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4.1. Coherent growth: EuO on YSZ (100) 65 Figure 4.6.: Thickness dependent magnetic properties in single-crystalline EuO thin films. (a) depicts the in-plane magnetization under applied saturation field. (b) shows the coercive field for every thickness of EuO, with the original hysteresis loop as insets. (c) summarizes the Curie temperature for every EuO thickness, together with model predictions. Starting with bulk-like EuO, for the EuO thickness d = 20–30 nm no change in the magnetization curves is observable, as depicted in Fig. 4.6, and a bulk-like saturation and Curie temperature is observed. If the EuO thickness is reduced, the coercive field increases from an optimum value of 23 Oe of the bulk-like EuO film up to ∼130 Oe for an ultrathin (d = 2 nm) EuO film (Fig. 4.6b). This variation of coercivity with thickness also has been found in Suran et al. (1995) for soft ferromagnetic thin films. 153 A reduction of the EuO thickness to d = 15 nm does not affect the Curie temperature, but the magnetization curve is altered from the shape of a Brillouin function to a reduced magnetization (Fig. 4.6a). The field dependence changes towards a mixture of ferromagnetic (rectangular-shaped hysteresis) and paramagnetic (linear response) behavior. This trend is continued in the course of a further reduction of EuO thickness down to d = 2 nm. Bulk-near magnetic ordering temperatures (T C = 64 K) are maintained down to d = 2 nm and the saturation magnetization is still approximately half the bulk EuO value for this thickness. Finally for 1 nm EuO, the magnetization curve strongly deviates from the Brillouin shape and we estimate T C as 18 K. In this case, the remanence is near zero and the field dependence is mainly paramagnetic. We conclude, that the threshold for establishing sizable ferromagnetic properties of coher-
66 4. Results I: Single-crystalline epitaxial EuO thin films on cubic oxides ently grown single-crystalline EuO lies between one and two nanometers film thickness. In particular, obtaining a bulk-like T C down to 2 nm is remarkable, and more robust than for polycrystalline EuO films. 48 This thickness dependence can be described by a mean field model after Jensen et al. (1992) or the Heisenberg model by Schiller and Nolting (1999) with no need for corrections due to interface intermixing as presented in Fig. 4.6c. 154,155 4.1.2. Preparation on conductive YSZ substrates Figure 4.7.: Conductive YSZ characterized by high- and low-energy electron diffraction. LEED (green) is only observable after an e-beam treatment of YSZ. Coherent growth of EuO on YSZ (100) provides the advantages of single-crystalline EuO thin films with bulk-like magnetic properties. However, any investigation based on high-energy electrons – either by electron scattering or electron emission techniques – is usually hampered by charging of YSZ, as depicted in Fig. 4.7a. Since magnetic oxides like EuO on top of YSZ are also insulating, there arises an experimental need for a certain electrical conductivity in YSZ (100) substrates. The physical origin of a finite electrical conduction in otherwise highly insulating YSZ is usually described by ionic conduction in literature, 156 however this effect is working only at elevated temperatures (usually above T = 500 ◦ C). For a large concentration of oxygen vacancies, electronic defect bands may form which allow for a metallic conduction. 157 This conductive effect of metallic donor bands is particularly predicted for Ti-doped YSZ, 158 which renders Ti-doped YSZ promising as a future substrate which might combine electrical contact with a coherent seed for epitaxial EuO thin films. In the frame of this work, however, doped YSZ (100) substrates were not available, and we developed a treatment with the aim to increase the electronic conduction in otherwise insulating YSZ. Conductive YSZ (we refer to is as cYSZ) with a large oxygen deficiency is created as a conductive substrate for EuO heterostructures. For this, we bombard a cleaned and oxygen-annealed YSZ substrate with hot electrons from a tungsten filament with an energy of U = 1000 eV We remark, that our experimental investigation of the Curie temperature does not provide a highly resolved curve from which we can judge a best fitting model. The included models serve as a guide for the eye.
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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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2.3. Magnetic properties of EuO 13
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Page 36 and 37: 2.4. Hard X-ray photoemission spect
Page 46 and 47: 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 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
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5.4. Interface engineering II: Eu p
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5.5. Interface engineering III: SiO
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5.5. Interface engineering III: SiO
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5.6. Summary 121
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6. Conclusion and Outlook In the th
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125 Outlook and perspectives In the
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A.1. EuO-related phases and cubic o
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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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