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4.3. Lateral compressive strain: EuO on MgO (100) 81 Figure 4.22.: EuO/MgO (100): different epitaxial arrangements. In (a), a cube-on-cube relation is illustrated inducing large compressive strain to EuO. In the EuO/MgO 4:5 configuration, a minimum lateral strain of +2.4% acts on EuO (b). If however, EuO keeps its native lattice parameter (c), no simple cube-on-cube relation is realized and electrostatic distortions will readily occur. fraction, a certain roughness remains and hampers observation of a surface crystal structure in low energy electron diffraction. We elucidate the observed initial polycrystalline fraction and the quasi-relaxed EuO growth at the EuO/MgO (100) interface in terms of geometrical and electrostatical aspects: In Fig. 4.22, three configurations of the EuO/MgO (100) heteroepitaxy are illustrated assuming different lateral lattice parameters (e. g. in [010] direction) of the initial EuO layers. From our structural investigation, we can exclude the direct cube-on-cube heteroepitaxy (in Fig. 4.22a) comprising large compressive strain, which can be explained as follows: the deposition of one atomically flat EuO layer on an MgO (100) surface is equivalent to a substitution of the Mg atoms of the top layer. Here, the ion-size difference 167 forms different sizes of the Mg–O and the Eu–O bond: in the MgO surface, replacing the Mg 2+ ion with a Eu 2+ ion changes the cation ionic diameter to 0.234 nm and increases the O–O nearest neighbor bond by 22% from 0.298 nm to 0.363 nm. 168 In such a case, the ion-size difference would force the Eu or O atoms to find equilibrium positions in a roughened structure. 27 Considering a second configuration, a EuO/MgO lateral arrangement of 4:5 as sketched in Fig. 4.22b has a lateral strain with a minimum value of +2.4%. From an electrostatic view point, strong Coulomb repulsion between O 2− –O 2− and Eu 2+ –Mg 2+ ions at alternating position would certainly lead to structural roughening at the interface. From our electron diffraction results, we observe in-
82 4. Results I: Single-crystalline epitaxial EuO thin films on cubic oxides dications for a third configuration: relaxed epitaxial EuO/MgO (100) with a small fraction of EuO crystal defects at the interface, as observed in the 60 s-RHEED photograph in Fig. 4.21. Here, the in-plane heteroepitaxy is maintained, however no simple integer relation of EuO and MgO lattices can be realized, as sketched in Fig. 4.22c. Starting from an EuO on MgO matching point, after several unit cells of relaxed EuO a structural distortion likely occurs in order to newly arrange a EuO/MgO ionic match. This leads to misfit dislocations in the initial layers of the EuO crystal and is observed as polycrystalline fraction next to epitaxial relaxed EuO in RHEED. As a result, no compressive strain is acting on the EuO layer, but rather structural dislocations which are observed in the first two nanometers of heteroepitaxial EuO/MgO (100). In conclusion, we observe a good heteroepitaxy of unstrained EuO on MgO (100) beginning with one monolayer EuO, which initially comprises a fraction of crystal defects and develops towards EuO single-crystalline quality. The following magnetic measurements investigate EuO/MgO (100) heterostructures of different EuO thickness to elucidate the relative effect of misfit dislocations from interfacial EuO on the magnetic properties. Magnetic bulk characterization of epitaxial EuO on MgO (100) Figure 4.23.: Magnetic properties of heteroepitaxial EuO on MgO (100). A Brillouin-shaped magnetization curve is observed for all heteroepitaxial EuO/MgO (100) layer structures. The ordering temperature for 4 nm-thin EuO/MgO is comparably reduced as for coherently grown epitaxial 4 nm EuO/YSZ. We investigate the influence of the MgO (100) substrate with a large lateral mismatch on the magnetic properties of heteroepitaxial EuO films by SQUID magnetometry, as depicted in Fig. 4.23. An ultrathin EuO/MgO (100) heterostructure exhibits a magnetization curve with a deviation from the optimum EuO/YSZ curve less than 5%. Here, the Curie temperature is reduced to 67 K, which is 2 K below the bulk value of optimum EuO/YSZ. These small deviations from bulk are caused by an interplay of reduced dimensionality (only 4 nm EuO) and the crystal distortions of interface-near EuO as discussed above. However, the hysteresis loop encloses a rectangular area with a well-defined switching point as expected for a ferromagnetic film, but the coercive field is double the value of an optimum EuO/YSZ heterostructure. This highlights the difference in crystalline quality: coinciding with the polycrystalline fraction of EuO in RHEED, these structural defects may contribute to pinning of magnetic domains in EuO. For EuO/MgO (100) of bulk-like thickness, however, we observe a bulk-like magnetization curve similar to that of a coherently grown reference sample EuO/YSZ (100). In conclusion, the magnetic properties of EuO/MgO are relatively robust against structural misfit disloca-
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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
Page 48 and 49: 2.5. Magnetic circular dichroism in
Page 50 and 51: 2.5. Magnetic circular dichroism in
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
Page 94 and 95: 4.2. Lateral tensile strain: EuO on
Page 96 and 97: 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
Page 114 and 115: 5.2. Thermodynamic analysis of the
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
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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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