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3.4. Ex situ characterization techniques 45 vations to the surface (as presented in Ch. 5). 3.4. Ex situ characterization techniques In addition to the in situ synthesis and characterization of magnetic oxide heterostructures, we apply a variety of ex situ characterization methods which permit an in-depth analysis of magnetic properties, crystalline structure and the electronic structure. After a brief introduction of bulk-sensitive SQUID magnetometry and X-ray diffraction techniques, photoemission spectroscopy with advanced analysis methods are discussed, which allows for a depthselective profiling. We present a model to determine the information depth by HAXPES and derive a formula to quantify chemical species located at a buried interface. We introduce the HAXPES beamlines and endstations where the photoemission experiments were conducted. Finally, we present the photoemission experiment conducted with circularly polarized X-ray light, which allows to investigate the magnetic circular dichroism effect in magnetized thin films. 3.4.1. Superconducting quantum interference device (SQUID) magnetometry Figure 3.10.: Measurement principle of a superconducting quantum interference device (SQUID). The thin film sample is reciprocating through the superconducting detection loops (a). Variations of the flux Φ (b) are converted by a Josephson contact into an RF signal (c). In order to characterize the bulk magnetic properties of thin magnetic oxide films, SQUID magnetometry is perfectly suited, as it provides a sensitivity down to the nano emu regime. Moreover, a He atmosphere prevents the degradation of the air-sensitive samples. The probing of the sample magnetization takes place in the SQUID sensor, which consists of a superconducting ring interrupted by one Josephson contact. Due to the fundamental magnetic flux quantization (Meißner-Ochsenfeld effect), only a magnetic flux of multiples of Φ 0 = h 2e is allowed to pass through the area of the ring. During measurement, a magnetic sample The electromagnetic notation “emu” measures magnetic moment in the Gaussian cgs system. It converts to SI as: 1 emu = 10 -3 Am 2 =10 -3 J/T.
46 3. Experimental details is moved through the ring (Fig. 3.10a), which induces a ring current. At the same time, the magnetic flux through the ring has to result in an integer multiple of Φ 0 . To fulfill this condition, the Josephson contact generates voltage oscillations with the time period of one ΔΦ 0 (Fig. 3.10b, c). These voltage oscillations are detected by an RF coil and further processed (see Fig. 3.10). In practice, when ˜m(T ) denotes a measured magnetization, ˜m off the magnetization offset from zero when only a vanishing signal is expected (i. e. at T ≫ T C ), ˜m 0 = m(T →0) the measured saturation magnetization, and m theo the theoretically expected magnetization, then the magnetic moment m in μ B per magnetic formula unit can be expressed as m( ˜m(T )) = ˜m(T ) − ˜m off fμ B d = ˜m 0 − ˜m off fμ B m theo · Vcell A . · Vcell , or (3.6a) Ad (3.6b) Here, μ B denotes Bohr’s magneton, V cell = a 3 the volume of the cubic unit cell of the magnetic oxide, f the number of magnetic units per cubic cell, A the area and d the thickness of the magnetic thin film. Application of eq. (3.6a) with known geometry of the magnetic thin film reveals the specific magnetic moment m. Alternatively, by assuming an optimum sample with reference magnetization ˜m 0 = m theo , the thin film thickness d can be obtained from eq. (3.6b). In this work, a Quantum Design MPMS XL SQUID magnetometer is used to determine the magnetic properties of magnetic oxide thin films. The magnetization m(T ) is recorded as field warm curve at a minimum saturation field (usually ∼100 Oe), which we determined by hysteresis measurements for every thin film sample. 3.4.2. X-ray diffraction methods (XRD, XRR, RSM) Based on the Bragg equation nλ =2d sinθ, X-ray diffraction (XRD) is a versatile technique which allows for the determination of structural properties such as the distance between crystal lattice planes, the thickness of layers, interface and surface roughness, texturing, and mosaicity, to mention the most common properties. In a simple Bragg-Brentano setup, in which one axis (2θ) is moved in order that the sample surface is always at ω = θ, a standard 2θ–ω diffractogram is obtained (Fig. 3.11). Here, only crystallites oriented parallel to the surface contribute to the diffraction peak. The observable peaks directly arise from the diffraction after Bragg’s law, and the exact peak position on the 2θ scale allows to calculate the distance between diffracting planes (c: out-of-plane lattice parameter). The width of those peaks provides information about the orientation distribution of the crystallites, i. e. the mosaicity. Thus, already simple XRD diffractograms provide information about orientation and quality of thin films with good crystallinity. 135 Using small angle scattering in the 2θ–ω setup, also known as X-ray reflectivity (XRR), a simulation of Kiessig fringes using the Parratt model 136 allows for the determination of the thickness of several layers from roughly two up to hundreds of nanometers. Moreover, from the damping of these fringes, the interface and surface roughness can be modeled.
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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
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 26 and 27: 2.2. Electronic structure of EuO 9
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
Page 64 and 65: 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
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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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Schlüsseltechnologien / Key Techno
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