Overview of oxygen incorporation eects in doped lanthanum manganites 

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X-Ray Re ectometry (XRR)

X Ray Re ectometry is an non-destructive technique that allows the investigation of thin lms in relation to their thickness, composition and roughness. It consists of performing an X-ray re ection experiment around the total re ection angle, also called the critical angle. A typical XRR measurement is shown in Figure 2.9, performed on a thin layer of LSMO deposited on an STO substrate. As it can be seen for angles that are smaller than the critical angle the intensity of the re ected radiation is maximum as one would expect since there is no refraction in the lm. As the critical angle is surpassed, the intensity decreases as refraction occurs. The typical features of an XRR measurement are the fringes visible at low 2 angles. The origin of the fringes stems from the interference of the radiation re ected from the lm and the radiation that is re ected from the substrate, at the lm-substrate interface. The condition for a constructive interference between the two is that the di erence in optical path of the two rays be an integer of . This requirement is the analogous of the Bragg relation, where the distance between the crystal planes, in case of the latter is replaced by the thickness of the lm in the case of the former. Thus, from the distance between two consecutive intensity maxima, one can calculate the lm thickness accordingly. Also, it can be noticed that the critical re ection angle is dependent on the material of the thin lm. This is due to the fact that the refraction index, n, is material dependent, and more speci cally is dependent on the material density and the electronic density. The critical re ection angle and the refraction index are connected by Snell’s law. In this sense, XRR is considered to be a technique that may be used for compositional studies, through the measurement of the critical re ection angle. As far as thin lm and interface morphology characterization are concerned, the overall intensity decay of the re ected X-ray radiation is related to the surface roughness through dispersion. Roughness, understood as large deviations of the lm surface normal on a lateral scale of a few nanometers, leads to the dispersion of the re ected radiation which in turn leads to a more rapid decay of the radiation intensity as a function of the incidence angle. Fitting of the XRR measurements on the deposited lms, in order to extract quantitative information about the lm thickness and morphology, was performed using the LEPTOS software.

Atomic Force Microscopy

Atomic force microscopy is a very powerful technique used to provide quantitative infor-mation about the surface topography of thin lms. The principle behind this technique is to use the forces that act between a small probe and the studied surface, when the two are very close or in contact with each other, in order to trace the topography of the surface. In practice this is achieved using an experimental set-up that is summarized in Figure 2.10a. The simplest operation mode of an AFM is the so-called contact mode, in which the probe, i.e. tip, is brought into contact with the sample surface and, by means of piezoelectric drives, it is moved in the x-y, surface in-plane direction. As the tip moves, it is de ected, together with the cantilever, by surface irregularities. The de ection amplitude, which is in fact a pro le line of the surface, is measured by a re ecting a laser beam from the cantilever into a set of four photodiodes. The topographic information is thus transformed into an electric signal, which is in turn used to recreate a surface image on a display, providing at the same time information for the feedback loop controlling the tip movement. Although simple, the contact operation mode has some drawbacks. Because the tip and surface are in contact, this may lead to the damaging of the tip, creating artifacts and degrading image resolution. Also, the surfaces that have been exposed to air tend to form a thin layer of water and other contaminants on top. An AFM tip touching the contaminated surface will cause the formation of a water meniscus. The surface tension thus created provides an additional force that in uences the de ection of the tip, and thus the out-come of the AFM measurement. To prevent tip and sample damage, but most importantly to increase image resolution and provide the possibility of imaging uid surfaces, the dynamic modes of AFM were developed. As opposed to the static, i.e. contact mode, in the dynamic regime the cantilever is oscillated at, or near the resonance frequency. The oscillation parameters of the cantilever are then used to provide topographical information about the surface. Depending on the parameters used to provide for the feedback loop there are two separate dynamic modes: amplitude modulated mode (AM-AFM) and frequency modulated mode (FM-AFM). Besides the di erence in the feedback parameters there is another fundamental di erence between the two modes, which consists in the region of the Lennard-Jones potential in which they operate. The Lennard-Jones potential is tip-surface interaction potential as a function of the distance between the two. It is composed of two terms: a repulsive and an attractive component. The repulsive interaction is of short range and stems from the Pauli repulsion as the electronic orbitals of the tip and surface overlap. The attractive long range interaction is due to the Van der Waals forces which are a sum of dipolar interaction forces, the most common of which being the London dispersion force, which is the interaction between instantaneous, self-polarized, dipoles [8]. It is quite clear that in the static, contact regime, the AFM operates in the repulsive branch of the interaction potential, corresponding to very small separation of the tip and surface. The amplitude modulated mode is also known as intermittent contact mode or the Tapping mode. The name comes from the fact that the cantilever oscillates above the sample and then lowered until it comes into contact with the surface, experiencing thus only intermittent contact with it. Consequently, this mode operates both in repulsive and attractive branches of the potential. As the tip is lowered its oscillation amplitude is reduced. During the operation the controller acts as to maintain the oscillation amplitude constant. In the frequency modulated mode, or the non-contact mode, the tip oscillates at a certain height above the surface, experiencing only the attractive van der Waals forces. The presence of the interaction potential produces shifts in the resonance frequency of the cantilever. In this mode, the controller is set to maintain a constant oscillation frequency. The non-contact mode is operated in vacuum and it’s the method that produces routinely atomic resolution images. However, the most common techniques for thin lm surface investigations are the Contact and Tapping modes. Throughout the present thesis the AFM images were obtained in the contact or Tapping mode, unless speci ed otherwise.

Magnetic Force Microscopy

Depending on the nature of the tip and the surface under investigation, the principles of the AFM can be used to probe other forces that appear between the two, besides the van der Waals forces. Such forces include electric, magnetic or friction forces, and they give rise to additional modes of AFM operation. When dealing with magnetic thin lms, one can have access to the magnetic domain structure by performing AFM measurements using a tip that has a magnetic coating and measuring the magnetic interaction force between the magnetization of the tip and the stray eld of the domains within the lm. This AFM mode bears the name of Magnetic Force Microscopy, MFM. The operation of the MFM is summarized in Figure 2.12a. First, the tip traces the topography of the surface (1) and then it is raised at a certain distance above the sample (2), retracing the same line scan, (3). As it is shown in Figure 2.12b, van der Waals forces are short range, while magnetic interaction is long range, so in order to eliminate the van der Waals component of the tip sample interaction, the tip is raised above the surface where only the magnetic force is present. Finally, the tip is then lowered (4) to proceed to the next line scan.

X-Ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) is an analysis technique which allows for the qualitative and quantitative determination of the electronic structure of materials. The physical phenomenon that lies behind this technique is photoemission. Essentially, photoe-mission is the process in which electrons are emitted from a bulk material after absorbing the energy of photons of an incident radiation beam, represented by X-rays, in the case of the XPS. This phenomena is described, from the conservation of energy point of view, by the Einstein equation: h = E1 + S+EF (k); (2.4). where S represents the sample work function, h the incident photon energy, which for most of the XPS systems lies in the 1-10 keV region. Ekin1 is the photoelectron kinetic energy and EBF (k) is the binding energy of the electron. The binding energy is given by Koopman’s theorem as EBF (k) = Etotf Etoti (2.5).
That is to say, the binding energy represents the di erence between the system in the initial state and the energy of the system in the nal state, which has one electron less. Since every chemical element has a unique electronic structure, the electrons will be emitted having speci c kinetic energies. In photoelectron spectroscopy, one analyzes the kinetic energy of the emitted electrons, from which the binding energy spectrum of the electrons is determined. The determination of the binding energy from the corresponding kinetic energies of the expelled electrons is schematically presented in Figure 3.37. In the case of solid samples, as was the case for our thin lm structures, electrical contact is made between the sample and the spectrometer. This ensures that the sample and spectrometer are in thermodynamic equilibrium, so that the electrochemical potentials or Fermi levels are equal [11]. An electron having Ekin1 at the sample surface, passing to the spectrometer will feel a potential energy di erence of spec S, Figure 3.37. Hence, the spectrometer will measure a di erent kinetic energy, Ekin, given by the expression Ekin = Ekin1 + ( S spec) (2.6).

Table of contents :

1 Magnetic ux pinning in superconductors 
1.1 Flux ow and energy dissipation
1.2 Magneticux pinning in hybrid SC/FM bi-layer structures
1.3 Magneticux pinning in SC/FM structures with periodic arrays of FM micro,- and nanostructures
1.4 Resume de chapitre en francais
2 Experimental Methods 
2.1 Thin Film Deposition
2.1.1 DC Magnetron Sputtering
2.1.2 Pulsed Laser Deposition
2.2 Photolithography and Ion Beam Etching
2.3 High Resolution X-Ray Diraction
2.3.1 HRXRD Diractometer
2.3.2 Symmetric and asymmetric X-ray diraction measurements
2.3.3 Reciprocal Space Mapping
2.3.4 Epitaxial thin lm texture characterization
2.3.5 X-Ray Reectometry (XRR)
2.4 Atomic Force Microscopy
2.4.1 Magnetic Force Microscopy
2.5 X-Ray Photoelectron Spectroscopy
2.6 Vibrating Sample Magnetometry
2.7 SQUID Magnetometry
2.8 Resume de chapitre en francais
3 Elaboration and characterization of epitaxial La1􀀀xSrxMnO3 thin lms 
3.1 Introduction to La1􀀀xSrxMnO3
3.1.1 Structural properties
3.1.2 Magnetic Properties
3.1.3 Electric Properties
3.2 Elaboration and characterization of La1􀀀xSrxMnO3 epitaxial thin lms
3.2.1 Morphological properties of LSMO thin lms
3.2.2 The inuence of target-substrate distance on the morphology of the lms
3.2.3 Structural properties of epitaxial LSMO thin lms
3.2.4 Electrical characterization of LSMO epitaxial thin lms
3.3 Oxygen incorporation eects on La1􀀀xSrxMnO3 epitaxial thin lms
3.3.1 La1􀀀xSrxMnO3+d defect model
3.3.2 Overview of oxygen incorporation eects in doped lanthanum manganites
3.3.3 Experimental
3.3.4 Results and Discussion
3.4 Resume de chapitre en francais
4 Magnetic structures with perpendicular magnetization 
4.1 Permalloy (Ni80Fe20) thin lms
4.1.1 Magnetic force microscopy of stripe domain patterns
4.1.2 Permalloy thin lm growth
4.1.3 Structural Properties of Permalloy thin lms
4.1.4 Magnetic Properties of Permalloy thin lms
4.2 Cobalt thin lms
4.2.1 Co thin lm growth
4.2.2 Magnetic properties of Cobalt thin lms
4.3 Magnetic micro,- and nanostructures
4.3.1 Micro,- and nanostructures elaboration techniques
4.3.2 Magnetic properties of the micro,- and nanostructures
4.4 Resume de chapitre en francais
5 Ferromagnetic/high temperature superconductor hybrid structures 
5.1 LSMO/YBCO hybrid structures
5.1.1 Surface decoration and vortex pinning
5.1.2 Experimental procedure
5.1.3 Single YBCO lm
5.1.4 Morphology of the LSMO/YBCO structures
5.1.5 Structural properties of the LSMO/YBCO structures
5.1.6 Superconducting properties of LSMO/YBCO structures
5.2 YBCO/Permalloy Structures
5.2.1 Experimental procedure
5.2.2 MFM on the Py lm
5.2.3 Superconducting properties of the YBCO/Py structure
5.3 Resume de chapitre en francais
6 Conclusions and Perspectives 
A Vortex state and dynamics 
A.1 Vortex state
A.1.1 Ginzburg-Landau equations
A.1.2 Fluxoid quantization
A.1.3 London equations
A.1.4 Type I and Type II superconductors
A.1.5 The Abrikosov vortex
A.1.6 The vortex energy
A.1.7 The critical elds
A.2 Vortex interaction and dynamics
A.2.1 Vortex interaction
A.2.2 Fluxow
A.2.3 Vortex pinning and critical currents
A.2.4 Thermally activateduxow. Flux creep
A.2.5 Irreversibility line

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