Johanna Nordlander

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Nordlander

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Johanna

ORCID

0000-0002-9899-6795

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Publications 1 - 10 of 13

Epitaxial integration of improper ferroelectric hexagonal YMnO3 thin ﬁlms in heterostructures
Nordlander, Johanna; Rossell, Marta D.; Campanini, Marco; et al. (2020)
Physical Review Materials
We report on multiple fundamental qualitative improvements in the growth of improper ferroelectric hexagonal YMnO3 (YMO) thin films and heterostructures by pulsed laser deposition (PLD). By a combination of pre-growth substrate annealing and low-energy-fluence PLD, we obtain a two-dimensional growth mode of the YMO films on yttria-stabilized zirconia (YSZ) with ultralow roughness and devoid of misoriented nanocrystalline inclusions. By inserting a hexagonal manganite buffer layer capped with conducting indium-tin oxide between the substrate and the final film, the latter is grown in a fully lattice-relaxed mode and, thus, without any misfit dislocations while maintaining the extraordinary flatness of the films grown directly on pre-annealed YSZ. This provides a template for the fabrication of heterostructures based on hexagonal manganites as a promising class of multiferroics with improper room-temperature ferroelectricity and the implementation of these into technologically relevant epitaxial metal ferroelectric-type multilayers.
In situ investigation of the ferroelectric phase transition in improper ferroelectric YMnO3 thin films by electron energy loss spectroscopy
Vogel, Alexander; Ruiz Caridad, Alicia; Nordlander, Johanna; et al. (2023)
Physical Review B
Origin of the Critical Thickness in Improper Ferroelectric Thin Films
Vogel, Alexander; Ruiz Caridad, Alicia; Nordlander, Johanna; et al. (2023)
ACS Applied Materials & Interfaces
Improper ferroelectrics are expected to be more robust than conventional ferroelectrics against depolarizing field effects and to exhibit a much-desired absence of critical thickness. Recent studies, however, revealed the loss of ferroelectric response in epitaxial improper ferroelectric thin films. Here, we investigate improper ferroelectric hexagonal YMnO3 thin films and find that the polarization suppression, and hence functionality, in the thinner films is due to oxygen off-stoichiometry. We demonstrate that oxygen vacancies form on the film surfaces to provide the necessary charge to screen the large internal electric field resulting from the positively charged YMnO3 surface layers. Additionally, we show that by modifying the oxygen concentration of the films, the phase transition temperatures can be substantially tuned. We anticipate that our findings are also valid for other ferroelectric oxide films and emphasize the importance of controlling the oxygen content and cation oxidation states in ferroelectrics for their successful integration in nanoscale applications.
Design and Manipulation of Ferroic Domains in Complex Oxide Heterostructures
Strkalj, Nives; Gradauskaite, Elzbieta; Nordlander, Johanna; et al. (2019)
Materials
The current burst of device concepts based on nanoscale domain-control in magnetically and electrically ordered systems motivates us to review the recent development in the design of domain engineered oxide heterostructures. The improved ability to design and control advanced ferroic domain architectures came hand in hand with major advances in investigation capacity of nanoscale ferroic states. The new avenues offered by prototypical multiferroic materials, in which electric and magnetic orders coexist, are expanding beyond the canonical low-energy-consuming electrical control of a net magnetization. Domain pattern inversion, for instance, holds promises of increased functionalities. In this review, we first describe the recent development in the creation of controlled ferroelectric and multiferroic domain architectures in thin films and multilayers. We then present techniques for probing the domain state with a particular focus on non-invasive tools allowing the determination of buried ferroic states. Finally, we discuss the switching events and their domain analysis, providing critical insight into the evolution of device concepts involving multiferroic thin films and heterostructures.
The ultrathin limit of improper ferroelectricity
Nordlander, Johanna; Campanini, Marco; Rossell, Marta Dacil; et al. (2019)
Nature Communications
The secondary nature of polarization in improper ferroelectrics promotes functional properties beyond those of conventional ferroelectrics. In technologically relevant ultrathin films, however, the improper ferroelectric behavior remains largely unexplored. Here, we probe the emergence of the coupled improper polarization and primary distortive order parameter in thin films of hexagonal YMnO3. Combining state-of-the-art in situ characterization techniques separately addressing the improper ferroelectric state and its distortive driving force, we reveal a pronounced thickness dependence of the improper polarization, which we show to originate from the strong modification of the primary order at epitaxial interfaces. Nanoscale confinement effects on the primary order parameter reduce the temperature of the phase transition, which we exploit to visualize its order-disorder character with atomic resolution. Our results advance the understanding of the evolution of improper ferroelectricity within the confinement of ultrathin films, which is essential for their successful implementation in nanoscale applications.
Ultrathin regime growth of atomically flat multiferroic gallium ferrite films with perpendicular magnetic anisotropy
Homkar, Suvidyakumar; Preziosi, Daniele; Devaux, Xavier; et al. (2019)
Physical Review Materials
Complex polar states in ultrathin epitaxial oxides
Nordlander, Johanna (2020)
The diverse functional properties of transition-metal oxides have given rise to entire fields of research spanning fundamental aspects such as quantum phenomena to the highly applied field of oxide electronics. In the latter, electrically ordered oxides, such as ferroelectrics, stand out as prominent materials for implementation in low-energy-consuming oxide applications due to their characteristic response under the application of mechanical stress, electric field or optical irradiation. The significant progress in oxide thin-film engineering over the last 20 years has enabled studies of such oxide functionality to go beyond bulk crystals and include characterization under confinement of the materials to the nanoscale. However, in epitaxially grown ultrathin films, the manifestation of electric polarization specifically and functionality in general can greatly differ from the behavior of the corresponding bulk crystals. In order to make use of the exotic functionality of ultrathin oxide films, it is therefore essential to understand when and how the polar states are set with respect to the thin-film synthesis and, upon implementation in electronic devices, with respect to device operation. This is however a challenging task. So far, detailed understanding of the ultrathin limit of polarity in oxide thin films remains restricted to a few model systems. In this thesis, we present an approach to probe such polar states in ultrathin oxide layers with high sensitivity and in a nondestructive manner by nonlinear optics. We demonstrate the of use optical second harmonic generation both in situ, during thin-film synthesis, and operando, during device operation, to provide unique insight into the evolution of polar states in oxide films in these highly dynamic environments. Here, we focus on a set of oxide materials whose polar architecture becomes particularly involved in the nanometer limit. We first establish the sensitivity of our optical probe to distinguish between differently oriented polar states with a complex nanoscale microstructure in a single-phase material, and its evolution under electric-field application. We next exploit this sensitivity to follow the phase coexistence of emergent polar phases in an epitaxially strained system during the thin-film synthesis process. We thus obtain novel insight into an unusually robust, yet metastable high-temperature polar phase. We further investigate the peculiar coupling between polarization and structural order in a so-called improper ferroelectric in the ultrathin regime. Here, a combination of nonlinear optics and electron microscopy could reveal the crucial impact of epitaxial interfaces on the evolution of polarization. Finally, we reveal emergent symmetry-breaking in layered oxides during epitaxial growth, allowing the nanoscale design of symmetry and functionality beyond polar compounds. Common to all of these systems, we find that the mechanical and electrostatic boundary conditions set during the thin-film synthesis itself dictates the emergence of electric polarity in the oxide films, which can even result in the emergence of new material phases that are unique to the thin-film geometry and do not have a bulk counterpart. The results presented in this thesis hence point to the many possibilities of designing ultrathin oxides with unique, yet robust, polar properties of interest for both oxide electronics applications and the emergent field of thin-film quantum materials.
Combined Electrostatic and Strain Engineering of BiFeO3 Thin Films at the Morphotropic Phase Boundary
Nordlander, Johanna; Grosso, Bastien F.; Rossell, Marta D.; et al. (2024)
Advanced Electronic Materials
Multiferroic BiFeO3 exhibits a morphotropic phase boundary at large compressive strain that merges metastable phases of tetragonal (T) and rhombohedral (R) character resulting in giant ferroelectric and electromechanical responses. To utilize this functionality in devices, it is essential to understand the response of these ferroelectric phases to the environment of a nanoscale heterostructure. Here, the emergence of ferroelectricity near the morphotropic phase boundary in BiFeO3 is explored directly during thin-film growth, using optical second harmonic generation. It is found that the epitaxial films form at the growth temperature purely in the T phase with zero critical thickness for the spontaneous polarization. Signatures of monoclinic T-like and R-like phases only appear upon sample cooling. The robustness of a single-domain configuration in the high-temperature T phase is furthermore demonstrated during growth of capacitor-like metal | ferroelectric | metal heterostructures. Here, a reduction in tetragonality, rather than multidomain formation, lowers the electrostatic energy in the few-unit-cell thickness regime. For this lower tetragonality, density-functional calculations and scanning transmission electron microscopy point to the stabilization of a novel metastable monoclinic structure upon cooling toward room temperature. The synergistic combination of strain and electrostatic phase stabilization in BiFeO3 heterostructures hence provides a basis for designing new ferroelectric phases and ultrathin ferroelectric devices.
In-situ monitoring of epitaxial ferroelectric thin-film growth
Sarott, Martin F.; Gradauskaite, Elzbieta; Nordlander, Johanna; et al. (2021)
Journal of Physics: Condensed Matter
In ferroelectric thin films, the polarization state and the domain configuration define the macroscopic ferroelectric properties such as the switching dynamics. Engineering of the ferroelectric domain configuration during synthesis is in permanent evolution and can be achieved by a range of approaches, extending from epitaxial strain tuning over electrostatic environment control to the influence of interface atomic termination. Exotic polar states are now designed in the technologically relevant ultrathin regime. The promise of energy-efficient devices based on ultrathin ferroelectric films depends on the ability to create, probe, and manipulate polar states in ever more complex epitaxial architectures. Because most ferroelectric oxides exhibit ferroelectricity during the epitaxial deposition process, the direct access to the polarization emergence and its evolution during the growth process, beyond the realm of existing structuralin situdiagnostic tools, is becoming of paramount importance. We review the recent progress in the field of monitoring polar states with an emphasis on the non-invasive probes allowing investigations of polarization during the thin film growth of ferroelectric oxides. A particular importance is given to optical second harmonic generationin situ. The ability to determine the net polarization and domain configuration of ultrathin films and multilayers during the growth of multilayers brings new insights towards a better understanding of the physics of ultrathin ferroelectrics and further control of ferroelectric-based heterostructures for devices.
Ferroelectric domain architecture and poling of BaTiO3 on Si
Nordlander, Johanna; Eltes, Felix; Reynaud, M.; et al. (2020)
Physical Review Materials

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Johanna Nordlander

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