Cette thèse avait pour objectif d'augmenter la fréquence de travail d'un multicouche magnéto-diélectrique pour des applications des Nouvelles Technologies de l'Information et de la Communication (NTIC). Ainsi, deux types de structures ont été étudiés : des multicouches (CoO=CoFeB)n et des tricouches Py/Ru/Py. Dans les empilements (CoO=CoFeB)n, la montée en fréquence est assurée par une anisotropie de surface du CoFeB induite par une rugosité orientée à la surface de la couche CoO. Cette rugosité est générée par la géométrie de dépôt. La fréquence de résonance de ce système est ajustable sur toute la gamme de fréquence des NTIC par le choix des épaisseurs de CoO et de CoFeB. Ces propriétés magnétiques sont modélisées en ajoutant à l'anisotropie intrinsèque du CoFeB un terme démagnétisant. Celui-ci est calculé à partir des observations de la surface de la couche CoO par microscopie à force atomique. Les propriétés magnétiques obtenues sur le bicouche sont maintenues dans le cas d'un multicouche, montrant que la rugosité est peu affectée par l'empilement. Dans les tricouches Py/Ru/Py, le terme s'ajoutant à l'anisotropie intrinsèque du Py est induit par le couplage des deux couches de Py via les électrons de conduction de la couche de Ru (couplage RKKY). Selon les échantillons, le terme de couplage antiferromagnétique ou quadratique est prépondérant. La modélisation du comportement statique permet de quantifier ces termes de couplage. La modélisation du comportement dynamique prédit les deux fréquences de résonance caractéristiques observées expérimentalement. / The aim of this thesis was to increase the working frequency of a magneto-dielectric multilayer for ICT applications. Two structures were studied : (CoO=CoFeB)n multilayers and Py/Ru/Py trilayer. In (CoO=CoFeB)n stacks, the CoFeB resonance frequency is increased thanks to a surface anisotropy induced by the CoO oriented roughness. This roughness is generated by the deposition geometry. The resonance frequency of this system is adjustable over the entire ICT frequency range by choosing the CoO and CoFeB thicknesses. These magnetic properties are simulated by adding a demagnetizing term to the CoFeB intrinsic volume anisotropy. This term is calculated from AFM observations of CoO surface. The magnetic properties of the bilayer are not degraded in multilayers because the roughness is poorly affected by the stacking. In trilayer Py/Ru/Py, the term added to the Py intrinsic anisotropy is induced by the coupling of the two Py layers via the conduction electrons of Ru (RKKY coupling). Depending on the samples, the quadratic or antiferromagnetic coupling term is dominant. The hysteresis loop fitting leads to the coupling terms values. The dynamic properties calculus predicts the two resonance frequencies experimentally observed.

In this thesis multidomain states in magnetically ordered systems with competing long-range and short range interactions are under consideration. In particular, in antiferromagnetically coupled multilayers with perpendicular anisotropy unusual multidomain textures can be stabilized due to a close competition between long-range demagnetization fields and short-range interlayer exchange coupling. These spatially inhomogeneous magnetic textures of regular multidomain configurations and irregular networks of topological defects as well as complex magnetization reversal processes are described in the frame of the phenomenological theory of magnetic domains. Using a modified model of stripe domains it is theoretically shown that the competition between dipolar coupling and antiferromagnetic interlayer exchange coupling causes an instability of ferromagnetically ordered multidomain states and results in three possible ground states: ferromagnetic multidomain state, antiferromagnetic homogeneous and antiferromagnetic multidomain states. The presented theory allows qualitatively to define the area of existence for each of these states depending on geometrical and material parameters of multilayers. In antiferromagnetically coupled superlattices with perpendicular anisotropy an applied magnetic bias field stabilizes specific multidomain states, so-called metamagnetic domains. A phenomenological theory developed in this thesis allows to derive the equilibrium sizes of metamagnetic stripe and bubble domains as functions of the antiferromagnetic exchange, the magnetic bias field, and the geometrical parameters of the multilayer. The magnetic phase diagram includes three different types of metamagnetic domain states, namely multidomains in the surface layer and in internal layers, and also mixed multidomain states may arise. Qualitative and quantitative analysis of step-like magnetization reversal shows a good agreement between the theory and experiment. Analytical equations have been derived for the stray field components of these multidomain states in perpendicular multilayer systems. In particular, closed expressions for stray fields in the case of ferromagnetic and antiferromagnetic stripes are presented. The theoretical approach provides a basis for the analysis of magnetic force microscopy (MFM) images from this class of nanomagnetic systems. Peculiarities of the MFM contrast have been calculated for realistic tip models. These characteristic features in the MFM signals can be employed for the investigations of the different multidomain modes. The methods developed for stripe-like magnetic domains are employed to calculate magnetization processes in twinned microstructures of ferromagnetic shape-memory materials. The remarkable phenomenon of giant magnetic field induced strain transformations in such ferromagnetic shape memory alloys as Ni-Mn-Ga, Ni-Mn-Al, or Fe-Pd arises as an interplay of two physical effects: (i) A martensitic transition creating competing phases, i.e. crystallographic domains or variants, which are crystallographically equivalent but have different orientation. (ii) High uniaxial magnetocrystalline anisotropy that pins the magnetization vectors along certain directions of these martensite variants. Then, an applied magnetic field can drive a microstructural transformation by which the martensitic twins, i.e. the different crystallographic domains, are redistributed in the martensitic state. Within the phenomenological (micromagnetic) theory the equilibrium parameters of multivariant stripe patterns have been derived as functions of the applied field for an extended single-crystalline plate. The calculated magnetic phase diagram allows to give a detailed description of the magnetic field-driven martensitic twin rearrangement in single crystals of magnetic shape-memory alloys. The analysis reveals the crucial role of preformed twins and of the dipolar stray-field energy for the magnetic-field driven transformation process in magnetic shape-memory materials. This work has been done in close collaboration with a group of experimentalists from Institute of Metallic Materials of IFW Dresden, Germany and San Jose Research Center of Hitachi Global Storage Technologies, United States. Comparisons between theoretical and experimental data from this cooperation are presented throughout this thesis as vital part of my work on these different subjects.

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The subject of this thesis is to study the interaction between carbon nanotubes (CNTs) and antiferromagnetic tetrametallic molecules attached to them. By employing quantum transport measurements, the sensitivity to sense the interactions is greatly increased, because the quantum dot is very susceptible to changes in its environment. The properties of carbon nanotubes can be altered by chemical functionalization with the aforementioned molecules, where the attachment is performed covalently via a ligand exchange with the CNT. The thesis is partitioned into two main parts: the first part presents experiments performed on tetramanganese functionalized CNTs, whereas for the second similar studies are conducted, except manganese is replaced by cobalt. Both complexes exhibit an antiferromagnetic ground state, yet the metal spin of manganese (S=5/2) is reduced to S=3/2 for cobalt. Additionally, an altered device preparation has been employed during the second part, leading to a strong suppression of the background signal. Quantum transport measurements at T=4K on manganese-functionalized CNTs show a very regular pattern of Coulomb diamonds, indicating only a mild disturbance of the quantum dot's electron system by the covalent bond. Moreover, the charging energy reveals a wave function extending over the entire device dimensions. However, at T=30mK in the tunneling current a strong noise emerges, when repeatedly measuring over an hour while keeping external biases constant. Additionally, these time traces are superimposed by a long-term background, which is removed by a correction algorithm plus a subsequent digitization. The remaining signal reveals a random telegraph signal (RTS) which is extensively studied and from its statistics the equivalent temperature of T=654mK for the excitation of the system is extracted. The quantum transport experiments conducted on cobalt-functionalized CNTs show a much better data quality of the coulomb diamonds, which is ascribed to the alteration in the device's preparation. From the line shape of the Coulomb oscillations as well as from the Coulomb staircases an electron temperature of about T=500mK is extracted. Moreover, a magnetic field dependence of the stability diagrams is apparent, attributable to Zeeman splitting. The respective Landé factor of g=1.73 is, compared to similar CNT quantum dot systems, unusually low. It is as attributed to an increased spin-orbit interaction between the conduction electrons and the cobalt's nuclei. The respective time traces exhibit or lack an RTS signal, depending on their external biases. Regarding the Coulomb diamonds, an essential prerequisite for the occurrence of an RTS is the proximity to a resonance, which is equatable to a high sensitivity of the quantum dot detector. Considering the available energy, the underlying process that is the cause for the emergence of the RTS is ascertained to be an internal excitation of the antiferromagnetic states of the metallic core.

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In dieser Dissertation wird die gute Einstellbarkeit von synthetischen Antiferromagneten mit dem kollektiven Surface Spin-Flop-Verhalten kombiniert. Es wird der Einfluss der Gesamtschichtdicke untersucht, welche mit dem Abstand der magnetische Oberflächen korreliert. Zudem werden die Dicken der ferromagnetischen Untereinheiten an den Außenseiten verändert, womit die Beiträge der Oberflächen unterdrückt oder verstärkt werden können. Darauffolgend wird die Kontrolle der Oberflächenbeiträge angewendet, um Exchange Bias-Strukturen auf Basis synthetischer Antiferromagnete zu erzeugen. Da diese nicht aus Heterostrukturen intrinsischer Antiferromagnete und Ferromagnete bestehen, wird nicht nur eine gute Abstimmbarkeit erreicht, sondern auch die Materialwahl wird potentiell vereinfacht. Zudem kann der Exchange Bias in synthetischen Antiferromagneten vollständig bei Raumtemperatur beobachtet und gesteuert werden. Im Weiteren wird ein zuvor untersuchtes Konzept zur Stabilisierung der vom Surface Spin-Flop erzeugten vertikalen antiferromagnetischen Domänenwände erweitert. Es wird demonstriert, wie ein Paar koexistierender antiferromagnetischer Domänenwände in Abwesenheit äußerer Magnetfelder und bei tiefen Temperaturen stabil gehalten werden kann. Damit können in Erweiterung der ursprünglichen Konzeptes nun acht anstatt sechs remanenter Zustände durch geeignete Magnetfeldroutinen eingestellt werden.:1. Introduction 2. Theoretical background 2.1. Micromagnetic energy terms 2.1.1. Zeeman energy 2.1.2. Demagnetization energy 2.1.3. Anisotropy energy 2.1.4. Exchange energy 2.2. Magnetic multilayers 2.2.1. Magnetic anisotropy in magnetic multilayers 2.2.2. Synthetic antiferromagnets 2.3. Exchange Bias 2.4. The bulk and surface spin-flop 3. Methods 3.1. Sputter deposition 3.2. X-ray diffraction and reflectometry 3.3. Magnetometry 3.4. Magnetic force microscopy 3.5. Micromagnetic simulations 4. Results 4.1. From collective reversal to exchange bias 4.1.1. Total thickness dependency of the surface spin flop 4.1.2. Influence of the surface block thickness 4.1.3. Exchange bias in synthetic antiferromagnets 4.2. Tailoring the surface spin flop 4.2.1. Coexistence of two vertical domain walls 4.2.2. Alternative anisotropy profile 5. Conclusions and Outlook A. Supplemental material A.1. Supplemental material for Section 2.4 A.2. Supplemental material for Section 4.1.1 A.3. Supplemental material for Section 4.1.2 A.4. Supplemental material for Section 4.1.3 A.5. Supplemental material for Section 4.2.1 A.6. Supplemental material for Section 4.2.2 A.7. Supplemental material for the outlook in Chapter 5 A.7.1. Synthetic ferrimagnets ans ferromagnetic resonance A.7.2. Synthetic antiferromagnets based on Co/Ni A.7.3. Initial magneto-resistance measurements A.8. Micromagnetic simulations MuMax3 code B. Bibliography C. List of Samples D. Selbstständigkeitserklärung E. Danksagung F. Lebenslauf G. Publikationsliste / In this thesis, the high degree of tunability in the SAFs is combined with the collective surface spin-flop reversal. The influence of the total thickness and thus the distance of the magnetic surfaces is explored. Furthermore, the thickness of the ferromagnetic surface subunits is altered to selectively suppress or enhance the surface contribution. The control of the surface contribution is subsequently employed to create magnetic exchange bias structures based on the synthetic antiferromagnets. If compared to conventional exchange bias systems in heterostructures of intrinsic antiferromagnetic and ferromagnetic materials, an exchange bias with full room temperature operation, high tunability and a potential potential much more flexible choice of materials becomes available. Additionally, a previously established concept to stabilize the vertical antiferromagnetic domain walls that originate from the surface spin-flop at remanence is extended to a coexisting pair of antiferromagnetic domain walls. At low temperatures, the coexisting vertical antiferromagnetic domain walls can be stabilized at remanence, too. Furthermore, the total number of different remanent states, which are accessible through different field routines, can be increased from six in the original concept to eight in the more sophisticated concept presented here.:1. Introduction 2. Theoretical background 2.1. Micromagnetic energy terms 2.1.1. Zeeman energy 2.1.2. Demagnetization energy 2.1.3. Anisotropy energy 2.1.4. Exchange energy 2.2. Magnetic multilayers 2.2.1. Magnetic anisotropy in magnetic multilayers 2.2.2. Synthetic antiferromagnets 2.3. Exchange Bias 2.4. The bulk and surface spin-flop 3. Methods 3.1. Sputter deposition 3.2. X-ray diffraction and reflectometry 3.3. Magnetometry 3.4. Magnetic force microscopy 3.5. Micromagnetic simulations 4. Results 4.1. From collective reversal to exchange bias 4.1.1. Total thickness dependency of the surface spin flop 4.1.2. Influence of the surface block thickness 4.1.3. Exchange bias in synthetic antiferromagnets 4.2. Tailoring the surface spin flop 4.2.1. Coexistence of two vertical domain walls 4.2.2. Alternative anisotropy profile 5. Conclusions and Outlook A. Supplemental material A.1. Supplemental material for Section 2.4 A.2. Supplemental material for Section 4.1.1 A.3. Supplemental material for Section 4.1.2 A.4. Supplemental material for Section 4.1.3 A.5. Supplemental material for Section 4.2.1 A.6. Supplemental material for Section 4.2.2 A.7. Supplemental material for the outlook in Chapter 5 A.7.1. Synthetic ferrimagnets ans ferromagnetic resonance A.7.2. Synthetic antiferromagnets based on Co/Ni A.7.3. Initial magneto-resistance measurements A.8. Micromagnetic simulations MuMax3 code B. Bibliography C. List of Samples D. Selbstständigkeitserklärung E. Danksagung F. Lebenslauf G. Publikationsliste