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5.1. Chemical stabilization of bulk-like EuO directly on silicon 93 to the valency quantification of the oxygen-rich Eu compound (ii). Thus, we estimate the mixed-valent Eu 3 O 4 to be the origin of the Eu 2+ signal in oxygen-rich Eu 1 O 1+x , which may also explain the Eu 2+ accumulation near the surface as a result of reduction by metallic Al from the capping layer. The results of this section are published in Caspers et al. (2011). 1 HAXPES: Analysis of the Eu 4s and Eu 4d core-levels In order to develop a consistent compositional analysis of the two complementary EuO/Si heterostructures (i) and (ii), we include the Eu 4s and 4d photoemission spectra into the quantitative analysis. In studying the 4s and 4d core-levels, one has to take into account the dominant exchange interactions with the magnetic 4f 7 open shell. Fig. 5.5a–d shows the Eu 4s and 4d core-levels for (i) stoichiometric EuO and (ii) oxygen-rich Eu 1 O 1+x on Si. A quantitative least-squares peak fitting analysis of the divalent and trivalent spectral contributions in the Eu 4s and 4d spectra is employed to quantify the EuO chemical composition. First, we identify the individual spectral features in the Eu 4s core-level. Figures 5.5 (a) and (b) show the Eu 4s core-level spectra for both (i) and (ii) EuO compounds. A clear doublepeak structure is caused by coupling of the 4s core-level electrons with the localized Eu 4f state, which leads to an exchange splitting ΔE = 7.4 eV of the 4s inner shell. The spins in the 4s photoemission final state |4s 1 4f 7 : 9 S〉|ɛl〉 are predicted to antiparallel for the lower binding energy peak and parallel for the higher binding energy peak ( |4s 1 4f 7 : 7 S〉|ɛl〉 ) with respect to 4s emitted spin. 94,184 For stoichiometric EuO (i), the 4s double-peak is assigned to divalent Eu 2+ spectral contributions, whereas an additional overlapping double-peak feature appears in Fig. 5.5b, that is chemically shifted by 8.1 eV towards higher binding energy. This shifted structure corresponds to trivalent Eu 3+ ions. Figure 5.5.: Ultrathin EuO directly on HF-Si investigated by HAXPES. In Al/EuO/HF-Si heterostructures, HAXPES spectra of the Eu 4s core-level (a, b) and of the Eu 4d core-level (c, d) are analyzed quantitatively. For curve fitting, calculated Eu 2+ and Eu 3+ multiplets serve as reference for the final state binding energies. 110,111,172,174,175
94 5. Results II: EuO integration directly on silicon Proceeding with the most complex photoemission final state structure among the Eu corelevels, we present the Eu 4d core-levels in Fig. 5.5c and d. Their complex multiplet structure is distributed in a wide energy range from 125–170 eV and we focus on the most prominent double peak structure at lower binding energy. Due to the strong 4d–4f exchange interaction and much weaker 4d spin–orbit splitting, these two 4d peak shapes cannot be assigned 4d 3/2 and 4d 5/2 spin–orbit components. By assuming a J = L − S multiplet splitting caused by the 4d–4f interaction, the two peaks are denoted as 7 D J and 9 D J multiplets, respectively. 112,114,115 The fine structure of the 7 D final state is not resolved, whereas the J = 2–6 components in the 9 D state are easily identified. In agreement with the other Eu core-levels, we clearly observe a mainly divalent Eu 2+ valency in EuO (i), but significant spectral contributions from Eu 3+ cations in oxygen-rich EuO (ii). Finally, we performed a quantitative peak analysis by least-squares fitting of the spectral contributions with convoluted Gaussian-Lorentzian curves. The best fit is shown by the solid lines in Fig. 5.5a–d and matches the experimental spectra quite well. From the integrated spectral intensities of the divalent Eu 2+ and trivalent Eu 3+ components, we derive a relative fraction of Eu 3+ cations of only 2.8% for stoichiometric EuO (i) and of 59% for oxygen-rich Eu 1 O 1+x (ii), both in excellent agreement with the Eu 4s and 3d analysis, as summarized in Tab. 5.2. Again, the larger ratio of Eu 3+ from the Eu 4f spectra is explained by the summation of Al and Si valence band spectral weight to the trivalent Eu peaks (as illustrated in Fig. 5.4 on p. 90). Table 5.2.: Binding energies E B and Eu 3+ valency ratios r Eu3+ for (i) stoichiometric EuO and (ii) oxygenrich Eu 1 O 1+x , as obtained by least-squares fitting the Eu 4s and 4d spectra. ( Eu 4s E B Eu 2+ ) ( E B Eu 3+ ) r4f Eu3+ 4s spins ‖ 4f 4f Estates spin ‖ 4f 4f Estates spin (i) EuO 355.4 eV 362.7 eV 7.3 eV 363.5 eV 370.7 eV 7.2 eV 2.2% (ii) EuO 1+x 355.4 eV 362.6 eV 7.2 eV 363.5 eV 370.7 eV 7.2 eV 53.0% ⇆ ( Eu 4d E B Eu 2+ ) ( E B Eu 3+ ) r3d Eu3+ S+1 D J 7 D J 9 D 6 Estates spin 7 D J 9 D 6 Estates spin 7 D J + 9 D 6 (i) EuO 134.2 eV 128.0 eV 6.2 eV 141.9 eV 135.4 eV 6.5 eV 2.8% (ii) EuO 1+x 134.3 eV 128.0 eV 6.3 eV 141.9 eV 135.4 eV 6.5 eV 59.0% ⇆ HAXPES: Impact of the EuO phases on the EuO/Si interface chemical state, observed by Si 2p photoemission Having confirmed the stoichiometric quality of the EuO film on top of Si, we now proceed to study the chemical state of the interface formed between the EuO layer and the Si substrate, which is crucial for any electrical and spin transport. Here, we probe the local chemistry and bonding at the EuO/silicon transport interface by HAXPES. Photoemission from the Si 2p core-level was recorded in normal (0 ◦ ) and off-normal (45 ◦ ) emission geometry with hard X-ray (hν = 4.2 keV) excitation. In this way, the information depth of the escaping photoelectrons is substantially varied between ∼18.4 nm and ∼13.2 nm, respectively, which allows to distinguish bulk and interface-like electronic states of the buried Si substrate. In oxygen-rich EuO/HF-Si (ii), next to the well-resolved Si 0 2p peak, a broader feature can
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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
Page 60 and 61: 3.3. Experimental developments at t
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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
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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
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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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.
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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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