The roles of rare-earth ions in ABO3 compounds: More than meets the eye
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2024
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Doctoral Thesis
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Abstract
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.
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Examiner: Fiebig, Manfred
Examiner : Weber, Mads C.
Examiner: Lottermoser, Thomas
Examiner : Viret, Michel
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ETH Zurich
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Subject
Multiferroics; SHG microscopy; Rare earth elements; Magnetism; Electronic transitions
Organisational unit
03918 - Fiebig, Manfred / Fiebig, Manfred
Notes
Funding
215423 - Magnetoelectric Properties of Multiferroic Domain Walls (SNF)
178825 - Dynamical processes in systems with strong electronic correlations (SNF)
178825 - Dynamical processes in systems with strong electronic correlations (SNF)
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