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4.3. Lateral compressive strain: EuO on MgO (100) 79 Figure 4.20.: MCD-PE of Eu 3d and 4d core-levels for ultrathin EuO under tensile strain on LAO (100). magnetization in the EuO/LAO (100) heterostructure. This may partly be explained by a small applied field during SQUID measurements, while during photoemission for the MCD- PE measurement, the EuO thin film is magnetized only remanently. Apart from this, SQUID measures the averaged spin alignment of the 4f 7 orbitals, thus the macroscopic magnetization. MCD-PE probes the photoemission final states of a core hole which is under exchange interaction with 4f 7 . Thereby, under external strain the intra-atomic exchange may be significantly altered, while the spin alignment itself (i. e. the origin of the magnetization) of the 4f 7 is mainly unaltered. This may explain the largely reduced MCD signal. A further analysis of single final state J components is subject to present work. Here, we have presented the MCD effect in a EuO/LAO (100) heterostructure with 4.2% tensile strain. The MCD asymmetry shows a large reduction with respect to an unstrained reference sample. This reduction is even larger than expected from SQUID measurements, and outlines changes in intra-atomic exchange interactions. 4.3. Lateral compressive strain: EuO on MgO (100) Lateral compressive strain can be induced by substrates with a smaller cubic lattice parameter than EuO. Magnesium oxide (MgO) has a smaller lattice parameter of a = 0.42 nm of the cubic unit cell. Thus, EuO on MgO (100) is exposed to 18% compressive biaxial strain in the lateral dimension. Several works have reported the successful growth of EuO/MgO (100), where epitaxy and a bulk-like magnetization of EuO has been observed. 32,166 However, these EuO films on MgO (100) were thick by means of many tens of nanometers, and the initial stages of growth and the properties of strained EuO in the few-nanometer regime were not adressed yet. A recent A summary of MgO and other substrates regarding lattice parameters and strain can be found in Tab. A.2 on p. 128.
80 4. Results I: Single-crystalline epitaxial EuO thin films on cubic oxides study 27 investigates ultrathin EuO directly on MgO (100) and finds that epitaxial EuO can be observed only after 2 nm of EuO deposition. This study also reveals a formation of EuO on MgO (100) in the monolayer regime without oxygen supply, a similar effect as observed for YSZ, 32,45 which supplies ionic oxygen at elevated temperatures. Facing the problem of “interrupted epitaxy”, Swartz et al. (2012) focus on an additional oxide interlayer in order to integrate the EuO unstrained lattice with the MgO substrate. 27 In this section, we focus on high-quality epitaxial EuO grown directly MgO (100) in order to investigate structural and magnetic effects of the large lateral compressive strain. Structural characterization of epitaxial EuO on MgO (100) Figure 4.21.: EuO/MgO (100): EuO deposition under large compressive strain. Beginning from one monolayer, EuO shows its native in-plane lattice parameter. A seamless EuO/MgO heteroepitaxy is observed by RHEED, accompanied by a small fraction of polycrystalline EuO as indicated by circular intensities. From 4 nm EuO on, reciprocal pattern improve towards largely singlecrystalline EuO; however, the accumulated roughness prevents the observation of the EuO surface structure by LEED after growth. In Fig. 4.21, the in situ deposition of EuO directly on MgO (100) is monitored by electron diffraction. The annealed MgO substrate shows sharp RHEED and LEED pattern indicating a single-crystalline and smooth surface. Accounting for possible oxygen diffusion from the substrate, one monolayer of Eu metal is deposited onto the heated MgO just before EuO growth as a seed preventing over-oxidized EuO phases. When EuO growth begins, for 1–2 monolayers EuO we observe a RHEED pattern of MgO and EuO simultaneously without any polycrystalline or amorphous phase. This is the first time, that a good heteroepitaxy of EuO directly on MgO (100) without any non-crystalline stages is achieved. The observed RHEED streaks of the first monolayer EuO exhibit the native EuO lattice parameter, thus large areas of unstrained and epitaxial EuO have formed directly on MgO. Proceeding with the EuO synthesis, from two monolayers EuO on, a small polycrystalline fraction indicated by circular intensities next to the unstrained EuO pattern is observed in the RHEED photograph. Thus, a small fraction of disorder or cracks must be present to allow the unstrained EuO to arrange heteroepitaxially directly on MgO (100). From two monolayers EuO until the end of EuO deposition, the surface crystal structure improves, and only EuO fcc pattern are observable when EuO growth is finished (d = 15 nm). However, inherited from the initial polycrystalline
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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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Page 46 and 47: 2.5. Magnetic circular dichroism in
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Page 58 and 59: 3.2. In situ characterization techn
Page 60 and 61: 3.3. Experimental developments at t
Page 62 and 63: 3.4. Ex situ characterization techn
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Page 76 and 77: 59 Table 4.1.: Parameters for stoic
Page 78 and 79: 4.1. Coherent growth: EuO on YSZ (1
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Page 98 and 99: 4.3. Lateral compressive strain: Eu
Page 100 and 101: 4.3. Lateral compressive strain: Eu
Page 102 and 103: 5. Results II: The integration of t
Page 104 and 105: 5.1. Chemical stabilization of bulk
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Page 122 and 123: 5.3. Interface engineering I: Hydro
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Page 134 and 135: 5.5. Interface engineering III: SiO
Page 136 and 137: 5.5. Interface engineering III: SiO
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Page 140 and 141: 6. Conclusion and Outlook In the th
Page 142 and 143: 125 Outlook and perspectives In the
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A.1. EuO-related phases and cubic o
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A.2. In situ analysis of the Si (00
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Bibliography 133 [13] S. S. P. Park
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Bibliography 135 [36] R. E. Dickers
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Bibliography 137 [65] R. Sutarto, S
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Bibliography 139 [91] P. Braun, M.
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Bibliography 141 [116] S. Hüfner.
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Bibliography 143 [144] R. Feder and
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Bibliography 145 [171] H. Miyazaki,
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Bibliography 147 [197] J. Qi, T. Ma
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Software and Internet Resources [21
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List of own publications 151 [10] R
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List of conference contributions 15
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Schriften des Forschungszentrums J
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Schlüsseltechnologien / Key Techno
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