Multiferroic materials that exhibit simultaneous - and strongly coupled - magnetic and ferroelectric order above room temperature offer exciting potential for room temperature device integration. In particular, magnetoelectric multiferroic films are ideal candidates for applications in next-generation memory devices, which use low-power electric fields to control magnetic order. However, structural defects can drastically change the behavior of multiferroic materials. Crystal imperfections can, for example, induce local polarization reversal, alter domain kinetics or even modify phase transition temperatures. Thus, a better understanding of the structure and properties of structural defects is needed to help drive devices based on multiferroic materials towards their technological application. Therefore, a major challenge is their investigation at the atomic scale. Recent advances in aberration-corrected scanning transmission electron microscopy (STEM), as well as new faster and more sensitive detectors and cameras, have opened up a wide range of new opportunities for atomic resolution studies. In this contribution, I will show recent advances in the field of investigation of ferroic order in thin films by state-of-the-art aberration-corrected STEM. For this purpose, different examples of multiferroic systems will be discussed.

Subtle changes in stoichiometry and crystal symmetry govern fundamental physics of perovskite oxides. Particularly in thin films, the interplay between chemistry and structure can be altered by fine-tuning the conditions for epitaxial growth. A good example is the family of multiferroics AMnO3 (A= Ca, Sr, Ba) perovskite, antiferromagnets in which ferroelectricity driven by the off-centering of the magnetic cation Mn4+ can emerge through selective crystal distortions.

In this talk we will review our research on the family of Sr1-xBaxMnO3 (SBMO) thin films grown on LSAT and STO (001) substrates by pulsed laser deposition (PLD). We have investigated the interplay between the polar states, and strain and stoichiometry (i.e. Ba content and oxygen vacancies). We have observed that epitaxial growth is key to the stabilization of the polar perovskite phase of SBMO. In epitaxially strained thin films, it is possible to tailor the polar atomic displacements as a function of the composition, by tuning key growth conditions. Aberration-corrected scanning transmission electron microscopy (STEM) combined with X-ray diffraction (XRD) has allowed us to determine the local polarization at the nanoscale as a function of the induced crystalline structure and composition. Polarization is intimately linked to the sign and magnitude of epitaxial strain, thickness and oxygen stoichiometry. It can be tuned either in-plane or out-of-plane with respect to the substrate plane by the adequate choice of the substrate-induced strain, Ba doping and the O content ─induced by controlled annealing.

This chemistry-mediated engineering of the polarization orientation of oxide thin films opens new venues for the design of functional multiferroic architectures and the exploration of novel physics and applications of ferroelectric textures with exotic topological properties.

Magnetoelectric multiferroics are the materials that simultaneously show magnetic and electric orders. Interest in them largely originates from the possibility of controlling one order using the stimulus that usually controls the other, offering great potential for development of multifunctional devices. BiFeO₃ is among the most exciting representatives of this family because it displays both orders at room temperature. Moreover, a deterministic reversal of magnetization by an electric field has been experimentally observed in BiFeO₃ thin films. It has been proposed that this magnetoelectric switching occurs as a result of a peculiar polarization switching process that occurs in two steps: 109⁰ out-of-plane polarization rotation is followed by 71⁰ in-plane rotation or vice versa. However, the origin of the two-step polarization switching process is still not understood, which hampers its optimization (faster switching at a smaller coercive field). In this work we combine the phenomenological Landau theory, the density functional theory (DFT) and the Landau-Khalatnikov time-evolution equation (LKE) to elucidate the origin of the two-step polarization switching process in BiFeO₃. First, we introduce the simplest, lowest order Landau-like potential for bulk BiFeO₃ and ensure that this potential accurately reproduces the DFT energies and the distortion amplitudes for a set of structural BiFeO₃ polymorphs. Then, we extend our model by introducing additional constraints which account for the presence of the substrate and multidomain configuration observed experimentally in BiFeO₃ films. We combine this model with the LKE to investigate the role of the introduced constraints on ferroelectric switching in BiFeO₃. We show that our simple model allows us to reproduce two-step polarization switching in multidomain system. Finally, we discuss the most likely cause of this switching process and potential ways to optimize the switching characteristics.