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2.2. Electronic structure of EuO 9 Figure 2.3.: The rare earth electronic structure as a shell scheme for Eu metal. The open shell 4f 7 and conduction electrons of Eu are highlighted. The conduction band is composed of the 5d and 6s levels. The Eu 4f 7 levels are highly localized inside the [Xe] core. In ferromagnetic Eu 2+ O, the seven f electrons carry the spin-only magnetic moment M. After Dionne (2009). 37 2.2.1. The open shell 4f 7 The half-filled Eu 4f 7 orbitals constitute valence-like levels near the Fermi edge in EuO which are often represented as bands, as depicted in Fig. 2.4. However, the 4f levels do not take part in the ionic bondings. The narrow radial distribution of n = 4 shells is responsible for the highly localized character of the 4f shell. 38 These properties justify a treatment of the Eu 4f orbital as core-level rather than a valence band, in particular with regard to core-level photoemission (see Ch. 2.4). In Eu 4f , the spin–orbit interaction is weak and the electron exchange strong. This justifies the Russel-Saunders-coupling (LS) for which the Hund’s Rules predict the filling of the 4f 7 level as follows: 1. The spin multiplicity S =2S + 1 is maximal. S 4f 7 = ∑ s i = 7/2 (S = 8), in the FM state due to spin alignment. 2. With given spin multiplicity, the total angular momentum L is largest. Due to Pauli’s exclusion principle, there is only one way to distribute seven electrons with parallel spins in an f shell: l i = −3 ...+3, L 4f 7 = ∑ l i =0. 3. For a half-filled shell, the total angular momentum is J = L + S. ForEu4f 7 : J 4f 7 =0+7/2 = 7/2. Thus, the Eu 4f orbital is in the initial state with ferromagnetic order in a 8 S7/2 configuration. The ferromagnetic moments in EuO originate from the 8 S7/2 spin-only states of the 4f orbital. Correlations between the localized magnetic states 4f 7 and the itinerant electrons † are the cause of the ferromagnetic exchange, as discussed in Ch. 2.3. The correlations in the 4f shell are further discussed in literature. 18,39–42 This is the term symbol notation, S L J . The spin multiplicity S =2S + 1 quantifies the amount of unpaired electron spins. † Of course, in EuO there are no itinerant electrons. However, interactions between Eu 2+ sites work via 4f spins and virtual excitations to 5d conduction bands, often referred to as d–f interaction, or as s–f model in which s denotes the virtual itinerant electron in the conduction band.
10 2. Theoretical background 2.2.2. Valence band, band gap, and the 5d 6s conduction bands Figure 2.4.: Band structure of EuO calculated in the LSDA+U approximation. The Eu 5d conduction band and the top of the Eu 4f band are indicated. The 4f band is narrow in energy and shows almost no dispersion, it is clearly distinguishable from the O 2p level at E B ≈−4.5 eV. Adapted from Ghosh et al. (2004). 41 EuO, as an ionic crystal, possesses the valence band O 2p, and the conduction bands (5d 6s). Between the lower edge of the conduction band and the O 2p valence band is an electrical gap of ∼5 eV, thus, conduction levels and the anionic O 2p level are well separated. The optical gap between Eu 4f and the conduction band has been determined by optical absorption to be 1.12 eV. 29,43 Spin-splitting of the conduction band As one of the most noteworthy properties of a ferromagnetic insulator, EuO exhibits an exchange spitting of the conduction bands due to exchange coupling with the aligned spin state 8 S7/2 of the 4f 7 orbital. This splitting was first determined by Wachter (2011) and confirmed by Steeneken et al. (2002) (2ΔE exch ≈ 0.6 eV) and is depicted schematically in Fig. 2.5. 29,45 In order to obtain an image of the spin-split conduction band of ferromagnetic EuO, Fig 2.6 shows the calculated one-electron Bloch density of states (B-DOS) for the first five conduction bands in Eu 2+ of the majority spin system. 44 They are of 5d character. Below T C , the m =1 peak is shifted by about 1 eV to lower energies. This spin-selective shift of conduction bands is the basis for electronic transport effects in EuO: (i) spin-selective tunnel transmission be- Figure 2.5.: Temperature-dependent scheme of the EuO band gap. If T < T C , the exchange interaction arising from the aligned 4f spins splits the conduction band and lowers the band gap. Adapted from Wachter (2011). 29
Page 1 and 2: Member of the Helmholtz Association
Page 4 and 5: Forschungszentrum Jülich GmbH Pete
Page 6: Zusammenfassung In dieser Doktorarb
Page 10 and 11: Contents 1. Introduction 1 2. Theor
Page 12 and 13: List of Figures 2.1. Phase diagram
Page 14: List of Figures xi 5.12. Resulting
Page 18 and 19: 1. Introduction Smart and powerful
Page 20 and 21: 3 In the thesis at ha
Page 22 and 23: 2. Theoretical background . . . The
Page 24 and 25: 2.1. The challenge of stabilizing s
Page 28 and 29: 2.3. Magnetic properties of EuO 11
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
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Page 74 and 75: 4. Results I: Single-crystalline ep
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59 Table 4.1.: Parameters for stoic
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4.1. Coherent growth: EuO on YSZ (1
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4.2. Lateral tensile strain: EuO on
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4.2. Lateral tensile strain: EuO on
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4.3. Lateral compressive strain: Eu
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5. Results II: The integration of t
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5.1. Chemical stabilization of bulk
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5.2. Thermodynamic analysis of the
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5.3. Interface engineering I: Hydro
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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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