented Aurivillius phase. Detailed scanning transmission electron microscopy analysis of 120 nm film revealed a predominantly five-layered structure with a coexistence of four layers stacking. Such stacking defects are found to be pertinent to high structural flexibility of Bi-rich Aurivillius phases, alleviated by lattice strain. Raman spectral features at ambient temperatures depict the signature of orthorhombic-tetragonal phase transition. SBLFT films have a strong ferroelectric nature (remanent polarization 2Pr, of 35 μC/cm2) with fatigue endurance up to 1010 cycles and strongly improved, switchable magnetization opposed to its antiferromagnetic bulk counterpart. The scaling behavior of dynamic hysteresis reveals that ferroelectric domain reversal has good stability and low energy consumption. We observed the presence of SBLFT nanoregions (1 – 5 nm), distributed across the film, with Bi, Fe-rich compositions and oxygen vacancies that contribute to the week ferromagnetic behavior mediated by the Dzyaloshinskii-Moriya interactions. Subtle changes in structural strain and lattice distortions of thin films with varied thickness lead to distinct ferroic properties. Stronger ferroelectric polarization of 80 and 120 nm films compared to thicker ones can be due to structural strain and possible rearrangement of BO6 octahedra. Observation of improved magnetoelectric coefficient of 50 mV/cm-Oe for 120 nm film, compared to several Aurivillius oxides, indicates that structural strain modification in SBLFT is beneficial for fatigue-free magnetic field switching of ferroelectric polarization. The structural strain of the unit cell as well as the presence of Bi- and ferromagnetic Fe-rich nanoregions were found to be responsible for the improved multiferroic behavior of SBLFT films.