Lea Forster


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Forster

First Name

Lea

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0000-0001-7079-6401

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Publications 1 - 4 of 4
  • The roles of rare-earth ions in ABO3 compounds: More than meets the eye
    Item type: Doctoral Thesis
    Forster, Lea (2024)
    Rare-earth ABO3 compounds are known to show a multitude of intriguing phenomena ranging from colossal magnetoresistance [1] over metal-to-insulator transitions [2] to multiferroicity [3]. The rare-earth elements in such compounds can influence the properties of the material in various ways, which is the focus of the investigation in this thesis. Three different influence pathways of the rare-earth ion are discussed here: via the rare-earth ionic radius, by the rare-earth magnetic moment, and by the rare-earth electronic structure. At first, the focus is laid on the tendency of the rare-earth ionic radius to affect the magnetic order in the compound series of hexagonal manganites h-RMnO3 with R = Sc, Y, In, Dy–Lu. In this family of materials, while the literature agrees on the magnetic order of the Mn3+ ions in compounds with smaller ionic radii to be B2 (P6′3c′m), in h-YMnO3, which has a large ionic radius, there is a debate between B1 (P6′3cm′) and B2 ⊕ B1 (P6′3) order [4, 5]. With this work, it is shown by the use of optical second-harmonic generation (SHG) that h-YMnO3 orders into the B1 arrangement. Thereby this work shows the tendency that hexagonal manganites with larger rare-earth ionic radii order into the B1 configuration. In a second project, the effect of the rare-earth magnetic order on the magnetic phase diagram of h-ErMnO3 is investigated, especially with regards to the coupling between the Er3+ and Mn3+ order. This work finds that under increasing magnetic fields, both the Er3+ and Mn3+ magnetic orders undergo a phase transition simultaneously. However, under decreasing fields, the Mn3+ order does not instantaneously follow the changes in the Er3+ order, but it shows a two-step phase transition. With lower temperatures, the full completion of both steps in this phase transition can require magnetic fields in the reverse direction or can no longer be completed. This unusual phase transition is caused by the intricate interplay between the Er3+ and the Mn3+ magnetic moments which effectively delays the phase transition of the Mn3+ order. This underlines the role of the rare-earth magnetic moments on the overall magnetic order in rare-earth ABO3 compounds. The third effect of the rare-earth ion on the properties of ABO3 structures discussed here is the influence of the rare-earth electronic levels on the properties of NdGaO3. For this compound, several anomalies in structural, electronic, and magnetic properties or even a structural phase transition have previously been reported to appear around 150–200 K [6–12]. The results presented in this thesis do not show any indication of a structural phase transition occurring. However, due to temperature-dependent changes in the SHG response, these low-temperature anomalies can be correlated to changes in the population of the rare-earth electronic levels. This emphasizes the importance of the consideration of the rare-earth electronic levels when investigating the properties of rare-earth compounds. Overall, this work emphasizes the importance of the role of rare-earth elements in ABO3 compounds, which can have different pathways to influence the material behavior. Due to their ionic sizes, magnetic and electronic properties, rare-earth elements can enrichen the physics of these materials and lead to new complex phenomena. Therefore, this work can serve as a foundation for further exploration and design of complex rare-earth compounds.
  • Magnetoelectric domain engineering from micrometer to Ångstrøm scales
    Item type: Journal Article
    Giraldo Castaño, Leidy Marcela; Simonov, Arkadiy; Sim, Hasung; et al. (2024)
    Physical Review Research
    The functionality of magnetoelectric multiferroics depends on the formation, size, and coupling of their magnetic and electric domains. Knowing the parameters guiding these criteria is a key effort in the emerging field of magnetoelectric domain engineering. Here we show, using a combination of piezoresponse-force microscopy, nonlinear optics, and x-ray scattering, that the correlation length setting the size of the ferroelectric domains in the multiferroic hexagonal manganites can be engineered from the micron range down to a few unit cells under the substitution of Mn3+ ions with Al3+ ions. The magnetoelectric coupling mechanism between the antiferromagnetic Mn3+ order and the distortive-ferroelectric order remains intact even at substantial replacement of Mn3+ by Al3+. Hence, chemical substitution proves to be an effective tool for domain-size engineering in one of the most studied classes of multiferroics.
  • Strain-Driven Thermal and Optical Instability in Silver/Amorphous-Silicon Hyperbolic Metamaterials
    Item type: Journal Article
    Ocana-Pujol, Jose L.; Forster, Lea; Spolenak, Ralph; et al. (2022)
    Advanced Optical Materials
    Hyperbolic metamaterials show exceptional optical properties, such as near-perfect broadband absorption, due to their geometrically-engineered optical anisotropy. Many of their proposed applications, such as thermophotovoltaics or radiative cooling, require high-temperature stability. In this work, Ag/a-Si multilayers are examined as a model system for the thermal stability of hyperbolic metamaterials. Using a combination of nanotomography, finite element simulations, and optical spectroscopy, the thermal and optical instability of the metamaterials is mapped. Although the thermal instability initiates at 300 degrees C, the hyperbolic dispersion persists up to 500 degrees C. Direct finite element simulations on tomographical data provide a route to decouple and evaluate interfacial and elastic strain energy contributions to the instability. Depending on stacking order the instability's driving force is either dominated by changes in anisotropic elastic strain energy due thermal expansion mismatch or by minimization of interfacial energy. These findings open new avenues to understand multilayer instability and pave the way to design hyperbolic metamaterials able to withstand high temperatures.
  • Writing of strain-controlled multiferroic ribbons into MnWO4
    Item type: Journal Article
    Toyoda, Shingo; Fiebig, Manfred; Forster, Lea; et al. (2021)
    Nature Communications
    Local and low-dimensional structures, such as interfaces, domain walls and structural defects, may exhibit physical properties different from the bulk. Therein, a wide variety of local phases were discovered including conductive interfaces, sheet superconductivity, and magnetoelectric domain walls. The confinement of combined magnetic and electric orders to spatially selected regions may be particularly relevant for future technological applications because it may serve as basis of electrically controllable magnetic memory devices. However, direct observation of magnetoelectric low-dimensional structures cannot readily be done partly because of the lack of experimental techniques locally probing their physical nature. Here, we report an observation of multiferroic ribbon-like domains in a non-multiferroic environment in MnWO4. Using optical second harmonic generation imaging, we reveal that a multiferroic phase is stabilized by locally generated strain while the bulk magnetic structure is non-multiferroic. We further find that the confined multiferroic state retains domains with different directions of electric polarization and we demonstrate deterministic writing of a multiferroic state embedded in a non-multiferroic environment.
Publications 1 - 4 of 4