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6. Conclusion and Outlook In the thesis at hand, we explore ultrathin films of the magnetic oxide europium oxide (EuO) with two complementary goals. First, we investigate fundamental structural and magnetic properties of EuO as a model Heisenberg ferromagnet. We tune the lateral lattice parameter of EuO, which allows us to elucidate the response of the ferromagnetic coupling of the localized 4f magnetism by biaxial strain. Our second goal is to establish ultrathin EuO layers for spin-functional tunnel contacts on silicon. Until today, a challenging bulk and interface chemistry of the highly reactive materials EuO and Si has hampered a seamless integration into silicon heterostructures. Therefore, we develop the integration of ultrathin EuO directly with silicon focusing on the structural and chemical control of the spin-functional EuO/Si interface. In the first part of this thesis, we discuss the synthesis and throughout characterization of high-quality ultrathin EuO films on cubic oxides for our fundamental studies. We established the growth of epitaxial EuO thin films on cubic oxides by Oxide-MBE from bulk-like thicknesses down to one nanometer. By a precise control of synthesis parameters, we achieved a sustained layer-by-layer growth of EuO on conductive YSZ (001). During MBE growth, we verified the single-crystalline quality and epitaxy of the ultrathin films by electron diffraction techniques, and confirmed a bulk-like magnetization down to 2 nm thickness. We determined a nearly perfect stoichiometry of the metastable EuO thin films by HAXPES for various EuO thicknesses down to 1 nm. Thus, we could establish EuO ultrathin films as a magnetic oxide with reference quality. This is an important prerequisite for our further studies on fundamental electronic and magnetic properties. In order to further investigate the nature of the ferromagnetic coupling in EuO, we experimentally realized to apply tensile and compressive biaxial strain in epitaxial EuO/LaAlO 3 (100) and EuO/MgO (001) heterostructures, respectively. We compare their magnetic properties obtained by the SQUID technique – yielding an averaged macroscopic moment – with the local and element-sensitive magnetic circular dichroism (MCD) in photoemission; this technique gives insight into the intra-atomic exchange coupling of EuO. Tensile biaxial strain causes a larger reduction of the core-level MCD asymmetry than of the averaged magnetic moment of the ultrathin strained EuO film as observed by SQUID. Thus, by strain-engineering we could tune the magnetic exchange in ultrathin epitaxial EuO, and quantify this effect by MCD in hard X-ray core-level photoemission, which is significantly different from the averaged macroscopic magnetization. In the second part of this thesis, we investigate heterostructures of ultrathin EuO grown directly on silicon. EuO is the only binary magnetic oxide thermodynamically stable in direct contact with silicon, and it also permits up to 100% spin filter efficiency due to its exchangesplit conduction bands. Thus, we are aiming towards a controlled integration of ultrathin EuO tunnel barriers directly with Si (001) without additional buffer layers. In particular, 123
124 6. Conclusion and Outlook we focus on the characterization and careful optimization of the spin-functional EuO/Si heterointerface. First, we explore how to stabilize metastable EuO on Si using Oxide-MBE. We adapt a common strategy from silicon technology, the surface etching with hydrofluoric acid (HF), in order to remove native SiO 2 and chemically passivate the Si surface (H-Si). On such passivated Si substrates, we synthesized polycrystalline EuO using different growth parameters. By means of HAXPES, we clearly distinguished two EuO valency phases: stoichiometric EuO and over-oxidized Eu 1 O 1+x . For the stoichiometric EuO heterostructures, we confirmed a bulk-like ferromagnetism – this is an important prerequisite for magnetic EuO tunnel contacts. This result paves the way for a following interface study of ultrathin EuO tunnel barriers directly on Si (001). In the next step of our integration of ultrathin EuO directly with silicon, we focus on experimentally realizing a high structural and chemical quality of the EuO/Si heterointerface. Such atomically sharp interfaces of high crystalline quality may lead to coherent tunneling. Thus, we carefully studied how to chemically control the highly reactive EuO/Si heterointerface in order to permit an epitaxial integration of EuO on Si (001). Our thermodynamic analysis of the EuO/Si interface – taking into account the high chemical reactivity of Eu, EuO, and Si during EuO synthesis at elevated temperatures – gives us a guideline of probable reaction paths. In this way, we decided to compare three complementary in situ passivation procedures of the Si (001) surface: (i) hydrogen passivation, (ii) Eu passivation in the monolayer regime, and (iii) SiO x formation in the Ångström regime. We apply these passivations selectively against interfacial silicon oxides and silicide formation – both being major antagonists to efficient spin filter tunneling. In order to quantify the impact of different chemical passivations of the Si (001) surface on the EuO/Si interface properties, we conducted optimization studies comprising the control of surface crystalline structure and chemical interface properties by electron diffraction techniques and HAXPES, respectively. We conclude, that the respective Si surface passivations have minimized silicon oxides to below 1 nm and interfacial silicides to 2 Ångströms, which is clearly in the sub-nanometer regime. In particular, for an optimum passivation using the SiO x method, the ultrathin EuO layer showed a magnetic saturation moment of 5μ B /EuO, comparable to that of bulk EuO. Thus, our work demonstrates how to successfully apply chemical passivations to the Si (001) surface and – at the same time – maintain a mainly heteroepitaxial integration of EuO with the Si (001) wafer. This is an important result for possible spin-dependent coherent tunneling in EuO/Si contacts. Such optimized EuO/Si heterointerfaces may be used as spin-functional EuO tunnel contacts for silicon spintronic devices in the near future.
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Member of the Helmholtz Association
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Forschungszentrum Jülich GmbH Pete
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Zusammenfassung In dieser Doktorarb
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Contents 1. Introduction 1 2. Theor
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List of Figures 2.1. Phase diagram
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List of Figures xi 5.12. Resulting
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1. Introduction Smart and powerful
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3 In the thesis at ha
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2. Theoretical background . . . The
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2.1. The challenge of stabilizing s
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2.2. Electronic structure of EuO 9
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2.3. Magnetic properties of EuO 11
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2.4. Hard X-ray photoemission spect
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2.5. Magnetic circular dichroism in
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3. Experimental details Die Pläne
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3.1. Molecular beam epitaxy for oxi
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3.2. In situ characterization techn
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3.2. In situ characterization techn
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3.3. Experimental developments at t
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3.4. Ex situ characterization techn
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4. Results I: Single-crystalline ep
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59 Table 4.1.: Parameters for stoic
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4.1. Coherent growth: EuO on YSZ (1
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Page 90 and 91: 4.1. Coherent growth: EuO on YSZ (1
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Page 94 and 95: 4.2. Lateral tensile strain: EuO on
Page 96 and 97: 4.3. Lateral compressive strain: Eu
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Page 142 and 143: 125 Outlook and perspectives In the
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Page 146 and 147: A.1. EuO-related phases and cubic o
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Page 150 and 151: Bibliography 133 [13] S. S. P. Park
Page 152 and 153: Bibliography 135 [36] R. E. Dickers
Page 154 and 155: Bibliography 137 [65] R. Sutarto, S
Page 156 and 157: Bibliography 139 [91] P. Braun, M.
Page 158 and 159: Bibliography 141 [116] S. Hüfner.
Page 160 and 161: Bibliography 143 [144] R. Feder and
Page 162 and 163: Bibliography 145 [171] H. Miyazaki,
Page 164 and 165: Bibliography 147 [197] J. Qi, T. Ma
Page 166 and 167: Software and Internet Resources [21
Page 168 and 169: List of own publications 151 [10] R
Page 170 and 171: List of conference contributions 15
Page 172 and 173: Schriften des Forschungszentrums J
Page 175: Schlüsseltechnologien / Key Techno
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