Effects of Interfaces on the Structure and Novel Physical Properties in Epitaxial Multiferroic BiFeO₃ Ultrathin Films

Repeatable and deterministic all electrical switching in a mixed phase artificial multiferroic

We demonstrate a repeatable all-electric magnetic switching behaviour in a PMN-PT/FeRh thin film artificial multiferroic. The magnitude of the effect is significantly smaller than expected from conventional thermomagnetic switching of FeRh thin films and we explore properties of the PMN-PT/FeRh system in order to understand the origin of this reduction. The data demonstrate the importance of the crystallographic phase of PMN-PT and show how a phase transition at ~ 100 °C modifies the magneto-electric coupling. We demonstrate a large strain remanence effect in the PMN-PT substrate, which limits the magnetoelectric coupling on successive cycling of the applied electric field.

Surface Modification and Enhancement of Ferromagnetism in BiFeO3 Nanofilms Deposited on HOPG

BiFeO3 (BFO) films on highly oriented pyrolytic graphite (HOPG) substrate were obtained by the atomic layer deposition (ALD) method. The oxidation of HOPG leads to the formation of bubble regions creating defective regions with active centers. Chemisorption occurs at these active sites in ALD. Additionally, carbon interacts with ozone and releases carbon oxides (CO, CO2). Further annealing during the in situ XPS process up to a temperature of 923 K showed a redox reaction and the formation of oxygen vacancies (Vo) in the BFO crystal lattice. Bubble delamination creates flakes of BiFeO3-x/rGO heterostructures. Magnetic measurements (M-H) showed ferromagnetism (FM) at room temperature Ms ~ 120 emu/cm3. The contribution to magnetization is influenced by the factor of charge redistribution on Vo causing the distortion of the lattice as well as by the superstructure formed at the boundary of two phases, which causes strong hybridization due to the superexchange interaction of the BFO film with the FM sublattice of the interface region. The development of a method for obtaining multiferroic structures with high FM values (at room temperature) is promising for magnetically controlled applications.

Precession electron diffraction tomography on twinned crystals: application to CaTiO3 thin films

Strain engineering via epitaxial thin-film synthesis is an efficient way to modify the crystal structure of a material in order to induce new features or improve existing properties. One of the challenges in this approach is to quantify structural changes occurring in these films. While X-ray diffraction is the most widely used technique for obtaining accurate structural information from bulk materials, severe limitations appear in the case of epitaxial thin films. This past decade, precession electron diffraction tomography has emerged as a relevant technique for the structural characterization of nano-sized materials. While its usefulness has already been demonstrated for solving the unknown structure of materials deposited in the form of thin films, the frequent existence of orientation variants within the film introduces a severe bias in the structure refinement, even when using the dynamical diffraction theory to calculate diffracted intensities. This is illustrated here using CaTiO3 films deposited on SrTiO3 substrates as a case study. By taking into account twinning in the structural analysis, it is shown that the structure of the CaTiO3 films can be refined with an accuracy comparable to that obtained by dynamical refinement from non-twinned data. The introduction of the possibility to handle twin data sets is undoubtedly a valuable add-on and, notably, paves the way for a successful use of precession electron diffraction tomography for accurate structural analyses of thin films.

Thickness-Dependent Evolution of Piezoresponses and Stripe 90° Domains in (101)-Oriented Ferroelectric PbTiO3 Thin Films

High-index ferroelectric films as (101)-orientated ones exhibit enhanced dielectric responses, piezoelectric responses, and exotic ferroelectric switching behaviors, which are potential candidates for applications in memories and capacitors. However, possible domain patterns and domain wall structures in (101)-oriented ferroelectric thin films are still elusive, which results in difficulties in understanding the origin and further modulating their special properties. In this work, a series of PbTiO₃ (PTO) thin films with 35, 50, 60, and 70 nm in thickness were grown on (101)-oriented (LaAlO₃)_0.29(SrTa_1/2Al_1/2O₃)_0.71 (LSAT(101)) substrates by pulsed laser deposition and investigated by both piezoresponse force microscopy (PFM) and (scanning) transmission electron microscopy ((S)TEM). PFM measurements reveal that periodic stripe domains are dominant in 50 nm thick PTO films. Besides stripe domains, a/ c domains appear in films with thickness more than 60 nm. A thickness-dependent evolution of piezoresponse amplitude indicates that the 50 nm thick PTO films demonstrate a superior piezoresponse. Electron diffraction and contrast analysis clarify that all these (101)-oriented PTO films contain periodic stripe ferroelectric 90° domains. The domain periods increase with the film thickness following Kittel's law. Aberration-corrected STEM imaging reveals that the stripe ferroelectric 90° domains have an alternate arrangement of wide and narrow c domains with polarization directions along [100] for c₁ domains and [001̅] for c₂ domains, forming a "head-to-tail" polarization configuration. Further strain analysis reveals that stripe domains have uniform strain distributions and distinct lattice rotations around domain walls. It is proposed that the periodic arrangement of high-density stripe 90° domains in 50 nm thick PTO films is the main contributor to the superior piezoresponse behavior. These results are expected to provide useful information to understand the domain structures in (101)-oriented PTO thin films and thus facilitate further modulation of the properties for potential applications.

BiFeO3 nanorings synthesized via AAO template-assisted pulsed laser deposition and ion beam etching

Well-ordered BiFeO₃ nanorings with epitaxial structure, strong ferroelectricity and polarization reversal have been fabricated using this novel and facile method.