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4.3. Lateral compressive strain: EuO on MgO (100) 83 tions. Even ultrathin EuO exposed to a large lateral mismatch by the MgO (100) surface deviates only in a weakly from the magnetization curve of bulk EuO. These results are published in Caspers et al. (2013). 2 In order to investigate the effect of large compressive strain and resulting structural rearrangements EuO/MgO (100) heterostructures, we conducted an MCD experiment using hard X-ray photoemission spectroscopy. However, no MCD asymmetry above the noise level could be observed (not shown). Since the EuO ultrathin film on MgO (100) is only of 3 nm thickness, the polycrystalline crystal structure may be responsible for a reduced T C , which lets the EuO layer be in the paramagnetic phase during the experiment. We assume experimental issues, for example a local warming of the heterostructure during the spectroscopy. One reason might be the incident X-ray beam, which excites the MgO resulting in a white glowing fluorescence. Further studies of biaxial compressive strain on epitaxially integrated EuO are desirable. A summary of alternative substrates regarding lattice parameters and strain can be found in Tab. A.2 on p. 128.
84 4. Results I: Single-crystalline epitaxial EuO thin films on cubic oxides 4.4. Summary In order to provide high-quality ultrathin EuO for our fundamental studies, we synthesized EuO thin films from bulk-like thicknesses down to one nanometer by reactive molecular beam epitaxy specialized for oxide growth (Oxide-MBE). The EuO thin films are of textbook-like single-crystalline quality, as confirmed by electron diffraction techniques. We developed a procedure to obtain a certain metallicity in YSZ substrates (“conductive YSZ”), which allows for experimental techniques involving high electron yields, e. g. hard X-ray photoemission spectroscopy (HAXPES) or transmission electron microscopy (TEM). On conductive YSZ, we confirmed epitaxial layer-by-layer growth of EuO. A sharp and well-defined EuO/cYSZ interface as well as a homogenous crystallinity of the EuO film are observed by high-resolution TEM. Our magnetic oxide EuO thin films exhibit a bulk-like magnetization down to 2 nm thickness. The stoichiometric chemical quality of EuO in Si/EuO/cYSZ (100) heterostructures was confirmed by HAXPES for various thicknesses. Even for 1 nm ultrathin EuO, no valence change or interface shifts are identified from Eu core-level peaks. We conducted an advanced magnetic characterization by magnetic circular dichroism (MCD) in core-level photoemission, providing insight into the intra-atomic exchange coupling of EuO. This reveals large MCD asymmetries of up to 44% in the Eu 3d and 49% in the Eu 4d photoemission multiplets. The temperature dependence of the MCD describes a Brillouin function with T C = 64 K. Thus, EuO thin films on conductive YSZ provide a fundament for ideal EuO heterostructures of reference quality. This allows to explore advances of selected physical properties of ultrathin EuO layers – like a tuned ferromagnetism. Biaxial lateral strain applied to single-crystalline EuO is of fundamental interest, since it alters the electronic structure and magnetic coupling in a controlled way. In order to investigate epitaxial EuO under 4.2% tensile biaxial strain, we synthesized high quality EuO/LAO (100) heterostructures by Oxide-MBE. A seamless heteroepitaxy of EuO is observed by electron diffraction. For 16 nm epitaxial EuO/LAO (100), reciprocal space maps reveal the adaption of the lateral LAO lattice parameter, while the perpendicular parameter c of EuO is the unchanged EuO bulk value, which corresponds to a Poisson ratio of ν EuO ≈ 0. While the magnetization of 4 nm EuO/LAO (100) heterostructures shows a Brillouin-shape as expected for a Heisenberg ferromagnet with a bulk-like saturation moment, the Curie temperature is reduced by 12.3 K with respect to the EuO reference value. However, a EuO/LAO (100) heterostructure of bulk thickness shows a T C reduction of only 1.7 K. Therefrom we anticipate, that a large thickness in the unstrained z dimension provides sufficient long range magnetic exchange for a bulk-like Curie temperature in laterally expanded EuO. MCD asymmetries in Eu 3d and 4d core-level photoemission multiplets show a maximum asymmetry of 58% relative to the unstrained EuO reference. This reduction is significantly larger than expected from SQUID measurements, which we interpret as a larger effect of the tensile strain on intra-atomic exchange (MCD) than on the long range order of the 4f 7 magnetic moments (SQUID).
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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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2.5. Magnetic circular dichroism in
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Page 52 and 53: 3. Experimental details Die Pläne
Page 54 and 55: 3.1. Molecular beam epitaxy for oxi
Page 56 and 57: 3.2. In situ characterization techn
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
Page 74 and 75: 4. Results I: Single-crystalline ep
Page 76 and 77: 59 Table 4.1.: Parameters for stoic
Page 78 and 79: 4.1. Coherent growth: EuO on YSZ (1
Page 92 and 93: 4.2. Lateral tensile strain: EuO on
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Page 96 and 97: 4.3. Lateral compressive strain: Eu
Page 98 and 99: 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
Page 128 and 129: 5.4. Interface engineering II: Eu p
Page 134 and 135: 5.5. Interface engineering III: SiO
Page 136 and 137: 5.5. Interface engineering III: SiO
Page 138 and 139: 5.6. Summary 121
Page 140 and 141: 6. Conclusion and Outlook In the th
Page 142 and 143: 125 Outlook and perspectives In the
Page 144 and 145: A.1. EuO-related phases and cubic o
Page 146 and 147: A.1. EuO-related phases and cubic o
Page 148 and 149: 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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