Publikation: Emerging phenomena at EuO ultrathin film interfaces in the spotlight of photoelectron spectroscopy
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Oxide interfaces host a rich plethora of novel functionalities and quantum phenomena. In multilayers of ultrathin films, the electronic properties may be determined by the interface. Especially interfaces of the semiconducting model Heisenberg ferromagnet europium monoxide (EuO) with other oxides or metals are of interest because, due to its bulk properties, EuO serves as an ideal template for basic research on tuning, creating or coupling spin-dependent functionalities in a unique manner. However, EuO is only metastable and highly reactive. Therefore, controlling its stoichiometry demands special care. A powerful toolbox for non-destructively investigating the interface phenomena is the variety of X-ray photoelectron spectroscopy (PES) techniques. Taking advantage of the magnetic dichroism, these techniques can even probe the element-specific magnetic order at buried interfaces.
In the work at hand, we apply PES and focus on two aspects of EuO-based thin-film interfaces: First, their synthesis using molecular beam epitaxy (MBE), where in situ lab-XPS is the key tool for probing the interface chemistry. Starting from the analysis of the growth process of EuO on its model substrate yttria-stabilized zirconia (YSZ), where we observed – unexpected – formation of Eu3+ due to oxygen diffusion from the substrate towards the EuO overlayer, we derived a generalized growth model for EuO on oxidic substrates. In addition, we give insights in the synthesis of EuO on LaAlO3 and CaF2. Here, we find, that LaAlO3 does not cause the formation of Eu3+, while on CaF2 EuF2 is formed. Following an analysis of the influence of deposition of various metal overlayers onto EuO on the stoichiometry of EuO – only Pt was found to cause a significant interfacial overoxidation –, we addressed the effect of sample degradation due to aging while being stored under ultra-high vacuum conditions. These results are of significant importance for tailoring novel functionalities at EuO-based interfaces. Second, we study the magnetic proximity effect at 3d ferromagnet (FM)/EuO interfaces by hard X-ray magnetic depth profiling (HAX-MDP), a novel approach taking advantage of the versatile abilities of synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). We discuss the theoretical aspects of angle-dependent measurements of the magnetic circular dichroism (MCD) in photoelectron spectroscopy from 3d core levels and thereby show that the method is applicable to the europium 3d core level. Previous studies showed that due to the proximity effect the magnetic order in EuO thin films persists above its bulk Curie temperature and even up to room temperature. However, the underlying mechanism and the dependence of the effect on the EuO film thickness or the depth inside the EuO film are not yet understood. Using HAX-MDP, we demonstrate that the enhancement of magnetic order is a short-ranged interface effect. Our results moreover suggest that in an initially in-plane magnetized 3d FM/EuO bilayer, both the 3d FM and EuO have a non-zero out-of-plane magnetization component at temperatures above the bulk Curie temperature of EuO. These findings improve the understanding of the magnetic proximity effect and may enable consecutive approaches on taking advantage of the unique properties of EuO in the technologically relevant temperature range. HAX-MDP is shown to be a powerful tool for probing the magnetic order of buried magnetic layers and is expected to be applied to other sample systems in near future.
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ROSENBERGER, Paul, 2024. Emerging phenomena at EuO ultrathin film interfaces in the spotlight of photoelectron spectroscopy [Dissertation]. Konstanz: Universität KonstanzBibTex
@phdthesis{Rosenberger2024Emerg-70594, year={2024}, title={Emerging phenomena at EuO ultrathin film interfaces in the spotlight of photoelectron spectroscopy}, author={Rosenberger, Paul}, address={Konstanz}, school={Universität Konstanz} }
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In multilayers of ultrathin films, the electronic properties may be determined by the interface. Especially interfaces of the semiconducting model Heisenberg ferromagnet europium monoxide (EuO) with other oxides or metals are of interest because, due to its bulk properties, EuO serves as an ideal template for basic research on tuning, creating or coupling spin-dependent functionalities in a unique manner. However, EuO is only metastable and highly reactive. Therefore, controlling its stoichiometry demands special care. A powerful toolbox for non-destructively investigating the interface phenomena is the variety of X-ray photoelectron spectroscopy (PES) techniques. Taking advantage of the magnetic dichroism, these techniques can even probe the element-specific magnetic order at buried interfaces. In the work at hand, we apply PES and focus on two aspects of EuO-based thin-film interfaces: First, their synthesis using molecular beam epitaxy (MBE), where in situ lab-XPS is the key tool for probing the interface chemistry. Starting from the analysis of the growth process of EuO on its model substrate yttria-stabilized zirconia (YSZ), where we observed – unexpected – formation of Eu<sup>3+</sup> due to oxygen diffusion from the substrate towards the EuO overlayer, we derived a generalized growth model for EuO on oxidic substrates. In addition, we give insights in the synthesis of EuO on LaAlO3</sub> and CaF<sub>2</sub>. Here, we find, that LaAlO<sub>3</sub> does not cause the formation of Eu<sup>3+</sup>, while on CaF<sub>2</sub> EuF<sub>2</sub> is formed. Following an analysis of the influence of deposition of various metal overlayers onto EuO on the stoichiometry of EuO – only Pt was found to cause a significant interfacial overoxidation –, we addressed the effect of sample degradation due to aging while being stored under ultra-high vacuum conditions. These results are of significant importance for tailoring novel functionalities at EuO-based interfaces. Second, we study the magnetic proximity effect at 3d ferromagnet (FM)/EuO interfaces by hard X-ray magnetic depth profiling (HAX-MDP), a novel approach taking advantage of the versatile abilities of synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). We discuss the theoretical aspects of angle-dependent measurements of the magnetic circular dichroism (MCD) in photoelectron spectroscopy from 3d core levels and thereby show that the method is applicable to the europium 3d core level. Previous studies showed that due to the proximity effect the magnetic order in EuO thin films persists above its bulk Curie temperature and even up to room temperature. However, the underlying mechanism and the dependence of the effect on the EuO film thickness or the depth inside the EuO film are not yet understood. Using HAX-MDP, we demonstrate that the enhancement of magnetic order is a short-ranged interface effect. Our results moreover suggest that in an initially in-plane magnetized 3d FM/EuO bilayer, both the 3d FM and EuO have a non-zero out-of-plane magnetization component at temperatures above the bulk Curie temperature of EuO. These findings improve the understanding of the magnetic proximity effect and may enable consecutive approaches on taking advantage of the unique properties of EuO in the technologically relevant temperature range. 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