The thesis deals with magnetic force microscopy of soft magnetic nanostructures, mainly NiFe nanowires and thin-film elements such as discs. The thesis covers almost all aspects related to this technique - i.e. from preparation of magnetic probes and magnetic nanowires, through the measurement itself to micromagnetic simulations of the investigated samples. We observed the cores of magnetic vortices, tiny objects, both with commercial and our home-coated probes. Even domain walls in nanowires 50 nm in diameter were captured with this technique. We prepared functional probes with various magnetic coatings: hard magnetic Co, CoCr and soft NiFe. Hard probes give better signal, whereas the soft ones are more suitable for the measurement of soft magnetic structures as they do not influence significantly the imaged sample. Our probes are at least comparable with the standard commercial probes. The simulations are in most cases in a good agreement with the measurement and the theory. Further, we present our preliminary results of the probe-sample interaction modelling, which can be exploited for the simulation of magnetic force microscopy image even in the case of probe induced perturbations of the sample.

Spintronics is the study of electron spins and their utilization in electronic devices. Within this field, spin-based oscillators have shown promise for mi- crowave signal generation as they can operate at high frequencies, are small in scale and are compatible with modern fabrication techniques. Among these oscillators are the spin Hall nano-oscillators (SHNOs). They are nanoscale thin-film structures driven by pure spin-current injection from a primary con- ductor into a ferromagnetic material. This process can be used to generate microwave signals through oscillations in the material’s magnetization. By constraining the current flow in the device to individual constrictions, an ar- ray arrangement of multiple oscillators can be realized. These oscillators can then be coupled together via their internal interactions to achieve mutual syn- chronization and improve their characteristics.In this work, a versatile micromagnetic modelling procedure for simulating constriction-based SHNOs and their synchronization dynamics in different ar- ray arrangements is presented. A case study of various 2x2 array geometries is conducted along with an exploration of higher-order networks of 4x4, 6x6 and 8x8 oscillators. A perturbative optimization algorithm is developed to improve excitation conditions and drive geometries into a synchronized regime. Lastly, a comparison to nonlinear auto-oscillator theory is presented to illustrate the dependence of generated signals on constriction sizes and the spacing between oscillators. Mutual synchronization between multiple oscillators is achieved and favourable geometry and excitation conditions are defined. The conducted simulations show good agreement with experimental results and illustrate the potential for future studies of SHNO characteristics through micromagnetic modelling. / Spinntronik är ett forskningsområde, som handlar om hur elektronens s.k. spinn kan användas i elektroniska komponenter. Inom detta område har spinnbaserade oscillatorer visat sig ha lovande egenskaper för generering av mikrovågssignaler, eftersom de har höga arbetsfrekvenser, liten storlek och är kompatibla med moderna tillverkningstekniker. En typ av dessa oscillatorer kallas spinn-Hall nano-oscillatorer (SHNO). De är nanometerstora tunnfilms- strukturer, vilka drivs av en ren spinnström, som injiceras från en (metallisk) ledare till en ett ferromagnetiskt material. Denna mekanism kan användas för att skapa mikrovågssignaler genom oscillationer i materialets magnetisering. Genom att begränsa strömflödet i komponenten till enskilda gap kan man skapa en matris med ett stort antal oscillatorer. Dessa oscillatorer kan sedan kopplas till varandra genom interna utbytesmekanismer och på så sätt uppnår man en ömsesidig koppling och förbättrade egenskaper.I detta arbete presenteras ett mångsidigt mikromagnetiskt modelleringsflö- de, för att simulera SHNO:er, baserade på nano-gap, och deras synkronisering i olika matriskonfigurationer. En fallstudie som inkluderar olika 2x2 matris- geometrier har genomförts tillsammans med explorativ utforskning av högre ordnings nätverk, såsom 4x4, 6x6 och 8x8 oscillatorer. En störnings-baserad optimerings-algoritm har utvecklats för att förbättra exciterings-parametrarna och för att tvinga geometrierna in i en synkroniserad regim. Som en avslutning presenteras en jämförelse med icke-linjär auto-oscillatorteori för att visa den genererade signalens beroende på gapens storlek och avståndet mellan dem. Ömsesidig synkronisering mellan flera oscillatorer kunde uppnås och en för- delaktig geometri samt lämpliga värden på exciterings-parametrarna kunde definieras. Simuleringarna i studien hade bra överensstämmelse med experi- mentella resultat och visar på potentialen för vidare studier av SHNO egen- skaper med hjälp av mikromagnetisk modellering.

No description available.

Die vorliegende Arbeit befasst sich mit der Untersuchung der mikromagnetischen Domänenstruktur und des gekoppelten magneto-strukturellen Phasenübergangs dünner epitaktischer MnAs-Filme auf GaAs. Im Besonderen wird der Einfluss der Substratorientierung, der Filmdicke und eines externen magnetischen Feldes auf die magnetischen und strukturellen Eigenschaften untersucht. Dabei kommen die komplementären Untersuchungsmethoden AFM (atomic force microscopy) / MFM (magnetic force microscopy) und LEEM (low energy electron microscopy) / XMCDPEEM (X-ray magnetic circular dichroism photoemission electron microscopy) zum Einsatz. Im Zuge des Phasenübergangs erster Ordnung zeigen MnAs Filme auf GaAs (001) und (311)A eine regelmäßige Anordnung ferromagnetischer alpha-MnAs und paramagnetischer beta-MnAs Streifen. Die Breite der Streifen ist eine Funktion der Temperatur, während die Periodizität eine lineare Funktion der Filmdicke ist. Die Domänenstruktur hängt stark von der Breite bzw. dem Abstand der ferromagnetischen Streifen ab, da diese direkt die Formanisotropie bzw. die magnetische Kopplung beeinflussen. Die Domänenstrukturen wird, abhängig von der Zahl der Subdomänen entlang der leichten Magnetisierungsrichtung, klassifiziert, wobei bis zu drei elementare Domänentypen beobachtet werden. Bei MnAs-Filmen die auf der GaAs (111)B Oberfläche gewachsen wurden, führt die Epitaxie zu einem geänderten Spannungszustands des Films, wobei eine erhöhte Phasenübergangstemperatur beobachtet wird. Durch temperaturabhängige XMCDPEEM-, AFM- und MFM-Messungen kann gezeigt werden, daß durch den lokalen Abbau der Verspannung in der Nähe eines Risses die Phasenübergangstemperatur lokal erhöht ist. Um Ummagnetisierungsprozesse auf einer mikroskopischen Skala untersuchen zu können und um den Einfluß eines magnetischen Feldes auf die Domänenstruktur sichtbar zu machen, wurde das temperaturvariable Rastersondenmikroskop um einen variablen Magnetfeldaufbau ergänzt. / This work presents the study of the micromagnetic domain structure and the coupled magneto-structural phase transition of epitaxial MnAs thin films on GaAs. In particular, the influence of substrate orientation, film thickness and external magnetic field on the magnetic and structural properties are investigated, employing the complementary measurement techniques atomic force microscopy (AFM) / magnetic force microscopy (MFM) and low energy electron microscopy (LEEM) / X-ray magnetic circular dichroism photoemission electron microscopy (XMCDPEEM. In the course of the first-order phase transition MnAs films on GaAs (001) and (311)A substrates show a regular array of ferromagnetic alpha- and paramagnetic beta-MnAs stripes. The width of the ferromagnetic stripes are a function of the temperature, whereas the periodicity of the stripe pattern is a function of the film thickness. The domain structure strongly depends on the width and the distance of the ferromagnetic stripes, as it directly affects the shape anisotropy and magnetic coupling, respectively. The domain patterns are classified depending on the number of subdomains along the easy axis direction. Up to three basic domain types can be distinguished. For MnAs films grown on GaAs (111)B, the epitaxy leads to a different strain state of the film, resulting in polygonal ferromagnetic structures embedded in a honeycomb-like paramagnetic network, and a higher phase transition temperature. Using temperature-dependent AFM, MFM and XMCDPEEM it is shown that the local strain relaxation in the vicinity of cracks in the MnAs film results in a locally increased phase transition temperature. In order to study magnetization reversal processes on a microscopic scale, as well as the influence of the magnetic field on the domain structure, a variable-magnetic field set-up is employed.

Im Rahmen der Dissertation wurde das Phänomen der Wechselwirkungsdomänen sowohl experimentell als auch unter Zuhilfenahme mikromagnetischer Simulationen untersucht. Gegenstand der Untersuchungen waren einphasige NdFeB-Magnete, die durch Heißpressen und anschließender Warmumformung hergestellt wurden. Zusätzlich wurden über den gleichen Herstellungsweg Kompositproben aus NdFeB und Fe mit unterschiedlichen Partikelausgangsgrößen erhalten und studiert. Korrelationsuntersuchungen verschiedener Messmethoden haben gezeigt, dass im thermisch entmagnetisierten Zustand die Grenzen der Wechselwirkungsdomänen an der Oberfläche größtenteils entlang von Korngrenzen verlaufen. Mittels in-situ MFM wurden erstmalig Untersuchungen von Wechselwirkungsdomänen an massiven NdFeB-Magneten im Magnetfeld durchgeführt. Die Ummagnetisierung erfolgt dabei über die Bewegung der Domänengrenze durch schrittweises Wandern von einer Korngrenze zur benachbarten. Die Beweglichkeit der Domänengrenzen ist durch das Haften an den Korngrenzen gehemmt, was sich in der geringeren Suszeptibilität der warmumgeformten Magnete im Vergleich zu Sintermagneten bemerkbar macht. Aufgrund der eingestellten Mikrostruktur in den warmumgeformten Magneten kann folglich gesagt werden, dass die Ummagnetisierungsprozesse sowohl Merkmale von klassischen Nukleations-, als auch von klassischen Pinningmagneten aufweisen. Mit Hilfe von mikromagnetischen Simulationen konnte eine Eindomänenteilchengröße prismatischer Partikel mit quadratischer Grundfläche ermittelt werden. Außerdem konnte gezeigt werden, dass der Winkel des Streufeldvektors eine entscheidende Rolle bei Ummagnetisiserungsprozessen in solchen Partikeln spielt. Die Superposition des Streufeldvektors mit dem Vektor des angelegten Feldes führt zu einem Gesamtfeldvektor, dessen Winkelabhängigkeit ein Stoner-Wohlfarth ähnliches Verhalten zeigt.

L'auteur n'a pas fourni de résumé en français / Nowadays in medicine and biotechnology a wide range of applications involves magnetic micro/nano-object manipulation including remote control of magnetic beads, trapping of drug vectors, magnetic separation of labelled cells and so on. Handling and positioning magnetic particles and elements functionalized with these particles has greatly benefited from advances in microfabrication. Indeed reduction in size of the magnet while maintaining its field strength increases the field gradient. In this context, arrays made of permanent micromagnets are good candidates for magnetic handling devices. They are autonomous, suitable for integration into complex systems and their magnetic action is restricted to the region of interest.In this thesis we have elaborated an original approach based on AFM and MFM for quantitative study of the magnetic force and associated force gradients induced by TMP micromagnet array on an individual magnetic micro/nano-object. For this purpose, we have fabricated smart MFM probes where a single magnetic (sub)micronic sphere was fixed at the tip apex of a non-magnetic probe thanks to a dual beam FIB/SEM machine equipped with a micromanipulator.Scanning Force Microscopy conducted with such probes, the so-called Magnetic Particle Scanning Force Microscopy (MPSFM) was employed for 3D mapping of TMP micromagnets. This procedure involves two main aspects: (i) the quantification of magnetic interaction between micromagnet array and attached microsphere according to the distance between them and (ii) the complementary information about micromagnet array structure. The main advantage of MPSFM is the use of a probe with known magnetization and magnetic volume that in combination with modelling allows interpreting the results ably.We conducted MPSFM on TMP sample with two types of microparticle probes: with superparamagnetic and NdFeB microspheres. The measurements carried out with superparamagnetic microsphere probes reveal attractive forces (up to few tens of nN) while MFM maps obtained with NdFeB microsphere probes reveal attractive and repulsive forces (up to one hundred of nN) for which the nature of interaction is defined by superposition of microsphere and micromagnet array magnetizations. The derived force and its gradient from MFM measurements are in agreement with experiments on microparticle trapping confirming that the strongest magnetic interaction is observed above the TMP sample interfaces, between the areas with opposite magnetization. Thanks to 3D MFM maps, we demonstrated that intensity of magnetic signal decays fast with the distance and depends on micromagnet array and microsphere properties.Besides the magnetic interaction quantification, we obtained new information relevant to TMP sample structure: we observed and quantified the local magnetic roughness and associated fluctuations, in particular in zones of reversed magnetization. The variation of detected signal can reach the same order of magnitude as the signal above the micromagnet interfaces. These results complete the experiments on particle trapping explaining why magnetic microparticles are captured not only above the interfaces, but also inside the zones of reversed magnetization.Quantitative measurements of the force acting on a single (sub)microsphere associated to the modelling approach improve the understanding of processes involved in handling of magnetic objects in microfluidic devices. This could be employed to optimize the parameters of sorting devices and to define the quantity of magnetic nanoparticles required for labelling of biological cells according to their size. More generally these experimental and modelling approaches of magnetic interaction can meet a high interest in all sorts of applications where a well-known and controlled non-contact interaction is required at micro and nano-scale.

Asymmetrically sandwiched ferromagnetic thin films display a large number of spin-orbit effects, including the Dzyaloschinsii-Moriya interaction (DMI), spin-orbit torques (SOT) and magnetoresistance (MR) effects. Their concurrence promises the implementation of interesting magnetic structures like skyrmions in future memory and logic devices. The complex interplay of various effects originating from the spin-orbit coupling and their dependencies on the microstructural details of the material system mandates a holistic characterization of its properties. In this PhD thesis, a comprehensive study of the spin-orbit effects in a chromium oxide/cobalt/platinum trilayer sample series is presented. The determination of the complete micromagnetic parameter set is based on a developed measurement routine that utilizes quasistatic methods. The unambiguous quantification of all relevant constants is crucial for the modeling of the details of magnetic structures in the system. In this context the necessity of a strict distinction of magnetic objects, that are stabilized by magnetostatics or the DMI, was revealed. Furthermore, a sample layout was developed to allow for the simultaneous quantification of the magnitudes of SOTs and MR effects from nonlinear magnetotransport measurements. In conjunction with a structural characterization, the dominating dependence of the effect magnitudes on microstructural details of the systems is concluded. Precisely characterized systems establish a solid groundwork for further investigations that are needed for viable skyrmion-based devices.

Asymmetrically sandwiched ferromagnetic thin films display a large number of spin-orbit effects, including the Dzyaloschinsii-Moriya interaction (DMI), spin-orbit torques (SOT) and magnetoresistance (MR) effects. Their concurrence promises the implementation of interesting magnetic structures like skyrmions in future memory and logic devices. The complex interplay of various effects originating from the spin-orbit coupling and their dependencies on the microstructural details of the material system mandates a holistic characterization of its properties. In this PhD thesis, a comprehensive study of the spin-orbit effects in a chromium oxide/cobalt/platinum trilayer sample series is presented. The determination of the complete micromagnetic parameter set is based on a developed measurement routine that utilizes quasistatic methods. The unambiguous quantification of all relevant constants is crucial for the modeling of the details of magnetic structures in the system. In this context the necessity of a strict distinction of magnetic objects, that are stabilized by magnetostatics or the DMI, was revealed. Furthermore, a sample layout was developed to allow for the simultaneous quantification of the magnitudes of SOTs and MR effects from nonlinear magnetotransport measurements. In conjunction with a structural characterization, the dominating dependence of the effect magnitudes on microstructural details of the systems is concluded. Precisely characterized systems establish a solid groundwork for further investigations that are needed for viable skyrmion-based devices.:1 Introduction 2 Fundamentals 2.1 Towards new devices 2.2 Spin-orbit effects 2.2.1 Spin-current sources 2.2.2 Magnetoresistanceeffects 2.2.3 Spin-orbit torques 2.2.4 Harmonic analysis 2.3 Micromagnetic model 2.3.1 Dzyaloshinskii-Moriya interaction (DMI) 2.3.2 Consequences of the DMI for magnetic structures 2.3.3 Interface-induced DMI in asymmetrically stacked ferromagnets 2.3.4 Quantification of the interface-induced DMI 2.3.5 Levy-Fert three-site model including roughness 3 The CrOx/Co/Pt sample system 3.1 Experimental techniques 3.2 Structural characterization 4 Complete micromagnetic characterization 4.1 Magnetometry 4.1.1 Static investigation 4.1.2 Ferromagnetic resonance 4.2 DMI quantification 4.2.1 Field-driven domain wall creep motion 4.2.2 Asymmetric domain growth 4.2.3 Winding pair stability 4.3 Determination of the exchange parameter 4.3.1 Generation of circular magnetic objects 4.3.2 Homochiral magnetic bubble domains 4.4 Results 5 Magnetotransport measurements 5.1 Measurement setup 5.2 Magnetoresistance effects 5.3 Spin-orbit torque quantification 5.4 Results 6 Discussion 6.1 Structural predomination of the DMI strength 6.2 Ultra-thin limit exchange parameter reduction 6.3 Magnetotransport properties 6.4 Magneticstructures in //CrOx/Co/Pttrilayers 7 Conclusion and Outlook A Appendix A.1 Calculation of the skyrmion diameter A.2 Micromagnetic simulation of the winding pair stability Bibliography Acknowledgements

Neuromorphic computing applications could be made faster and more power efficient by emulating the function of a biological synapse. Non-conventional spintronic devices have been proposed that demonstrate synaptic behavior through domain wall (DW) driving. In this work, current induced domain wall motion has been studied through micromagnetic simulations. We investigate the synaptic behavior of a head to head domain wall driven by a spin polarized current in permalloy (Py) nanostrips with shape anisotropy, where triangular notches have been modeled to account for edge roughness and provide pinning sites for the domain wall. We seek optimal material parameters to keep the critical current density for driving the domain wall at order 1011 A/m2.

В магистерской диссертации представлены результаты моделирования магнитных гистерезисных свойств ансамбля обменносвязанных частиц. Для выполнения компьютерного моделирования разработан пакет компьютерных программ в среде MATLAB, позволяющий моделировать и анализировать предельные и частные петли магнитного гистерезиса для однофазных и многофазных ансамблей с различными типами магнитной анизотропии, упорядочением фаз, а также при параметрической зависимости микроскопических констант от температуры. Все это реализовано в рамках модели однодоменных нанокристаллитов, имеющих однородную намагниченность, процесс изменения которой осуществляются за счет когерентного вращения. Разработан алгоритм параллельного расчета, позволяющий основные арифметические и логически операции выполнять на графических ускорителях. Полученные с его помощью численные результаты качественно соответствуют экспериментальным данным для системы Nd-Fe-B. / The results of magnetic properties modelling of the ensemble of exchange coupled particles are represented in this master's thesis. The computer modelling is realized in the MATLAB programming environment and allows performing calculations and analysis of major and minor magnetic hysteresis loops of single-phase or multiphase ensembles with different types of magnetic anisotropy, phase arrangement and with the parametric dependency of microscopic constants on temperature. The latter is based on the model of single domain nanocrystallites. Each of them has a uniform magnetization and any changes of one are happened by coherent rotation. The algorithm of parallel calculations using a graphic processing unit is also shown in the work. The obtained numeric results qualitatively are in compliance with the Nd-Fe-B experimental data.