In this investigation, the cation distribution in ferrites of spinel-type structure is spectroscopically investigated with respect to the observed magnetic behavior. The ferrite thin films were fabricated by pulsed laser deposition and consequently annealed at different temperatures as well as atmospheres. Structural properties were determined with various methods and the crystalline quality was examined. The dielectric function line-shape was parametrized based on empirical evidence and was found to be dominated by electronic transitions between d orbitals of Fe2+ cations as well as transitions from O 2p to 3d and 4s orbitals of iron and zinc cations. The strongest magneto-optical response was observed for transitions involving cations, which correspond to lattice disorder and inversion within the normal spinel structure. With the decrease in the substrate temperature during fabrication, a decrease in the magnetic response was observed. The diminishing ferrimagnetic order was directly correlated to the decrease in strength of the transitions, involving Fe3+ on tetrahedral lattice sites. After thermal treatment in argon atmosphere and at a temperature below the deposition temperature, the increase in the magnetic response was explained through the facilitation of oxygen vacancies. With the increase in treatment temperature, a decrease in ferrimagnetic order was related to the recrystallization of the disordered spinel structure toward a more stable normal configuration, evident in the dielectric function spectra. The cationic configuration distribution in the surface as well as the bulk region, as a function of Zn concentration, was determined from approximation of the XPS and the dielectric function spectra, respectively. The difference in the cation configuration distribution, in films of predominantly inverse configuration, was related to the weak magnetic response, as opposed to films of predominantly normal spinel configuration. Our results demonstrate that a defect-rich surface region could serve as a possible explanation for the ferrimagnetic order in a nominally non-magnetic normal spinel ZnFe2O4. In combination with structural property determination, the net magnetic behavior is explained through the local cationic disorder, determined from the parametrization of the dielectric function spectra in a wide spectral range.:1 Introduction 2 Theoretical background and fundamental considerations 2.1 Spinel ferrite crystal structure 2.2 Crystal field stabilization energy 2.3 Band structure description 2.4 Verwey transition 2.5 Magnetic exchange interactions 3 Sample preparation and modification 3.1 Macroscopic spinel film formation 3.2 Pulsed laser deposition 3.3 Thermal treatment 3.4 Sample overview 4 Methods and general properties 4.1 Structure characterization techniques 4.1.1 X-ray diffraction 4.1.2 X-ray reflectivity 4.1.3 Energy dispersive X-ray spectroscopy 4.1.4 Focused ion beam and scanning electron microscopy 4.1.5 Raman spectroscopy 4.2 Surface properties 4.2.1 Atomic force microscopy 4.2.2 X-ray photoelectron spectroscopy 4.3 Dielectric tensor properties 4.4 Spectroscopic ellipsometry 4.5 Magneto-optical Kerr effect 4.6 Magneto-static properties 5 Results and discussion 5.1 Magnetic and optical properties of Fe3O4 thin film and single crystal 5.2 Magneto-optical properties of ZnxFe3−xO4 thin films 5.3 Fabrication temperature dependent ferrimagnetic order 5.4 Thermally induced structural stabilization 5.5 Cation configuration in dependence on the Zn concentration 5.5.1 Structural property determination 5.5.2 Composition characterization 5.5.3 Magneto-static behavior 5.5.4 Section summary and discussion 6 Summary and outlook Bibliography List of article contributions Selbstständigkeitserklärung Acknowledgments

This work addresses the growth of ultrathin magnetite (Fe3O4) and cobalt ferrite (CoFe2O4) films and their thorough structural, electronic, and magnetic characterization. In a first step, ultrathin Fe3O4 films are grown on SrTiO3(001) substrates by reactive molecular beam epitaxy (RMBE) and the substrate-induced anomalous strain behavior of the films is investigated by complementary high-resolution transmission electron microscopy (HRTEM) and (grazing incidence) X-ray diffraction [(GI)XRD] measurements. Next, an additional CoO film is deposited on similar Fe3O4/SrTiO3(001) heterostructures to demonstrate an alternative route for the synthesis of cobalt ferrite films through the thermally mediated interdiffusion of both oxide films. The evolution from the initial bilayer stacks to completely reacted cobalt ferrite films is extensively monitored by soft and hard X-ray photoelectron spectroscopy (soft XPS and HAXPES) and (GI)XRD. Complete intermixing and formation of single cobalt ferrite films is confirmed by angular-resolved HAXPES (AR-HAXPES) and X-ray reflectivity (XRR). The study of the cationic distribution resulting from this novel synthesis technique and its effects on the magnetic properties of the cobalt ferrite films is the subject of the subsequent part. Here, X-ray magnetic circular dichroism (XMCD) and superconducting quantum interference device (SQUID) magnetometry serve as key investigation techniques, which are further complemented by AR-HAXPES and atomic force microscopy (AFM) measurements. In a final step, highly crystalline cobalt ferrite films with different cationic stoichiometries are grown on MgO substrates using RMBE while their growth behavior is captured in real-time using operando XRD. Further structural characterization of the films is carried out by low-energy electron diffraction and XRR, whereas HAXPES and SQUID provide fundamental information on the electronic, chemical, and magnetic properties of the films.

<p>Some molecular complexes exhibiting bistability between two different spin states have been subject to vast investigations. Spin crossover and valence tautomerism interconversion molecular compounds are such examples showing this dynamic switching behavior and are a route toward designing molecular devices with a facile readout due to the change in the spin state that accompanies the change in conductance. Due to their extensive potential applications in industry and research, they are among the most interesting topics in spintronics. Spin state switching processes provide the foundation for applications in molecule-based devices. The main goal is to study the parameters that affect the intramolecular electron transfer between different spin states in spin crossover molecular thin films and the electron transfer between the metal center and redox-active ligands in valence tautomer thin films. </p> <p>Because substrate effects are important for any molecular-based device, there are increasing efforts to study the influence of the substrate on spin state transition. While some non-metallic substrates like graphite seem to be promising from experimental measurements, theoretical and experimental studies indicate that 2D semiconductor surfaces will have minimum interaction with spin crossover molecules.</p> <p>In this work, the functionality of two different spin crossover molecules sublimated on the ferroelectric Polyvinylidene Fluoride Hexafluoropropylene (PVDF-HFP) layer and 2D Ti3C2 MXene thin film is studied. We report the temperature-dependent spin state transition of spin crossover molecules, [Fe{H2B(pz)2}2 (bipy)], thin films, and valance tautomer [Co(sq)(cat)(3-tpp)2] thin films. Using a UV-Vis spectrometer and a specific lab-built sample holder we were able to perform the measurement at temperatures as low as 90 K and as high as 440 K. Temperature-dependent UV-Vis data show that the transition temperature from the high spin state to the low spin state and vice versa is well below the room temperature. However, for isothermal switching purposes, we designed and fabricated a bilayer device with PVDF-HFP thin films as the substrate to facilitate isothermal switching close to room temperature. The retention of voltage-controlled nonvolatile changes to the electronic structure in bilayers of PVDF-HFP/[Fe{H2B(pz)2}2(bipy)] and PVDF-HFP/[Co(sq)(cat)(3-tpp)2] strongly depends on the thickness of the PVDF-HFP layer. </p> <p>While the electric transport measurement revealed that doping the SCO layer with Ti3C2 MXene flakes can significantly improve the conductivity of the spin crossover molecular thin films, the temperature-dependent UV-Vis measurements represent that the adjacent MXene layer can cause a lock in the given spin state or a change in the transition temperature dramatically.</p> <p><br></p>

Die vorliegende Arbeit befasst sich mit dem ferromagnetischen Kondo-Gitter-Modell (s-d-, s-f-Modell) für Filmstrukturen. Die Spin-Fermion-Wechselwirkung des Modells kommt in Materialien vor, in denen lokalisierte Spins mit beweglichen Ladungsträgern wechselwirken, wie etwa in (verdünnten) magnetischen Halbleitern, Manganaten, oder Seltene-Erd-Verbindungen. Die durch die Ladungsträger vermittelte, indirekte Wechselwirkung zwischen den lokalisierten Spins reicht von der langreichweitigen, oszillierenden RKKY-Austauschwechselwirkung im Falle schwacher Kopplung bis zur kurzreichweitigen Doppelaustausch-Wechselwirkung bei starker Spin-Fermion-Kopplung. Beide Grenzfälle werden in dieser Arbeit durch die Abbildung des Problems auf ein effektives Heisenberg-Modell erfasst. Der Einfluss von reduzierter Translationssymmetrie auf die effektive Austauschwechselwirkung und auf die magnetischen Eigenschaften des ferromagnetischen Kondo-Gitter-Modells wird untersucht. Curie-Temperaturen werden für verschiedene Parameterkonstellationen berechnet. Die Auswirkungen von Ladungstransfer und von Gitter-Relaxation auf die magnetische Oberflächenstabilität werden betrachtet. Die Diskussion bezieht sich auf die Modifizierungen der Zustandsdichte und der kinetischen Energie im dimensionsreduzierten Fall, da die effektiven Austauschintegrale eng mit diesen Größen verknüpft sind. Die Bedeutung von Spinwellen für den Magnetismus dünner Filme und an der Oberfläche wird gezeigt. Die Interlagen-Austauschkopplung stellt ein besonders interessantes und wichtiges Beispiel der indirekten Wechselwirkung zwischen lokalisierten Momenten dar. Im Rahmen einer RKKY-Behandlung wird die Kopplung zwischen Monolagen in dünnen Filmen untersucht. Sie wird entscheidend durch die Art der ebenen und senkrechten Ladungsträgerdispersion bestimmt und ist jenseits eines kritischen Wertes der Fermi-Energie stark unterdrückt. Schließlich wird die temperaturabhängige magnetische Stabilität von interlagen-gekoppelten dünnen Filmen behandelt und die Bedingungen für einen temperaturgetriebenen magnetischen Reorientierungsübergang werden diskutiert. / This thesis is concerned with the ferromagnetic Kondo lattice (s-d, s-f) model for film geometry. The spin-fermion interaction of this model refers to materials in which localized spins interact with mobile charge carriers like in (dilute) magnetic semiconductors, manganites, or rare-earth compounds. The carrier-mediated, indirect interaction between the localized spins comprises the long-range, oscillatory RKKY exchange interaction in the weak-coupling case and the short-range double-exchange interaction for strong spin-fermion coupling. Both limits are recovered in this work by mapping the problem onto an effective Heisenberg model. The influence of reduced translational symmetry on the effective exchange interaction and on the magnetic properties of the ferromagnetic Kondo lattice model is investigated. Curie temperatures are obtained for different parameter constellations. The consequences of charge transfer and of lattice relaxation on the magnetic stability at the surface are considered. Since the effective exchange integrals are closely related to the electronic structure in terms of the density of states and of the kinetic energy, the discussion is based on the modifications of these quantities in the dimensionally-reduced case. The important role of spin waves for thin film and surface magnetism is demonstrated. Interlayer exchange coupling represents a particularly interesting and important manifestation of the indirect interaction among localized magnetic moments. The coupling between monatomic layers in thin films is studied in the framework of an RKKY approach. It is decisively determined by the type of in-plane and perpendicular dispersion of the charge carriers and is strongly suppressed above a critical value of the Fermi energy. Finally, the temperature-dependent magnetic stability of thin interlayer-coupled films is addressed and the conditions for a temperature-driven magnetic reorientation transition are discussed.

L'étude des excitations magnétiques de faible amplitude existant dans des films minces à aimantation non uniforme dans la gamme des hautes fréquences, typiquement entre quelques dizaines de MHz et quelques dizaines de GHz, constitue le sujet de ce mémoire. Le cas idéal de films magnétiques possédant une anisotropie perpendiculaire et présentant une configuration d'équilibre de l'aimantation sous forme de rubans parallèles et périodiques est étudié en détail. Dans le cas de films à forte anisotropie perpendiculaire, les spectres théoriques de susceptibilité dynamique calculés à l'aide de modèles analytiques puis, à partir de simulations numériques 2D de micromagnétisme dynamique, font apparaître des excitations localisées soit à l'intérieur des domaines magnétiques soit à l'intérieur des parois magnétiques. Ces résultats ont été comparés de façon satisfaisante avec des résultats expérimentaux issus de mesures de résonance ferromagnétique réalisées sur des films monocristallins de grenats ferrimagnétiques. Toutefois, les largeurs de raie expérimentales associées aux excitations de parois excèdent celles calculées en utilisant le terme dissipatif de Gilbert. Dans le cas de films à faible anisotropie perpendiculaire, les spectres théoriques de susceptibilité dynamique présentent de multiples excitations magnétiques liées à la forte hétérogénéité des configurations d'équilibre de l'aimantation de type rubans faibles. Les analyses modales effectuées à partir des simulations micromagnétiques dynamiques indiquent l'existence de modes de surface et de volume localisés dans différentes régions de spins à l'intérieur du film. L'influence de différents paramètres (anisotropie perpendiculaire, épaisseur du film, champ magnétique statique) sur les principales caractéristiques des spectres (nombre de résonances, fréquences de résonance, intensités et largeurs de raie) est ensuite étudiée. Ces simulations micromagnétiques dynamiques permettent d'interpréter des spectres expérimentaux complexes de perméabilité dynamique mesurés sur des films ferromagnétiques amorphes. Quelques perspectives concernant la dynamique de l'aimantation de faible amplitude dans différents objets magnétiques sont ensuite présentées.

No description available.

Cette thèse porte sur l 'étude et le contrôle des caractéristiques de transition de phase dans les films magnétiques. Dans le cas des films pulvérisés par magnétron à base de FeRh, on a étudié l'influence de la composition et de la souche. La caractérisation magnétique, structurale et électrique a été effectuée en fonction de la position sur des films de composition graduelle. L'ajout d'un troisième élément (Pt ou Ni) a été utilisé pour décaler la température de transition. Nous avons établi qu'un changement dans les propriétés optiques se produit dans les films à base de FeRh en passant de l'état FM à l'état AFM. En étudiant la position dépendant de la température de la transition optique, nous avons pu construire un diagramme de phase du système Fe-Rh-Pt. Un tel diagramme de phase détaillé, basé sur une variation continue de composition, met en évidence une stabilité accrue (température de transition plus élevée) de la phase AFM pour une composition spécifique proche de celle équiatomique. Le contrôle de la souche du changement de phase dans les hétérostructures PMN-PT / FeRh a été étudié par des mesures de transport à l'aide des mises au point développées dans cette thèse. La température de transition pourrait être décalée de 22 K jusqu'à l'application d'un champ électrique faible (10 V ~ 0,32 kV / cm). Le changement de résistance induit par la déformation est important, ce qui est intéressant pour les applications. L'état AFM est stabilisé dans les films contraints. Le caractère anisotrope de la souche piézoélectrique joue un rôle important par rapport au changement de volume isotrope. Nous avons réussi à préparer des films de La (Fe, Si) 13 pour la première fois. Bien que les films optimaux présentent des valeurs de température de Curie, de constante de réseau, de variation d'entropie isotherme induite par le champ et d'aimantation spontanée comparables aux valeurs rapportées pour un matériau en vrac de composition similaire, elles présentent également des caractéristiques inhabituelles, c'est-à-dire des courbes M-T irréversibles et une hystérésis thermique inverse. L'élucidation de l'origine de ces effets nécessite une enquête plus approfondie / This thesis deals with the study and control of phase transition characteristics in magnetic films. In the case of FeRh-based magnetron sputtered films, the influence of both composition and strain were studied. Magnetic, structural and electrical characterization was made as a function of position on compositionally graded films. Addition of a third element (Pt or Ni) was used to shift the transition temperature. We established that a change in optical properties occurs in FeRh-based films on passing from the FM to AFM state. By studying the temperature dependent position of the optical transition, we were able to construct a phase diagram of the Fe-Rh-Pt system. Such a detailed phase diagram based on a continuous variation of composition, evidences an enhanced stability (higher transition temperature) of the AFM phase for a specific composition close to the equiatomic one. Strain control of the phase change in PMN-PT/FeRh heterostructures was investigated through transport measurements using set-ups developed in this thesis. The transition temperature could be shifted by up to 22 K upon application of a low electric field (10 V ~ 0.32 kV/cm). The strain-induced resistance change is large, which is attractive for applications. The AFM state is stabilized in strained films. The anisotropic character of the piezo-electric strain plays a significant role compared to the isotropic volume change. We succeeded in preparing films of La(Fe,Si)13 for the first time. While the optimal films show values of Curie temperature, lattice constant, field-induced isothermal entropy change and spontaneous magnetization comparable with values reported for bulk material of similar composition, they also display unusual features, i.e. irreversible M-T curves and an inverse thermal hysteresis. Elucidation of the origin of these effects requires further investigation.

No description available.

Το θέμα αυτής της διπλωματικής εργασίας είναι η παρασκευή με τη μέθοδο sputtering και ο χαρακτηρισμός μαγνητικών λεπτών υμενίων. Στo πρώτο κεφάλαιο, που αποτελεί και την εισαγωγή, γίνεται μια αναφορά στη τεχνολογία των λεπτών υμενίων και δίνεται το στοιχειώδες θεωρητικό υπόβαθρο των τεχνικών ανάπτυξης αυτών. Στο δεύτερο κεφάλαιο παρουσιάζεται η διάταξη sputtering που χρησιμοποιήθηκε για την ανάπτυξη των μαγνητικών υμενίων και δίνονται τα αποτελέσματα της βαθμονόμησης ενός μετρητικού πάχους υψηλής ακρίβειας. Το τρίτο κεφάλαιο αναφέρεται στην κατασκευή και πιστοποίηση μιας πλήρως αυτοματοποιημένης και χαμηλού κόστους διάταξης μέτρησης μαγνητικών βρόχων μέσω του φαινομένου Kerr, με μέγιστο πεδίο πόλωσης 2Τ. Το τέταρτο κεφάλαιο αναφέρεται στις πειραματικές λεπτομέρειες της παρασκευής των μαγνητικών λεπτών υμενίων Νικελίου και Κοβαλτίου και δίνονται τα αποτελέσματα του δομικού και μαγνητικού χαρακτηρισμού τους. Τέλος, στο πέμπτο κεφάλαιο, παρουσιάζονται τα συμπεράσματα και οι παρατηρήσεις που προέκυψαν κατά τη διάρκεια της ενασχόλησης με τη διπλωματική εργασία. Γίνονται, επιπλέον, και ορισμένες προτάσεις για τη μελλοντική εξέλιξη των συστημάτων sputtering και ΜΟΚΕ, ενώ τονίζεται και το τεχνολογικό ενδιαφέρον που παρουσιάζουν τα υμένια που παρασκευάσθηκαν. / The subject of this diploma thesis is the growth using the sputtering process and the characterization of thin magnetic films. The first chapter refers to thin films' technology and there is given the elementary theoretical background of the thin films’ growth. In the second chapter, there is presented the sputtering device, which is used for the growth of the magnetic films. Also, there is given the results of the calibration of a newly established thickness monitor, which is used for high accuracy thickness measurements. In the third chapter, there is given the description of the construction of a fully automatic and low cost magneto - optic Kerr effect magnetometer for the magnetic loops' measurement in a maximum magnetic field of 2T. There are, also, given the magnetic loops which are used for the construction’s certification. The fourth chapter refers to the experimental details about the preparation of the Nickel and Cobalt magnetic thin films. There are, also, given the results about the structural and magnetic characterization of the thin films. Finally, in the fifth chapter, there are presented the conclusions and the observations, which arose during the diploma thesis. There are, also, presented some proposals about the future progress of the sputtering and MOKE systems, whereas at the same, there is stressed the technical interest of the thin films, which are prepared.

This work investigates the growth dynamic of the reactive molecular beam epitaxy of Fe3O4 films, and its impact on the cation distribution as well as on the magnetic and structural properties at the surface and the interfaces. In order to study the structure and composition of Fe3O4 films during growth, time-resolved high-energy x-ray diffraction (tr-HEXRD) and time-resolved hard x-ray photoelectron spectroscopy (tr-HAXPES) measurements are used to monitor the deposition process of Fe3O4 ultrathin films on SrTiO3(001), MgO(001) and NiO/MgO(001). For Fe3O4\SrTiO3(001) is found that the film first grows in a disordered island structure, between thicknesses of 1.5nm to 3nm in FeO islands and finally in the inverse spinel structure of Fe3O4, displaying (111) nanofacets on the surface. The films on MgO(001) and NiO/MgO(001) show a similar result, with the exception that the films are not disordered in the early growth stage, but form islands which immediately exhibit a crystalline FeO phase up to a thickness of 1nm. After that, the films grown in the inverse spinel structure on both MgO(001) and NiO/MgO(001). Additionally, the tr-HAXPES measurements of Fe3O4/SrTiO3(001) demonstrate that the FeO phase is only stable during the deposition process, but turns into a Fe3O4 phase when the deposition is interrupted. This suggests that this FeO layer is a strictly dynamic property of the growth process, and might not be retained in the as-grown films. In order to characterize the as-grown films, a technique is introduced to extract the cation depth distribution of Fe3O4 films from magnetooptical depth profiles obtained by fitting x-ray resonant magnetic reflectivity (XRMR) curves. To this end, x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectra are recorded as well as XRMR curves to obtain magnetooptical depth profiles. To attribute these magnetooptical depth profiles to the depth distribution of the cations, multiplet calculations are fitted to the XMCD data. From these calculations, the cation contributions at the three resonant energies of the XMCD spectrum can be evaluated. Recording XRMR curves at those energies allows to resolve the magnetooptical depth profiles of the three iron cation species in Fe3O4. This technique is used to resolve the cation stoichiometry at the surface of Fe3O4/MgO(001) films and at the interfaces of Fe3O4/MgO(001) and Fe3O4/NiO. The first unit cell of the Fe3O4(001) surface shows an excess of Fe3+ cations, likely related to a subsurface cation-vacancy reconstruction of the Fe3O4(001) surface, but the magnetic order of the different cation species appears to be not disturbed in this reconstructed layer. Beyond this layer, the magnetic order of all three iron cation species in Fe3O4/MgO(001) is stable for the entire film with no interlayer or magnetic dead layer at the interface. For Fe3O4/NiO films, we unexpectedly observe a magnetooptical absorption at the Ni L3 edge in the NiO film corresponding to a ferromagnetic order throughout the entire NiO film, which is antiferromagnetic in the bulk. Additionally, the magnetooptical profiles indicate a single intermixed layer containing both Fe2+ and Ni2+ cations.