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4.1. Coherent growth: EuO on YSZ (100) 69 Figure 4.11.: Hard X-ray photoemission spectroscopy of 20 nm single-crystalline EuO/cYSZ (100). The Eu 3d in (a, b) and the Eu 4d (c) photoemission spectra are well-suited for an electronic structure analysis of EuO thin films. The calculated multiplet lines are taken from literature. 110,114 With the largest photoionization cross-section and one of the best separated doublet structures among the Eu core-levels, according to the survey in Fig. 4.10, the Eu 3d photoemission is perfectly suited for a characterization regarding chemical character and electronic states of the EuO layer. The Eu 3d final state (Fig. 4.11a, b) shows 3d 5/2 and 3d 3/2 groups, separated by a large spin-orbit splitting of 29.2 eV, in agreement with literature. 159 The peak of low intensity in the center is assigned to collective excitations of Eu 3d photoelectrons in the Si layer, a so-called extrinsic plasmon with a kinetic energy loss of 17 eV. 90 For stoichiometric EuO, one main peak is observable in the Eu 3d 5/2 and 3d 3/2 groups which originates from the Eu 2+ initial state (Fig. 4.11a, b). The observed asymmetry in the line shapes is perfectly consistent with theoretical calculations of the divalent Eu 3d multiplet 110 which is described by the ∣ ∣ ∣4f 7 : 8 S7/2〉 ∣ ∣∣3d 9 : j = 5 2 ,j= 3 2〉∣ ∣∣ɛl 〉 final states, where ɛl denotes the photoelectron. The multiplet structure of the final state has its origin in the core-hole interaction with the 4f open shell. A satellite peak in the high binding energy region of the Eu 2+ 3d 5/2 (Eu 2+ 3d 3/2 ) multiplet, which is separated by 7.8 eV (6.3 eV) from the main peak is also attributed to the ∣ ∣ ∣3d 9 4f 7〉 final state multiplet. 111 Both energy splitting and intensity ratios compare well with previous reports on calculated and measured multiplet spectra of divalent Eu compounds. 110,159,160 The Eu 4d core-levels are found at a relatively low binding energy of 126 eV and are therefore photoelectrons of high kinetic energy (Fig. 4.11c). The Eu 4d photoemission final states extend to the binding energy of 160 eV. 114,115 In this work, however, we focus on the two
70 4. Results I: Single-crystalline epitaxial EuO thin films on cubic oxides (a) 160x10 3 intensity (counts) 110 60 bulk vs. ultrathin EuO on cYSZ(001) Eu 3d hv=5.9 keV, normal emission, dE=0.2 eV, T=63 K Eu 2+ 3d 3/2 ultrathin (1 nm) EuO bulk-like (4 nm) EuO Si plasmon (Eu 3+ ) Eu 2+ 3d 5/2 1160 1150 1140 1130 1120 binding energy (eV) (b) bulk vs. ultrathin EuO on cYSZ(001) 50x10 3 hv=5.9 keV, normal emission, dE=0.2 eV, T=63 K intensity (counts) 30 Eu 4d ultrathin (1 nm) EuO bulk-like (4 nm) EuO 7 DJ (J=1..5) 9 DJ (J=2..6) 145 140 135 130 125 binding energy (eV) Figure 4.12.: HAXPES of ultrathin single-crystalline EuO/cYSZ (100) recorded at 63 K. Large crosssections of Eu 3d and 4d levels in (a) and (b) permit one to resolve even small intensity changes in the multiplet structure. peaks at low binding energy, as depicted in Fig. 4.11c. They resolve a complex multiplet structure. The 4d orbital shares the same principal quantum number with the 4f valence level which involves a strong d–f exchange interaction due to radial overlap. Therefore, we cannot separate 4d spectra into 4d J=3/2 and 4d J=5/2 components, but rather to a J = L − S multiplet splitting, and assign the peaks to the 7 D J and 9 D J multiplets, respectively. 114,115 Eu 4d final states are well described by the intermediate coupling scheme (LSJ) in which both spin-orbit and exchange splitting are treated as perturbations. 112 This leads to an intensity ratio of about 1:3 for the septet to the nonet peaks 161 of the final state configuration ∣ ∣4f 7 : S7/2〉∣ 8 ∣∣4d 〉∣ 9 : 9 D J , 7 D ∣∣ɛl 〉 J . Once more, we observe a clear Eu 2+ valency in EuO, indicative for the integral divalent state of EuO. In the HAXPES spectrum (Fig. 4.11c), 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. Energy losses due to the excitation of the 5p or the 4f electrons during photoemission are predicted to lead to satellites at 5–10 eV higher binding energy from the main line. 162 . Furthermore, smaller contributions of the 7 D final state exist at up to 20 eV higher binding energy than the 7 D main peak. 114 However, these higher binding energy features overlap within intensities of an extrinsic plasmon of silicon and the background of inelastic electron scattering. In a further step, we investigate the two core-levels from ultrathin single-crystalline EuO, and compare a bulk-like EuO thin film (4 nm) with a quasi-two dimensional EuO layer (1 nm). Such thicknesses are typical for tunnel barriers in future spin-selective tunnel contacts. In Fig. 4.12a, b, the HAXPES spectra of Eu 3d and Eu 4d core-levels are taken at LN 2 temperature in order to reduce thermal broadening. We observe a line shape of the core-level spectra comparable with thicker EuO, even the multiplet splitting originating from the single atomic angular momenta J is resolved with an identical distribution of spectral weight. In the spectra of 1 nm EuO, a small deviation can be observed at 8 eV shift towards higher binding energy with respect to the main multiplet features. We observe this small feature consistently in both the Eu 3d and 4d spectra, and identify this as chemical shift of Eu 3+ ions. 1 An explanantion may be ionic oxygen provided by the cYSZ substrate, to which the ultrathin EuO layer reacts sensitively with oxidation. In conclusion, we investigated Eu core-levels of heterostructures with single-crystalline EuO thin films in the range 1–20 nm by HAXPES. We selected the Eu 3d and the 4d orbitals for
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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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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
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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
Page 134 and 135: 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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