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Within this thesis, a comprehensive study of the initial growth process of pure Fe3O4 films and Fe3O4/NiO bilayers on Nb:SrTiO3(001) substrates including the thermal interdiffusion behavior of these bilayers is presented. The sensitive interplay between magnetic, electronic and structural properties of these materials has been investigated in detail. In the first study, the initial growth behavior of high-quality ultrathin magnetite films on SrTiO3(001) deposited by reactive molecular beam epitaxy depending on the deposition temperature has been analyzed. For this purpose, the growth process has been monitored in situ and during the deposition by grazing incidence x-ray diffraction (GIXRD). The second part provides a comparative study of Fe3O4/NiO bilayers grown on both MgO(001) and Nb:SrTiO3(001) substrates exploring morphological, structural and magnetic properties. These structures have been investigated by means of x-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), x-ray reflectivity (XRR) and diffraction (XRD), as well as vibrating sample magnetometry (VSM). Subsequently, thermal stability of these bilayers and the thermally induced interdiffusion process have been studied successively accompanied by a comprehensive characterization of the fundamental electronic, structural and magnetic properties using additional techniques such as angle resolved hard x-ray photoelectron spectroscopy (AR-HAXPES) and x-ray magnetic circular dichroism (XMCD). Finally, an alternative pathway for the preparation of ultrathin nickel ferrite films through interdiffusion is provided.

My dissertation research focuses on the investigation of the transport and magnetic properties of transition metal and rare earth doped oxides, particularly SnO2 and HfO2 thin films. Cr- and Fe-doped SnO2 films were deposited on Al2O3 substrates by pulsed-laser deposition. Xray- diffraction patterns (XRD) show that the films have rutile structure and grow epitaxially along the (101) plane. The diffraction peaks of Cr-doped samples exhibit a systematic shift toward higher angles with increasing Cr concentration. This indicates that Cr dissolves in SnO2. On the other hand, there is no obvious shift of the diffraction peaks of the Fe-doped samples. The magnetization curves indicate that the Cr-doped SnO2 films are paramagnetic at 300 and 5 K. The Fe-doped SnO2 samples exhibit ferromagnetic behaviour at 300 and 5 K. Zero-field-cooled and field-cooled curves indicate super paramagnetic behavior above the blocking temperature of 100 K, suggesting that it is possible that there are ferromagnetic particles in the Fe-doped films. It was found that a Sn0.98Cr0.02O2 film became ferromagnetic at room temperature after annealing in H2. We have calculated the activation energy and found it decreasing with the annealing, which is explained by the increased oxygen vacancies/defects due to the H2 treatment of the films. The ferromagnetism may be associated with the presence of oxygen vacancies although AMR was not observed in the samples. Pure HfO2 and Gd-doped HfO2 thin films have been grown on different single crystal substrates by pulsed laser deposition. XRD patterns show that the pure HfO2 thin films are of single monoclinic phase. Gd-doped HfO2 films have the same XRD patterns except that their diffraction peaks have a shift toward lower angles, which indicates that Gd dissolves in HfO2. Transmission electron microscopy images show a columnar growth of the films. Very weak ferromagnetism is observed in pure and Gd-doped HfO2 films on different substrates at 300 and 5 K, which is attributed to either impure target materials or signals from the substrates. The magnetic properties do not change significantly with post deposition annealing of the HfO2 films.

The preparation of layered magnets needs the knowledge of growth techniques which are focused on the growth of Fe/V(001) superlattices. Such films have been structurally investigated by X-rays reflectivity and diffraction. The magnetic investigations have been carried out by magneto-optic Kerr effect (MOKE), Superconducting Quantum Interference Device (SQUID) magnetometry and polarized neutron reflectivity (PNR). This latter technique has been used in cooperation with the Institute Laue Langvin (Grenoble, France) and Ruhr Universität (Bochum, Germany). The cross-over in universality class is shown in a series of layered magnets where a δ-doping layer of Fe has been embedded between two layers of Pd showing that the magnetization depends on the effective magnetic thickness of the polarized Pd. A model for the cross-over has been developed in terms of magnetic excitations. The interlayer exchange coupling (IEC) mediated by a non-magnetic spacer has been reviewed focusing the attention on the recent theoretical and experimental works based on Fe/V(001) superlattices. The IEC can be tailored at will by reversibly alloying of the spacer with H: this has been proved in Fe/V(001) double layers showing that in the two dimensional limit, the universality class is not affected by the coupling. The magnetic order-disorder transitions in Fe/V(001) superlattices do not seem to belong to any universality class. A phenomenological model which accounts for the effective coupling at the boundaries has been developed. The influence of the inherent ordering temperatures of single magnetic layers has been investigated in Fe/V(001) superlattices proving that the weakest ferromagnetic layer affects the overall magnetic ordering. A new kind of layered magnet has been developed to increase the effect of the boundaries. PNR measurements show that the universality class depends on which length-scale is investigated.

No description available.

Within this thesis, (ultra)thin NiFe2O4 (NFO) and CoFe2O4 (CFO) films are prepared via reactive molecular beam epitaxy (RMBE) on MgO(001) and SrTiO3(001) substrates and are characterized in terms of their structural, electronic, and magnetic properties. In a first step, the structural properties of ultrathin off-stoichiometric NixFe(3-x)O4 films (0<x<1.5) deposited via RMBE on MgO(001) are investigated in situ during film deposition by means of synchrotron radiation-based x-ray diffraction (XRD) and ex situ after film growth by high energy surface x-ray diffraction (HESXRD). In the second major step of this work, a more extensive study on the dependence of the cationic ratio in NixFe(3-x)O4 thin films (0<x<2.07) grown on MgO(001) is conducted. The film surface structure and chemical composition is characterized in situ by low energy electron diffraction (LEED) and laboratory-based soft x-ray photoelectron spectroscopy (XPS), respectively. Film thicknesses are determined via analysis of x-ray reflectivity (XRR) data, while the film structure is analyzed by XRD measurements. Further, chemical properties and the electronic structure of the NFO films with focus on the cationic valencies of Ni and Fe cations with varying x is investigated by means of (angle-resolved) hard x-ray photoelectron spectroscopy [(AR-)HAXPES]. Complementary x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) investigations are conducted to obtain information on the cationic site occupancies and on the element-specific magnetic moments. The latter are compared to magnetic properties characterized via superconducting quantum interference device (SQUID) magnetometry. In a third step, the type of substrate is changed to SrTiO3(001) to investigate the influence of a larger strain applied by the substrate to NFO films with varying thicknesses. Structural characterization at the surfaces and in the films is conducted by means of LEED, XRR, and (grazing incidence) XRD, whereas XPS and HAXPES provide information on the chemical composition and electronic structure in the near-surface region and in deeper subsurface layers, respectively. Magnetic properties are characterized by SQUID magnetometry. In a fourth step, an alternative pathway for the formation of ferrite thin films is demonstrated exemplarily for CoFe2O4 films on SrTiO3(001), which are formed by interdiffusion of Fe3O4/CoO bilayers. The interdiffusion process was monitored via XRR, soft XPS and AR-HAXPES to determine the bilayer/film structure, stoichiometry, and chemical properties. Analysis of complementary XAS measurements provides additional information on the occupancies of Fe and Co cations during interdiffusion. Final SQUID magnetometry measurements are performed to gain information on the magnetic properties before and after complete interdiffusion. Overall, within this thesis, it was demonstrated that NFO and CFO thin films can be prepared in high structural quality with sharp interfaces and surfaces, which is crucial for the applicability in the fields of spintronics and spincaloritronics.

The research in permanent magnet thin films focuses on the search of new materials and methods to increase the prevalent data storage limit. In the recent past the work towards the application of these films to micro systems have also gained momentum. Materials like samarium cobalt with better magnetic properties and temperature stability are considered to be suitable in this regard. The essential requirement in miniaturization of these films is to deposit them on silicon substrates that can alleviate the micro fabrication process. In this work, an effort has been made to deposit SmCo films with better magnetic properties on silicon substrates. In the deposition of SmCo, the composition of the deposited films and the structural evolution are found to play an important role in determining the magnetic properties. Proper control over these parameters is essential in controlling the magnetic properties of the deposited films. SmCo being a two component material the composition of the films is dependent on the nature of the source and the transport of the material species from source to substrate. On the other hand, structural evolution is dependent on the energetical considerations between the SmCo lattice and substrate lattice. This most often is dominated by the lattice match between the condensing lattice and the substrate lattice. As such Si does not provide good lattice match to SmCo lattice. Hence suitable underlayers are essential in the deposition of these films. Materials like W, Cu, Mo and Cr were used as underlayers. Out of all these Cr is found to provide good lattice match and adhesion to SmCo lattice. Sputtering being the common deposition tool, SmCo could be sputtered either from the elemental targets of Sm and Co or from the compound target of SmCo5. Sputtering of elemental targets of Sm and Co provides the flexibility of varying the composition whereas sputtering from the SmCo alloy target provides to flexibility of controlling the structural evolution by different process parameters. In this work two different techniques namely Facing Target Sputtering (FTS) and Ion Beam Sputter Deposition (IBSD) were followed in depositing SmCo films. In FTS technique, SmCo films were directly deposited on silicon substrates by simultaneous sputtering of samarium and cobalt targets facing each other. This sputtering geometry enabled to achieve films with a wide composition range of 55 – 95 at. % of cobalt in single deposition. The resulting composition variation and material property variation were investigated in terms of process parameters like pressure, temperature, SubstrateTarget Distance (STD) and InterTarget Distance (ITD). The composition distribution of the films was found to be dependent on the thermalisation distances and the mean free path available during the transport. To explain the process and the composition variation, a simulation model based on Monte Carlo method has been employed. The simulated composition variation trends were in good agreement with that of the experimental observations. IBSD, known for its controlled deposition, was employed to deposit both Cr (as an underlayer) and SmCo films. Cr with close epitaxial match with SmCo induces structural evolution in deposited films. The initial growth conditions were found to play a dominant role in the structural evolution of these Cr films. Hence, initial growth conditions were modified by means of oblique incidence and preferential orientation of (200) plane was obtained. With three different angles of incidence, three different surface orientations of Cr films were achieved. These films were then used as structural templates in the deposition of SmCo films. The influence of parameters like composition, impurities, film thickness, beam energy, ion flux, annealing, angles of incidence and underlayer properties on the structural and magnetic properties of SmCo was studied. The structural evolution of SmCo has been found to depend on the structural orientation of Cr underlayers. This followed the structural relation of SmCo(100)||Cr(110)||Si(100) and SmCo(110)||Cr(100)||Si(100). A mixed surface plane orientation was observed in the case of mixed orientation Cr template. The magnetic coercivities were found to increase from 50 Oe to 5000 Oe with the change in the structure of the deposited films.

State-of-the art Fourier transform holography (FTH) techniques use x-ray magnetic circular dichroism (XMCD) as a contrast mechanism for element-specfi c imaging of magnetic domains. With the soft x-ray Nanoscience beamline at Diamond Light Source in the UK, and the Dragon beamline at the European Synchrotron Radiation Facility (ESRF) in France, the possibility of new methods to study nanostructured magnetic systems has been demonstrated. The ability to record images without the use of lenses, in varying magnetic fi elds and with high spatial resolution down to 30 nm has been used to study in-plane magnetism of 50 nm thin permalloy (NiFe alloy) nanoelements. The holographic technique used extended reference objects rather than conventional pinhole references, which allowed a high flexibility on the direction of magnetisation that is probed. The element specific nature of the imaging, with the additional choice in the directions of magnetisation that are probed has been used to study dipolar interactions in a hard/Ta/soft [Co/Pt]30/Ta/Py multi-layered system. Images of the out-of-plane magnetised domains of [Co/Pt]30 were found to bare strong spatial resemblance to the in-plane domains of the permalloy. The domain structure is thought to be magnetostatically imprinted into permalloy during the growth stage of the lm, where stray elds generated by the adjacent Co/Pt multilayer influence the formation of domains in the permalloy. Strong resemblance between the two layers could be found at remanence within a pristine sample, however the similarities disappear after the sample was exposed to a saturating magnetic field. This disagreed with micromagnetic simulations performed in The Object Oriented MicroMagnetic Framework (OOMMF) program, and an explanation for the observations has been sought in the growth process of the multi-layered fi lm, with conditions that are diffi cult to recreate in the model. Optical holography has been used for preliminary insight into implementing a method of FTH in a reflective geometry at soft x-rays wavelength. With scattering chambers at BESSY II in Germany and at the Stanford Synchrotron Radiation Lightsource (SSRL) in California the possibility of reducing scattered noise in a hologram recorded in a reflective geometry has been investigated. Studies into specular and dif use reflections have been performed optically however the use of extended references alone may alleviate the current problem at x-ray wavelengths which lie in the weak signal given by a reflective point-like reference source.

The continuous phase transition in thin magnetic films and superlattices has been studied using the magneto-optical Kerr effect (MOKE) and polarized neutron scattering (PNR).  It has been shown that the critical behavior of amorphous thin films belonging to the 2D XY universality class can be described within the same theory as crystalline sample. This means that quenched disorder only serves as a marginal perturbation in systems with this symmetry. The connection between interlayer exchange coupling and the observed critical behavior in Fe/V superlattices was explored. The results prove that the origin of unusually high values of the exponent β can be traced to a position dependence of the magnetization at elevated temperatures. The magnetization of the outermost layers within the superlattice shows a more pronounced decrease at lower temperatures, compared to the inner layers, which in turn have a more abrupt decrease in the vicinity of the critical temperature. This translates to a high exponent, especially when the layers are probed by a technique where more weight is given to the layers close to the surface, e.g.MOKE.  The interlayer exchange coupling as a function of spacer thickness and temperature was also studied in its own right. The data was compared to the literature, and a dependence on the thickness of the magnetic layers was concluded. The phase transition in amorphous FeZr/CoZr multilayers, where the magnetization emanates from ferromagnetic proximity effects, was investigated. Even though the determined exponents of the zero-field magnetization, the susceptibility and the critical isotherm did not correspond to any universality class, scaling plots displayed an excellent data collapse. Samples consisting of Fe δ-layers (0.3-1.4 monolayers) embedded in Pd were studied using element-specific resonant x-ray magnetic scattering. The magnetization of the two constituents showed distinctly different temperature dependences.

[Truncated abstract] Although exchange bias was discovered more than four decades ago, a satisfactory understanding of every instance of exchange bias observed in experiment has not yet emerged. Understanding exchange bias is complicated by many factors. For example, details of the antiferromagnet interface structure set up during the initial field cooling, thermal activation processes in the ferromagnet and antiferromagnet, and domain formation and domain wall movement in the antiferromagnet are all important in determining features associated with exchange bias. Two exchange bias systems are investigated in this thesis. One is a disordered system: a single layer Co/CoO film with random interfaces prepared by a reactive RF sputtering technique. The other is a ‘model’ system of Fe/KFeF 3 bilayers with compensated interfaces prepared by molecular beam epitaxy (MBE). The central theme of this work is to understand exchange bias and other related magnetic properties in these two very different systems. The Co/CoO exchange bias system studied here is different in structure from conventional exchange bias systems such as bilayer and multilayer systems where interfaces between ferromagnet and antiferromagnet are reasonably well defined. In this Co/CoO system, the Co and CoO is in the form of particles distributed randomly in a sputtered film. The interfaces between the Co and CoO are randomly distributed and may not be continuous over a large length scale. More importantly, the interface area is dependent on the shape and size of the particles and on their distribution. Many unique magnetic properties are related to the random interface in this system. For example, exchange bias and coercivity obtained at low temperatures are very large due to the large interface area between Co and CoO particles. The interface area can be controlled by changing the Co/CoO mass ratio in the film. Unlike in bilayer systems, film thickness in this single layer Co/CoO system turns out not to be critical for exchange bias and coercivity. The independence of film thickness may be technically important. More interestingly, because the interface is random, exchange bias can be setup by field cooling in any direction. Both training and magnetic viscosity effects were studied and provided evidence of thermal activation processes in this Co/CoO system. Training is explained as formation of a domain wall in the CoO with motion limited locally due to limited continuity of Synthesis and Characterization of Magnetic Thin Films - Exchange Bias Systems interfaces between the Co and CoO. Specific magnetization measurements over time were made and studied using viscosity theory. The magnetic viscosity was found to be strongly temperature dependent. There is a broad distribution of blocking temperatures which might be due to a broad distribution of Co particle sizes