Highly ordered magnetic materials with high perpendicular magnetic anisotropy (PMA), such as the L10 ordered FePt, and new recording technologies, such as bit patterned media (BPM), have been proposed as solutions to the media trilemma problem and provide promising strategies towards future high-density magnetic data storage media. L10 ordered FePt thin films can provide the necessary high PMA. However, the ordering of this material perpendicular to the plane of the films remains challenging since high-temperature and time-consuming processes are required. In this work, a remote plasma sputtering system has been used for the investigation of FePt thin films in order to understand if the greater control of process parameters offered by this system can lead to enhanced ordering in L10 FePt thin films at low temperatures compared with conventional dc magnetron approaches. More specifically, the effect of the different substrate temperatures and the target bias voltages on the ordering, the microstructure and the magnetic properties of FePt thin films was investigated. Highly ordered FePt thin films were successfully fabricated after post-annealing processes and were patterned into arrays of FePt islands. This patterning process was carried out with e-beam lithography and ion milling. Initial MFM measurements of these islands showed their single-domain structure for all the island sizes, which indicated the high PMA of the FePt. Magnetometry measurements were also carried out with a novel polar magneto-optical Kerr effect (MOKE) system which was designed and built during this project. This system has unique capabilities which are: a) the application of uniform magnetic field up to 2 Tesla, b) the rotation of the field to an arbitrary angle and c) the use of lasers of four different wavelengths. The combination of these abilities enabled measurements on ordered FePt thin films and patterned media which can pave the way for further highly sensitive measurements on magnetic thin films and nanostructures.

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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.

The static and dynamic properties of spin distributions within domain walls(DWs) confined by Permalloy nanowire conduits are investigated by numerical simulations and high-speed magneto-optic polarimetry. Phase boundaries and critical points associated with DW spin distributions of various topologies are accurately determined using high-performance computing resources. Field-driven mobility curves that characterize DW propagation velocities in 20 nm thick nanowires are calculated with increasing the width of nanowires. Beyond the simple one-dimensional solution, the simulations reveal the four distinct dynamic modes. Oscillations of the field-driven DW velocity in Permalloy nanowires are observed above the Walker breakdown condition using high-speed magneto-optic polarimetry. A one-dimensional analytical model and numerical simulations of DW motion and spin dynamics are used to interpret the experimental data. Velocity oscillations are shown to be much more sensitive to properties of the DW guide structure (which also affect DW mobility) than the DW spin precessional frequency, which is a local property of the material. Transverse bias field effects on field-driven DW velocity are studied experimentally and numerically. DW velocities and spin configurations are determined as functions of longitudinal drive field, transverse bias field, and nanowire width. For a nanowire that supports vortex wall structures, factor of ten enhancements of the DW velocity are observed above the critical longitudinal drive-field (that marks the onset of oscillatory DW motion) when a transverse bias field is applied. The bias-field enhancement of DW velocity is explained by numerical simulations of the spin distribution and dynamics within a propagating DW that reveal dynamic stabilization of coupled vortex structures and suppression of oscillatory motion in the nanowire conduit resulting in uniform DW motion at high speed. Current-driven and current-assisted field-driven domain wall dynamics in ferromagnetic nanowires have thermal effects resulting from Joule heating, which make difficult to separate the spin-torque effects on DW displacements. To understand the thermal effects on DW dynamics, the temperature dependence of field-driven DW velocity is explored using high-bandwidth scanning Kerr polarimetry. Walker critical fields are decreased with increasing temperature and temperature-induced dynamic mode changes are observed. The results show that Joule heating effects are playing an important role in current-driven/current-assisted field-driven DW dynamics. / text

Mon travail de thèse concerne l'étude des propriétés magnétiques des alliages de cobalt et de gadolinium (CoGd) ainsi que l'étude de la transition de réorientation de spin (SRT) dans les films ultra minces de cobalt. Les alliages entre terres rares et métaux de transition CoGd possèdent des propriétés magnétiques uniques. Bien étudiés à l'état massif, ils ont connu récemment un regain d'intérêt, en particulier pour l'étude de la dynamique ultrarapide de l'aimantation ainsi que pour tester les mécanismes du transfert de spin. La diversité des propriétés magnétiques observées dans des films minces de ces alliages est liée à une connaissance incomplète de la structure de ces échantillons. J'ai développée au cours de ma thèse une chambre de dépôt en ultra vide pour produire des couches minces d'alliage par coévaporation. J'ai montré comment, en utilisant plusieurs techniques complémentaires qui vont de l'effet Kerr magnéto-optique au dichroïsme magnétique en absorption et de l'EXAFS à la spectroscopie des photoélectrons, j'ai pu corréler les propriétés structurales et magnétiques des films. Une étude des propriétés magnétiques en fonction de la température d'un échantillon présentant une inhomogénéité latérale de composition illustre les potentiels ouverts par ces films dont le comportement est reproduit par un calcul de champ moyen. Les films ultra minces Pt/Co(5Å)/Pt présentent une anisotropie perpendiculaire au plan de la couche. En augmentant la température, l'anisotropie diminue jusqu'à une valeur critique pour laquelle elle ne permet plus de contrebalancer les effets dipolaires tendant à ramener l'aimantation dans le plan de la couche. Ce passage d'une aimantation hors du plan à une aimantation dans le plan s'appelle la transition de réorientation de spin (SRT). Dans les couches ultra minces Pt/Co (5Å)/Pt il est possible de contrôler les propriétés magnétiques grâce à un processus d'irradiation par des ions He et ainsi d'obtenir un échantillon présentant une SRT au voisinage de la température ambiante, permettant une observation plus aisée. Dans cette partie j'ai exposé les résultats expérimentaux, obtenus essentiellement par microscopie et magnétométrie Kerr, permettant d'identifier le mécanisme impliqué dans la SRT et de mettre en avant le rôle des fluctuations thermiques. L'étude se conclue par une comparaison des résultats expérimentaux aux modèles théoriques disponibles.

Increased complexity of novel magnetic materials in the last decade has placed high demands on the manufacturing process as well as on the characterization. One of the possibilities for characterization of magnetic samples is to exploit the magneto-optical effects. The presented work uses the magneto-optical Kerr effect as a major characterization technique to probe the magnetic properties of samples. We have developed a mathematical model describing the effect of the magnetization on the polarized light and present an apparatus capable of measuring the response given by the light-matter interaction. The experimental results show the performance of the apparatus on the various magnetic systems including meta-stable iron layers, Stoner-Wohlfarth particles and magnetic vortices. The scanning vectorial Kerr magnetometer allowed us to probe the vector of magnetization with diffraction limited resolution below 500 nm.

<p>In the first part of this work, the influence of hydrogen on the structural and magnetic properties of Fe/V(001) superlattices was studied. The local structure of the vanadium-hydride layers was determined by extended x-ray absorption fine structure (EXAFS) measurements. The magnetic ordering in a weakly coupled Fe/V(001) superlattice was investigated using the magneto-optical Kerr effect (MOKE). The interlayer exchange coupling is weakened upon alloying with hydrogen and a phase with short-range magnetic order was observed.</p><p>The second part is concerned with first-principles calculations of magnetic materials, with a focus on magnetic recording applications. The uniaxial magnetic anisotropy energy (MAE) of Fe, Co, and Ni was calculated for tetragonal and trigonal structures. Based on an analysis of the electronic states of tetragonal Fe and Co at the center of the Brillouin zone, tetragonal Fe-Co alloys were proposed as a material that combines a large uniaxial MAE with a large saturation magnetization. This was confirmed by experimental studies on (Fe,Co)/Pt superlattices. The large uniaxial MAE of L1₀ FePt is caused by the large spin-orbit interaction on the Pt sites in connection with a strong hybridization between Fe and Pt. Furthermore, it was shown that the uniaxial MAE can be increased by alloying the Fe sublattice with Mn. The combination of the high-moment rare-earth (RE) metals with the high-<i>T</i>_C 3<i>d</i> transition metals in RE/Cr/Fe multilayers (RE = Gd, Tb, Dy) gives rise to a strong ferromagnetic effective exchange interaction between the Fe layers and the RE layer. The MAE of hcp Gd was found to have two principal contributions, namely the dipole interaction of the large localized 4<i>f</i> spins and the band electron magnetic anisotropy due to the spin-orbit interaction. The peculiar temperature dependence of the easy axis of magnetization was reproduced on a qualitative level.</p>

Due to the ever-increasing worldwide consumption of memory for digital information, new technologies for higher capacity and faster data storage systems have been the focus of research and development. A step towards achieving higher data storage densities or magnetic recording media is the concept of bit patterned media, where the magnetic recording layer is divided up into magnetically isolated bit units. This approach is one of the most promising technologies for increasing data storage densities and could be implemented by nanostructuring the wafer. Therefore, the fabrication of the appropriate nanostructures on a small scale and then be able to manufacture these structures on an industrial scale is one of the problems where science and industry are working on a solution. In addition, the answer to the open question about the influence that patterning on the nano length scale has on the magnetic properties is of great interest. The main goal of this thesis is to answer the open question, which magnetic properties can be tailored by a modification of the surface texture on the nanometre length scale. For this purpose the following properties: anisotropy, remanence, coercivity, switching field distribution, saturation magnetisation, Gilbert damping, and inhomogeneous linebroadening were compared between planar two dimensional thin ferromagnetic films and three dimensional magnetic structures. In addition, the influences of the tailored morphology on the intergranular or the exchange coupling between the structures, which is called interdot exchange coupling, was investigated. For the ferromagnetic thin films, the focus of the investigations was on the granular CoCrPt:SiO2 and [Co/Pd] layer, which currently are the state-of-the-art material for magnetic data storage media. These materials are characterised by their high coercivity and high perpendicular anisotropy, which has a low spatial distribution in the preferred direction of magnetisation. In this work the pre-structured GaSb(001) substrate with self-assembled periodic nanocone structures at the surface are used. The preparation by ion beam erosion of these structures is simple, fast, and highly reproducible and therefore this method is particularly beneficial for fundamental research. To compare the 2D thin films with the 3D magnetic structures, besides the pre-structured specimen, planar samples were also fabricated. The first sample series prepared was coated by Py. Due to the fact that the magnetic properties of this material are well-known, it was also possible to do some OOMMF simulations in addition to the VNA-FMR and MOKE measurements. Afterwards two planar samples with CoCrPt and CoCrPt:SiO2 were prepared. The planar CoCrPt:SiO2 samples were Co+ ion implanted to study the influence of such irradiation on the intergranular and interdot exchange coupling, switching field distribution, and in particular on the spin dynamics. Moreover, both samples were measured by TRMOKE in order to obtain information about the spin dynamics. Subsequently, the perpendicular storage media materials CoCrPt:SiO2 and [Co/Pd] were deposited on a prestructured GaSb(001) nanocone substrate surface. These sample series were measured by MOKE, SQUID, and vector-VSM. The measurements demonstrate the influence of the periodicity and height of the nanocones on the intergranular and interdot exchange coupling. They also show the reorientation of the magnetisation with respect to the curvature of the substrate template and furthermore, the morphology-induced influences on the magnetic domains. From the comparison between the results for the planar and the pre-structured samples, a decrease of the interdot exchange coupling was observed, which scales together with the periodicity of the nanocone pattern. In addition, it was shown that for all samples with thin magnetic films on nanocones,the magnetisation aligns along the curvature of the underlying nanocone structure. For Py on nanocones, planar granular CoCrPt:SiO2, and planar granular CoCrPt, measurements by VNA-FMR and TRMOKE could be carried out, which yielded information about the spin dynamics. The results obtained for both of the planar sample are comparable to values from the literature for the Gilbert damping. The results for the Py samples showed that the commonly used 2D model resonance condition is, in case of a 3D magnetic structure, no longer valid due to the alignment of the magnetisation along the underlying substrate structure and therefore an new model has to be derived. / Aufgrund des weltweiten, immer weiter steigenden Bedarfs an Speicherplatz von digitalen Information, sind neue Technologien für größere und schnellere Speichermedien im Fokus von Forschung und Entwicklung. Ein Schritt hin zu einer höheren Speicherdichte in der magnetischen Datenspeicherung ist dabei das sogenannte Konzept der ”Bit patterned media”, das definierte Informationseinheiten auf regelmäßig angeordneten Nanostrukturen beschreibt. Dieser Ansatz ist einer der derzeit vielversprechendsten Optionen die Speicherdichte zu erhöhen. Dabei ist die Herstellung der benötigten Nanostrukturen und deren Skalierung hin zu makroskopischen Dimensionen eines der Probleme an deren Lösung die Wissenschaft und Industrie derzeit arbeitet. Desweiteren ist die Antwort auf die noch offene Frage nach der Beeinflussung der nanoskaligen Strukturen auf die magnetischen Eigenschaften von großem Interesse. Das Hauptziel in dieser Arbeit ist es, einen Beitrag zur Beantwortung der Frage, welche magnetischen Eigenschaften sich durch eine Veränderung der Oberflächenstruktur im Nanometerbereich beeinflussen lassen, zu leisten. Hierzu wurden die folgenden Eigenschaften, wie zum Beispiel die Anisotropie, Remanenz,Koerzitivität, Schaltfeldverteilung, Sättigungsmagnetisierung, Gilbertdämpfung und inhomogene Linienverbreiterung von planaren zweidimensionalen dünnen ferromagnetische Schichten mit denen von dreidimensionalen magnetischen Strukturen verglichen. Zusätzlich wurde der Einfluss der angegpassten Morphologie auf die intergranularen- beziehungsweise auf die zwischen den Strukturen wirkende (interdot) Austauschkopplung untersucht. Der Hauptaugenmerk bei den ferromagnetisch dünnen Schichten lag dabei auf den granularen CoCrPt:SiO2 und [Co/Pd] Filmen, die heutzutage ein Standardmaterial für die magnetischen Speichermedien darstellen. Diese Materialien zeichnen sich durch eine hohe Koerzivität und senkrechte Anisotropie, mit geringer räumlicher Verteilung der Vorzugsrichtung der Magnetisierung, aus. Die hier vorgestellten vorstrukturierten GaSb(001) Substrate mit selbstordnenden periodischen Nanokegeln auf der Oberfläche, sind mittels Ionenstrahlerosion einfach, schnell und sehr gut reproduzierbar herzustellen. Deshalb ist diese Methode besonders für die Grundlagenforschung von Vorteil. Um einen Vergleich zwischen 2D Filmen und 3D Strukturen ziehen zu können, wurden neben den vorstrukturierten Substraten auch planare Proben beschichtet. Eine erste Versuchsreihe wurde mit einem dünnen Py Film präpariert. Da dessen magnetische Eigenschaften wohlbekannt sind, konnten neben den Untersuchungen mit VNA-FMR und MOKE auch einige OOMF Simulationen erstellt werden. Danach wurden zwei Proben mit planarem CoCrPt beziehungsweise CoCrPt:SiO2 untersucht. Bei den planaren CoCrPt:SiO2 Proben wurden außerdem noch Co+ Ionen implantiert, um deren Auswirkungen auf die intergranulare Austauschkopplung, Schaltfeldverteilung und besonders auf die Spindynamik zu bestimmen. Bei beiden Probensystemen konnte zusätzlich die Spindynamik mittels zeitaufgelöstem MOKE gemessen werden. Im Anschluss wurden die beiden senkrechten Speichermedien CoCrPt:SiO2 and [Co/Pd] auf Substraten mit Nanokegeln vorstrukturierten GaSb(001) Oberflächen abgeschieden. Diese Proben wurden mit MFM, MOKE, SQUID und Vektor-VSM vermessen. Aus den Messungen konnnten dann die Einflüsse auf die intergranulare- beziehungsweise interdot Austauschkopplung in Abhängigkeit von der Periodizität und Höhe der Nanokegel bestimmt werden, sowie die Umorientierung der Magnetisierung bezüglich der Substratkrümmung und den Morphologie induzierten Einfluss auf die magnetischen Domänen. Anhand der Vergleiche zwischen den Messungen der planaren und den vorstrukturierten Proben konnte eine Verringerung der Austauschkopplung zwischen den Strukturen gezeigt werden, die mit der Nanokegelstrukturperiodizität skaliert. Außerdem wurde in allen dünnen magnetischen Filmen auf Nanokegeln gezeigt, dass die Magnetisierung sich in Abhängigkeit der darunterliegenden Struktur ausrichtet. Bei den Py auf Nanokegeln, den planaren CoCrPt und dem planaren CoCrPt:SiO2 Proben konnten außerdem mit VNA-FMR und TRMOKE Informationen bezüglich der Spindynamik gemessen werden. Die erzielten Ergebnisse, der beiden planaren Proben, sind vergleichbar mit denen, aus der Literatur bekannten Werten, für die Gilbertdämpfung. Darüber hinaus wurde durch die Messungen an den Py Proben gezeigt, dass die Theorie, des bisher genutzten 2D Modells, nicht mehr gültig ist, da sich die Magnetisierung entlang der Substratstruktur ausrichtet, und deshalb ein neues Model aufgestellt werden muss.

In the first part of this work, the influence of hydrogen on the structural and magnetic properties of Fe/V(001) superlattices was studied. The local structure of the vanadium-hydride layers was determined by extended x-ray absorption fine structure (EXAFS) measurements. The magnetic ordering in a weakly coupled Fe/V(001) superlattice was investigated using the magneto-optical Kerr effect (MOKE). The interlayer exchange coupling is weakened upon alloying with hydrogen and a phase with short-range magnetic order was observed. The second part is concerned with first-principles calculations of magnetic materials, with a focus on magnetic recording applications. The uniaxial magnetic anisotropy energy (MAE) of Fe, Co, and Ni was calculated for tetragonal and trigonal structures. Based on an analysis of the electronic states of tetragonal Fe and Co at the center of the Brillouin zone, tetragonal Fe-Co alloys were proposed as a material that combines a large uniaxial MAE with a large saturation magnetization. This was confirmed by experimental studies on (Fe,Co)/Pt superlattices. The large uniaxial MAE of L10 FePt is caused by the large spin-orbit interaction on the Pt sites in connection with a strong hybridization between Fe and Pt. Furthermore, it was shown that the uniaxial MAE can be increased by alloying the Fe sublattice with Mn. The combination of the high-moment rare-earth (RE) metals with the high-TC 3d transition metals in RE/Cr/Fe multilayers (RE = Gd, Tb, Dy) gives rise to a strong ferromagnetic effective exchange interaction between the Fe layers and the RE layer. The MAE of hcp Gd was found to have two principal contributions, namely the dipole interaction of the large localized 4f spins and the band electron magnetic anisotropy due to the spin-orbit interaction. The peculiar temperature dependence of the easy axis of magnetization was reproduced on a qualitative level.

In this thesis the magnetic properties of novel amorphous magnetic materials grown on a flexible substrate of polyethylene naphthalate and a silicon wafer have been analyzed and characterized. The analyzed films are two films of amorphous Cobalt-Iron-Zirconium(Co36Fe53Zr11 &amp; Co37Fe55Zr8) grown on the flexible substrate and two films of amorphous (Fe89Zr11) doped with boron (B). The B is implanted in a lattice of rings with inner diameter of 10 μm and outer diameter of 20 μm and with the distance between the center of the rings of either 50 μm or 25 μm. The composition in the doped region is Fe80Zr10B10. Various magneto-optical Kerr effect(MOKE) magnetometers are used to measure hysteresis loops of the samples and a superconducting quantum interference device (SQUID) is used to find the volume magnetization of the flexible samples. To measure the anisotropy in the flexible films a series of sample holders has been developed to measure various amount of stress using the same sample in magneto-optical magnetometers. The stress induced uniaxial anisotropy is found by measuring hysteresis loops of the flexible samples while bending them with different curvatures. The induced anisotropy is related to the magnetostriction and the magnetostriction constants is estimated for the two flexible samples by assuming values for Young’s modulus and Poisson’s ratio. The estimated values for the magnetostriction constant are found to vary with the amount of Zr and to be in the correct order of magnitude for magnetic films. The implanted B rings with the short distance of 25 μm between the center showed to have some interaction between the rings. This conclusion is drawn after analyzing first order reversal curves of the samples and looking at the domains under a MOKE-microscope. At very low temperatures the (unimplanted) FeZr matrix is ferromagnetic and seem to have an anti-ferromagnetic coupling with the B rings. At room temperature the rings are still ferromagnetic and they couple to each other.