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Chiral magnetism has gained enormous interest in recent years because of the anticipated wealth of applications in nanoelectronics. The demonstrated stabilization of chiral magnetic domain walls and skyrmions has been attributed to the actively investigated Dzyaloshinskii-Moriya interaction. Recently, however, predictions were made that suggest dipolar interactions can also stabilize chiral domain walls and skyrmions, but direct experimental evidence has been lacking. Here we show that dipolar interactions can indeed stabilize chiral domain walls by directly imaging the magnetic domain walls using scanning electron microscopy with polarization analysis. We further show that the competition between the Dzyaloshinskii-Moriya and dipolar interactions can reverse the domain-wall chirality. Finally, we suggest that this competition can be tailored by a Ruderman-Kittel-Kasuya-Yosida interaction. Our work therefore reveals that dipolar interactions play a key role in the stabilization of chiral spin textures. This insight will open up new routes towards balancing interactions for the stabilization of chiral magnetism.

Recent experimental studies of magnetic domain expansion under easy-axis drive fields in materials with a perpendicular magnetic anisotropy have shown that the domain wall velocity is asymmetric as a function of an external in plane magnetic field. This is understood as a consequence of the inversion asymmetry of the system, yielding a finite chiral Dzyaloshinskii-Moriya interaction. Numerous attempts have been made to explain these observations using creep theory, but, in doing so, these have not included all contributions to the domain wall energy or have introduced additional free parameters. In this article we present a theory for creep motion of chiral domain walls in the creep regime that includes the most important contributions to the domain-wall energy and does not introduce new free parameters beyond the usual parameters that are included in the micromagnetic energy. Furthermore, we present experimental measurements of domain wall velocities as a function of in-plane field that are well decribed by our model, and from which material properties such as the strength of the Dzyaloshinskii-Moriya interaction and the demagnetization field are extracted., 6 pages, 4 figures, Supplemental Material (15 pages, 9 figures)

The frequency difference between two oppositely propagating spin waves can be used to probe several interesting magnetic properties, such as the Dzyaloshinkii-Moriya interaction (DMI). Propagating spin wave spectroscopy is a technique that is very sensitive to this frequency difference. Here we show several elements that are important to optimize devices for such a measurement. We demonstrate that for wide magnetic strips there is a need for de-embedding. Additionally, for these wide strips there is a large parasitic antenna-antenna coupling that obfuscates any spin wave transmission signal, which is remedied by moving to smaller strips. The conventional antenna design excites spin waves with two different wave vectors. As the magnetic layers become thinner, the resulting resonances move closer together and become very difficult to disentangle. In the last part we therefore propose and verify a new antenna design that excites spin waves with only one wave vector. We suggest to use this antenna design to measure the DMI in thin magnetic layers., 12 pages, 4 figures

This erratum includes three modifications to the original paper:2 • The units of RDW (y-axis) in Fig. 4 (in main text), Fig. S3 (in supplementary material), and Fig. S4 (in supplementary material) should be X instead of mX. • At the end of the right-top paragraph of page 3, a reference to the supplementary material should be added. .shunting through the Pt layer (see supplementary material). • The following section should be added to the supplementary material.

Focused-electron-beam-induced deposition (FEBID) is employed to create freestanding magnetic nanostructures. By growing Fe nanopillars on top of a perpendicular magnetic domain wall (DW) conduit, pinning of the DWs is observed due to the stray fields emanating from the nanopillar. Furthermore, a different DW pinning behavior is observed between the up and down magnetic states of the pillar, allowing to deduce the switching fields of the pillar in a novel way. The implications of these results are two-fold: not only can 3-dimensional nano-objects be used to control DW motion in applications, it is also proposed that DW motion is a unique tool to probe the magnetic properties of nano-objects.

The world as we know it today would be completely different without spintronics. It has revolutionized the way we carry, store and exchange information in our daily lives. What is it? It is a research realm that combines the fundamental property of the electron, spin, and the charge property driving conventional electronics, hence: spin-tronics. Spintronics has until recently concentrated mainly on the manipulation of chargecurrent through the control of the magnetic state. Currently, a new paradigm has emerged that reverses this idea, basically by applying Newton’s law of action implies reaction. Instead of manipulating the charge-current by the magnetic state, it manipulates the magnetic state by a spin-polarized current, and it is gradually gaining interest. Spin-polarized current induced motion of a magnetic domain wall is the subject of this thesis.We try to use a spin polarized current to push a magnetic domain wall. The interaction between spin-current and the magnetic domain wall is still an open area for exploration; there are still many unanswered questions on the fundamental physics that brings it about. The prospect of new data storage, memory and even bio-related devices makes it a very lively and competitive research topic possibly relevant in shaping our future world. We chose perpendicularly magnetized devices as our material, and for a reason. In this class of materials the magnetic domain walls are very narrow; and this, was our assumption, should increase the interaction between the spin polarized current and the magnetization. Our research has made use of a great variety of experimental and nano-fabrication techniques. Since the nano-fabrication techniques were particularly new to our research group, they have been given ample attention in chapter 2. The material used in our research are perpendicularly magnetized ultrathin Co(FeB) layers (

Recently there has been much interest in combining the fields of organic electronics and spintronics. This has been motivated by the fact that low atomic mass of organic materials are predicted to have long spin lifetimes. Also, spintronic devices could benefit from the chemical tunability, ease of fabrication, and mechanical flexibility of organic semiconductors. The nascent field of organic spintronics has already presented many new phenomena which must be explained with novel physics, here we explore one of these phenomena, organic magnetoresistance (OMAR). OMAR is a room temperature spintronic effect in organic devices without any magnetic materials. OMAR is a large change in resistance (up to 25%) at low magnetic fields ( 20mT). OMAR represents a scientific puzzle since no traditional magnetoresistance mechanisms can explain the combination of properties listed above. Another one of the remarkable properties of OMAR is that the sign of the MR can change based operating conditions of the device, like temperature and voltage. In this dissertation we focused in particular on resolving the origin of the sign change since understanding this unique property should be a major step in unraveling the microscopic origin of OMAR. We have explored the properties of the sign change experimentally with bipolar semiconducting small molecule and polymer devices, in which we observed sign changes as functions of voltage and temperature. These devices showed a strong correlation between the sign change and the onset of minority charge carrier injection and we could describe the lineshape and MR(V) behavior as a superposition of two MR effects of opposite sign. From this work we concluded the separate MR effects were from the mobilities of holes and electrons having different responses to magnetic fields, which is best described by the bipolaron model for OMAR. To test this conclusion, we employed analytical and numerical device models assigning separate magnetomobilities to holes and electrons. The models show, counter-intuitively, that in the case when the minority charge carrier contact is injection limited, a decrease in minority charge carrier mobility increases the current. This is a result of the minority carrier contact acting like a constant current source, and of the compensation of the majority carrier space charge by the oppositely charged minority carriers. We show that these models describe the observed MR(V) behavior very well, and if one assumes the magnetic field acts to reduce the mobility of electrons and holes, we observe that our models can reproduce all the sign changes observed in literature. The device model also predicts how different device parameters affect the observed MR, to test its predictions we performed experiments in which we increased the charge recombination by dye doping the organic active layer, we also observed how changing the charge injection by altering the organic semiconductor/ metal contacts experimentally compared with the device model. The fact that the current can increase when the minority carrier mobility decreases may explain the fact that in experiments the magnitude of the negative MR features has been much larger than the positive MR features, even though, microscopically, the bipolaron model predicts the opposite. Therefore, the presence of both signs of magnetoresistance may be related only to the device physics and not to the microscopic mechanism which causes OMAR.

A specific class of plastics and other organic materials is (semi)conducting and can be used for electronic applications. In this thesis, we investigated their use in certain applications using magnetic fields.These applications belong to the field of spin electronics, or ‘spintronics’, where next to the charge of the electrons, also their spin is utilized.This spin can be considered as a small magnetic moment, pointing either up or down. Organic semiconductors have several advantages that make them interesting for applications in spintronics.They are relatively cheap, are easy to process, and can be chemically adapted. Two different, but related, topics that combine organic materials with spintronics have been studied both experimentally and theoretically. The first topic is the recently discovered change in the current through an organic layer when a small magnetic field is applied. This effect is called organic magnetoresistance (OMAR). Large changes in the current (10–20%) have been found at relatively small magnetic fields (~10mT) and at room temperature.These properties make the effect interesting for applications like, for instance, cheap magnetic field sensors, opening the possibility to add magnetic functionality to existing organic electronic applications. Moreover, if its mechanisms are understood, the effect could be used to investigate processes in conventional organic electronic devices by studying their magnetic response. OMAR is observed in a wide range of organic materials, from small molecules to polymers. It is believed that OMAR originates from the interactions of a pair of charge carriers (for instance, electron–electron, hole–hole, or electron–hole).More specifically, it is believed that the possible interactions depend on the relative orientation of the spins of these two charges. Without an external magnetic field, small intrinsic magnetic fields in the organic layer (resulting from hyperfine coupling to nuclear spins) randomize the orientations of the two spins. This allows a change froma spin configuration that is less favorable for the current into amore favorable configuration. However, applying amagnetic field larger than these hyperfine fields results in a strong reduction of this spin randomization or spin mixing, causing a pair to remain locked in a less favorable spin configuration. Although there is agreement on the crucial role of hyperfine fields, the exact mechanisms behind OMAR are still heavily debated. Several models were proposed in literature that explain OMAR in terms of different charge pairs. In this thesis, we investigated a model based on pairs of equal carriers, called the bipolaron model. We used an elementary model of two neighboring sites, where, depending on the spins, one carrier might be preventing another one to pass. With this theoretical model we were able to successfully reproduce several characteristics of OMAR. Both a decrease and an increase in current, as found in experiments, could be obtained and also the universal shapes of the experimental OMAR curves could be reproduced. Additionally, we performed new types of experiments to gain better understanding of OMAR. We showed that when an oscillating magnetic field is applied,OMAR is reduced beyond a certain frequency threshold.This occurs when the slowest charges can no longer follow the oscillations, as we showed by measuring the frequency dependence of the capacitance. These findings are in agreement with recent interpretations in which these slowest carriers are expected to induce the largest OMAR effect. In literature, it was claimed that OMAR is independent of the orientation of the magnetic field. However, via sensitive measurements we demonstrated a small but systematic dependence on the angle between the magnetic field and the sample. We showed theoretically that this angle dependence can be explained in the different models by including an interaction between the spins. This interaction has to be direction dependent in order to explain the angle dependence.We identified dipole–dipole coupling or an anisotropy in the hyperfine fields as themost likely candidates. Furthermore,we outlined a first exploration of an alternative approach to describe OMAR curves. We introduced a function that allows us to extract information both about the hyperfine fields and about an additional broadening of the curves. Thereby, this approach could allow for amore quantitative analysis of changes in the OMAR curves resulting from changes in the operating conditions or the material properties. In the second topic, the spin of electrons is used in a different way. In many spintronics applications a difference between the number of spin-up and spin-down electrons, called spin polarization, is used to transport information through a device. For the functioning, it is essential that this polarization persists while transporting the charges.The main mechanism for loss of polarization inmost inorganic semiconductors, which is related to spin–orbit coupling, is negligible in organic materials. The absence of this loss mechanismmakes organic materials ideal candidates for these types of spintronics applications. However, there might still be other mechanisms that cause a smaller but non-zero loss of spin polarization. We conjecture that the hyperfine fields are the main source of polarization loss in organic materials, which results from mixing between the spin-up and spin-down electrons by precession of spins about these random fields. We theoretically investigated this effect of the hyperfine fields on the spin polarization. We explicitly included the hopping transport characteristic for organic semiconductors. Due to spatial and energetic disorder, the charges hop from one localized site to another. The longer the time they spend on a site, the larger the loss of spin polarization. We showed that an external magnetic field larger than the typical hyperfine-field strength reduces the loss of spin polarization. Hence, such an external field causes the polarization to persist over a larger distance, leading to an increase of the spin-diffusion length.We thus found a magnetic-field dependent spin-diffusion length. In addition, we found the spin-diffusion length to depend only weakly on temperature. A spintronics device that makes use of spin-polarized transport is the spin valve. Using the magnetic-field dependent spin-diffusion length obtained from our theory, we could very accurately fit experimental data on the magnetoresistance of organic spin valves reported in literature. However, there is still a hot debate on the interpretation of these and similar experiments.The question has been raised whether spins are indeed transported through the whole layer, or only through thin regions. A discriminating experiment would be the manipulation of spins during their transport through the organic layer.This can be done by applying a magnetic field perpendicular to the direction of the spin polarization. Using our spin-transport model, we made predictions about the results that can be expected from such an experiment. We showed that, in the case of transport through the organic layer, an effect of a perpendicular field should be observable, but that the strong oscillations in the signal that are typically seen in inorganic semiconductors will be absent. Finally, as an extension of the work presented in this thesis, we made predictions about possible future experiments in which spin polarization is combined with OMAR. Because for a spin-polarized current the majority of the spins point in the same direction, most charge pairs will have parallel spins.Therefore, within the bipolaron model, we expect an increase in the magnitude of OMAR when the injected current is spin polarized. Moreover, we showed that the shapes of the OMAR curves will also be changed.These experiments would provide a means to both prove spin-polarized transport and to validate the bipolaron model. In conclusion, both theoretical and experimental results on OMAR and spin polarized transport have been presented in this thesis. Contributions have been made to a new model for OMAR and new type of experiments have been performed that have added further insights to the puzzle of OMAR. The limiting role of the hyperfine fields on spin-polarized transport has been investigated theoretically, providing an explanation for the experimentally observed magnetoresistance curves of organic spin valves and providing suggestions for future experiments. Although the present work has led to better understanding of OMAR and spin-polarized transport, the field of organic spintronics still poses many theoretical and experimental challenges that should be resolved before a widespread emergence of organicspintronics applications will occur.

In the field of metallic and magnetic nanostructures, scanning tunneling microscopy (STM) is nowadays a powerful technique to detect and manipulate nano-objects that are located at or close to the surface of metallic systems. In this research area, we have used STM and scanning tunneling spectroscopy (STS) in order to: (1) manipulate Co atoms incorporated in a Cu(001) surface, potentially leading to formation of stable nanostructures "chapter 3", (2) investigate the mechanism of motion of surfaceembedded atoms, as well as the effect of tip-surface interaction and tip-velocity on the motion of embedded atoms, including experimental demonstrations "chapter 4", (3) study the possibility to detect subsurface argon-filled cavities buried several nanometers in a metallic substrate, including the effect of anisotropic propagation of electrons "chapter 5", and (4) measure and manipulate the conductance properties of triangular Co nano-islands deposited on Cu(111) "chapter 6". Below we will give a further explanation, including a brief overview of the obtained results. Chapter 3 covers the use of STM to study the tip-induced movement of Co atoms in a diluted Co/Cu(001) surface alloy. By varying the sample temperature from 4K up to room temperature, we measured that the threshold temperature at which an incorporated Co atom can be moved is approximately 150K. We propose that a vacancy-mediated mechanism is responsible for the observed movement, in which vacancies in the tip area exchange with an embedded Co atom. Finally we demonstrated for the first time a controlled positioning of single embedded Co atoms in a Cu(001) surface. Related to this in chapter 4, we introduced a microscopic model that successfully describes STM-tip induced motion of surface embedded atoms. Using these calculations, we have investigated the behavior of the system as a function of tip-sample interaction and velocity at which the tip drags atoms along. From the velocity dependence of trace-lengths achieved for Co atoms embedded in a Cu(001)surface, evidence is given for a combined pulling-pushing mode when comparing the calculations with experimental data. We have demonstrated that the tip-induced vacancy model is a most probable candidate to explain recent experiments on moving Co embedded atoms through a Cu(001) surface. To detect nanoscaled objects buried several nanometers below the surface in chapter 5, we used argon-filled nanocavities embedded in a Cu(001) crystal. These nanocavities are capable of reflecting electrons and to induce a localized quantum well between the nanocavity and the atomically flat Cu(001) surface. We have focused on mapping the spatial variation of conductance at the surface above the nanocavity using STM/STS. Our dI/dV spectra showed a quasi-periodic oscillation of electron density versus energy in a point above the nanocavity, from which the depth of the nanocavity can be estimated. We developed a simple electron interference model that is able to describe reasonably all the experimental details, including the possibility to extract a rough estimation of the relevant parameters that govern the shape and depth of the nanocavity. Due to rich details in our observations and the simplicity of our model, this work could open up new opportunities for a more detailed and systematic analysis of buried nano-objects. In chapter 6, we demonstrated the manipulation of electronic properties of triangular Co islands on a Cu(111) surface upon adsorption as well as desorption of adsorbates. When adsorbates are present on the surface of the islands, the surface states were shifted in energy by approximately 200 meV. Using current provided via the STM tip, electrons can stimulate the desorption of the adsorbates from the Co island, which can be clearly recorded by STS by looking at the shift of the surface state towards its original energy value. The recorded differential conductance maps showed the surface of the islands before and after removing the adsorbates. The structure of the adsorbates was resolved as well, which formed a 2×2 reconstruction. A similar behavior is demonstrated in our research group by intentionally introducing H2 to the system, by which the surface state is shifted to a lower energy. Furthermore, the removal of the hydrogen from the islands is done by the STM tip leading to a full recovery of the surface state of the Co islands.

Until recently, mainstream electronics was exclusively based on charge proper- ties. Apart from charge, an electron also possesses an intrinsic angular momen- tum (spin), and directly coupled to that a magnetic moment. A newly emerg- ing approach is to use this extra spin degree of freedom to provide additional performance and functionality in widely used commercial applications, although nowadays mostly in metallic structures. Semiconductors are of great interest for future spin-based electronics because of the predicted longer spin coherence. The ability to electrically inject, manipulate, and detect spin polarized carriers within the active semiconductor are essential components from the integration point of view for such a technology. Transport of spin polarized carriers is commonly demonstrated by a speci¯c resistance change as function of the external applied magnetic ¯eld (magnetoresistance). The realization of all electrical spin injection and detection in an inorganic (GaAs) and in an organic (Alq3) semiconductor will be the focus of this thesis. Besides, observed large magnetoresistance e®ects in silicon from di®erent origin will be extensively discussed. Before we address the experimental aspects of the hybrid semiconductor de- vices, a detailed theoretical analysis has been performed to the feasibility of all electrical spin injection and detection in semiconductors by means of ferro- magnetic electrodes and including spin-selective interface barriers to overcome the impedance mismatch. Based on the Poisson and the di®usion equation, in- cluding electric ¯eld e®ects, the expected resistance di®erence for parallel and anti-parallel magnetization alignment of the ferromagnetic electrodes has been analytically calculated. The in°uence of the sample and measurement geom- etry has been extensively investigated and the results will be used within our experimental design analysis. Moreover, we propose a new lateral measurement geometry for which the magnetoresistance closely approaches the magnetoresis- tance value obtained for a vertical device. Electric ¯elds created in di®erent regions via separate bias voltages minimizes the spin loss in the semiconductor side branches of the lateral device. Our ¯rst experimental device consists of electron-beam-lithography processed ferromagnetic electrodes crossing a GaAs transport channel. Although silicon is the industrially most relevant semiconductor, GaAs has been chosen for its di- rect band gap, which enables optical investigations of the spin polarization in our group. Control over the magnetization alignment of the electrodes is established via an external applied magnetic ¯eld. Di®erent switching ¯elds for di®erent electrode widths have been measured by magnetic force microscopy. Towards observation of magnetoresistance due to spin transport, we show that the dop- ing pro¯le under the ferromagnetic contacts and in the semiconductor transport channel is of critical importance for the e®ective detection and depolarization of the carriers. Organic semiconductors pro¯t from their °exibility and the ease of process- ing. While these synthetic organic materials are exploited for their tunability of charge-carrier transport properties, their spin transport properties form a less explored area. Spin transport through a hybrid Al2O3(1.5 nm)/Alq3 barrier has been studied as a function of the thickness of the organic (0-6 nm) layer at room temperature. From the current-voltage behavior we conclude that for hybrid bar- riers with more than 2 nm Alq3, transport by multiple step tunneling dominates over transport by direct tunneling. Because of weak spin-orbit interaction at the intermediate molecular site and a possible low hopping rate, spin should be sensitive to small local ¯elds, in particular the total hyper¯ne ¯eld originating from the local hydrogen nuclei. We suggest that in the regime of multiple step tunneling, hyper¯ne coupling could be a relevant source of spin randomization and could modify the characteristic hysteretic magnetoresistance by a symmetric contribution. A possible ¯ngerprint is indeed observed in our experiment. Silicon holds exceptional promise for spin-based electronics, by virtue of its compatibility with the current CMOS technology. As a possible implicit contri- bution to future silicon-based spintronics devices or as a magnetoresistive sensor, boron-doped Si-SiO2-Al structures are fabricated to study extremely large mag- netoresistance e®ects. Current-voltage characteristics show a strong non-linear behavior, dominated by an autocatalytic process of impact ionization. At low temperatures, the magnetic ¯eld postpones the onset of impact ionization to higher electric ¯elds. This results in a symmetric positive magnetoresistance of over 10,000% at magnetic ¯elds of 400 kA/m. A macroscopic transport model is introduced that is able to describe how the magnetoresistance is controlled by voltage, electrode spacing and oxide thickness. Utilizing that the e®ect of a magnetic ¯eld on an acceptor wave function becomes stronger with increas- ing distance from the impurity, we present a room temperature constant voltage magnetoresistance of over 1000% at 1 MA/m in low doped silicon.

The rapidly growing field of spintronics has recently attracted much attention. Spintronics is electronics in which the spin degree of freedom has been added to conventional chargebased electronic devices. A magnetic tunnel junction (MTJ) is an example of a spintronic device. MTJs consist of two ferromagnetic layers separated by a thin insulating barrier. The tunnel current that flows through the barrier depends on the relative alignment of the magnetization in the ferromagnetic layers. As a consequence of this dependence, the MTJ exhibits two different resistance values that distinguish a logical ‘0’ and a logical ‘1’, corresponding to anti-parallel and parallel magnetization. Due to these two distinct states, the MTJs can be used as magnetic memory elements, and serve as bits for information storage in magnetic random access memories (MRAMs). MRAMs can lead to instant-on computers and longer battery lifetimes for mobile devices, which gives MRAMs the potential to replace the current RAM technologies. However, for the current RAM technologies to be replaced by MRAMs, the dimensions of the MTJs have to decrease to sub - 100 nm in order to achieve a high enough areal density and to match the semiconductor technology. Therefore, the research in this Thesis aims at fabricating sub - 100 nm MTJs and investigating the influence of the reduced dimensions on the modification of magnetic and electronic properties. The MTJ is incorporated in an engineered multilayer stack to promote stability and reproducibility of the magnetic and electric response of the MTJ. For the structuring of these layers, we have used top-down nanofabrication techniques to produce the sub - 100 nm MTJs. For MTJs with a surface area of less than 0.01 µm2, the Al2O3 barrier has an approximate thickness of a nanometer to ensure an appropriate tunnel current. Therefore, we have concentrated on the plasm oxidation of sub -nm thin Al layers to produce Al2O3 barriers. We have shown that over-oxidation of sub -nm thin Al2O3 barriers of MTJs can be observed in real-time using in situ differential ellipsometry measurements. The change in ellipsometry signal of Al layers grown on CoFe films, is proportional to the amount of oxidized metallic material. As a result, the derivative of this signal is a direct measure of the oxidation rate. Further analysis of this oxidation rate allowed us to determine the onset of the CoFe oxidation. We have found the onset to be proportional to the deposited Al layer thickness. The amount of CoO determined from in situ X-ray Photoelectron Spectroscopy data on identical samples is found to be proportional to that obtained from ellipsometry. In short, this means that the point in time on which over-oxidation starts can be precisely determined. This is a critical necessity in producing exact, well-functioning MTJs. We have used electron beam (EB) lithography to pattern the sub - 100 nm features. With EB lithography features of only a few nanometer in size can be defined, hence EB lithography enables the exploration of the fundamental boundaries of magnetic and electric scaling properties. For the structuring of large-area samples with ultra-dense arrays of sub - 100 nm MTJs the throughput is limited because of the sequential writing process. However, with the developed special high-speed EB writing strategy we could pattern a sample area of 16 mm2 with ultra-dense arrays of sub - 100 nm elliptical features in 10 minutes. The strategy is employed to define the pattern of hard Ta masks with sub - 100 nm features. The Ta masks are etched at -50 ¿C in a SF6/O2 plasma to an etch depth that can be controlled with nanometer precision. Ar+ ion beam milling is used to transfer the pattern and to produce dense arrays of sub - µm MTJs. Insight in the magnetic switching behavior of nanoscale MTJs as a function of the size, shape and thickness is vital for MRAM application. Especially, the collective properties of high areal density arrays of MTJs are of interest, because magnetostatic coupling mechanisms between elements can be a limiting factor for applications. In order to understand the effects of geometry and coupling mechanisms, the switching of 5 nm thick polycrystalline nanoscale Co dots is examined using SQUID measurements and the switching of sub - µm MTJs is studied with MOKE measurements. An array consists of approximately 108 elements with a width ranging from 50 to 300 nm, and a length to width aspect ratios of 1.5 to 2.5, arranged on a rectangular lattice. The measured switching fields of the Co dots and MTJs were low compared to predictions using the Stoner-Wohlfarth model. The deviations of the Stoner-Wohlfarth behavior could be explained in term of interdot coupling and edge roughness. Comparison with the outcome of OOMMF simulation of the switching of a single dot revealed that the interdot coupling has a major influence on the magnetic switching behavior of arrays of nanoscale magnetic elements. This implies that for the feasibility of ultra-high areal density arrays of nanoscale MTJs for information storage new strategies are needed. For example, a more complex toggle MTJ multilayer stacks can be used for the nanostructuring of sub - µm MTJs. Faceting of the etch mask due to physical sputtering of the mask material is a problem during deep etching of ultra-high bit density arrays of sub - µm MTJs for MRAMs. Besides that, chlorinated etch residues can reduce the magnetization of patterning magnetic materials substantially, and therefore constitutes a considerable concern. To get more insight into the magnetization losses, CoFeB dots were etched in a high ion density Cl2- based plasma with a width ranging from 0.3 to 6.4 µm. The magnetic properties of the CoFeB dots were measured by SQUID magnetometry. The sub-µm CoFeB dots showed significant magnetization reductions, despite H2O rinsing. Scanning electron microscopy (SEM) studies revealed that etching in a Cl2-based plasma caused faceting of the masks, leading to sloped sidewalls. SEM pictures were used to determine the geometric volume which was compared to the effective magnetic volume resulting from the magnetometry measurements. The SEM data are in good agreement with the magnetometry data, and a chloride penetration depth of only a few nanometer could be derived, indicating that the postetch rinsing is sufficient to prevent considerable corrosion of the CoFeB dots. This means that the chlorinated etch residues could be removed from the samples, without severely effecting the magnetic properties. The I-V characteristics of nanometer thin AlOx barriers are measured by applying a voltage over a 1×1 µm2 square MTJ pillar located at the cross-point of the bottom and top electrode. The I-V response of the MTJs showed in principle three different characteristics, that is an ohmic like response, a response resembling breakdown, and a tunneling response. Approximately 30 % of the measured 32 MTJs showed a tunneling response and had a resistance-area product between approximately 20 and 50 k µm2. Furthermore, the conductance showed roughly a parabolic behavior implying an asymmetrical barrier. As a consequence, we used the Brinkman formula to fit the experimental data. The fits yielded average barrier heights between 1.2 and 2.3 eV, and an asymmetry parameter ranging from 0.3 to 0.8 eV, which are close to reported observations in literature. Resistance measurements yielded no significant magnetoresistance for the 1 µm2 MTJs. Probably, as a consequence of interface roughnesses, a strong N´eel coupling originates, through which an independent switching of the two magnetic layers of the MTJ is hindered. As an alternative electrical characterization technique, the conductive atomic force microscopy (c-AFM) technique can used to measure the local electrical transport properties of nanoscale MTJs. We have explored this c-AFM technique and performed I-V measurements by applying a constant bias voltage to the bottom electrode and measuring the tunnel current through the barrier. However, due to resist remains no significant bias voltage dependency was observed.

This thesis describes the results of a feasibility study on the use of the magnetic semiconductor europium sulfide (EuS) as a so-called "spin filter". A spin filter is a tunnel barrier with a barrier height that depends on the spin orientation of the tunneling electron. A whole myriad of applications exists for such a layer, in particular in the field of spintronics. Spintronics is an extension of current electronics, in which not only the electron charge but also its spin is exploited. This allows for many new applications, such as nonvolatile fast memories and reprogrammable logic, which is a type of electronics that can change functionality if a magnetic field is applied. For many applications, however, it is necessary to inject a current that consists of unequal amounts of spin-up and spin-down electrons into a semiconductor. In order to create such a current, a high and spin-dependent resistance is needed. A spin filter meets these requirements and therefore constitutes a challenging, but highly interesting solution. In this thesis several properties of spin filters will be treated that are of importance for their further development and implementation in electronics. An optimization study for a EuS spin filter tunnel barrier is presented and the magnetic properties of a spin filter will be discussed. Special attention is given to magnetic coupling effects: Both coupling between a spin filter and a neighboring magnetic electrode as well as coupling between two spin filter layers that are only separated by a thin nonmagnetic spacer layer are treated. Moreover, we will describe how a magnetoresistance resulting from two such spin filters, in a polarizer - analyzer fashion, depends on their resistance and on the spacer layer. The thesis is organized as follows. First, chapter 2 addresses the theory that is at the basics of the physics that is described in this thesis. This chapter is followed by a brief description in chapter 3 of the experimental techniques that were used. In the subsequent chapters the aforementioned issues, regarding the application of EuS as a spin filter, are addressed. Chapter 4 reports on the results of an optimization study on sputter-grown EuS tunnel barriers. Barriers grown at a temperature of 200±C and subsequently annealed at 400-450±C are best, both from magnetic and chemical point of view, as measured by SQUID an XPS, respectively. Combination of EuS as a tunnel barrier with a ferromagnetic electrode in principle leads to a magnetoresistance effect. Magnetoresistance is the dependence of the electrical resistance of a sys- tem on an applied magnetic field. In this case the magnetoresistance is given by the relative change in resistance resulting from a change in the mutual alignment of the magnetizations of the two magnetic layers, which can be modified by the application of a magnetic field. For the practical realization of such a magnetoresistance device aluminum (Al) appears to be most suitable as a nonmagnetic bottom electrode and gadolinium (Gd) appears to be the best magnetic top electrode. This configuration allows for magnetoresistances of more than 100 percent. However, it is not possible to reliably produce such magnetoresistance devices, due to the roughness of the EuS layer. Other combinations of spin filtering EuS with a magnetic electrode only led to small resistance changes as a function of the applied magnetic field (100%) can be obtained, as long as the resistance of the spin filter layer is between one and one hundred times higher than that of the spacer layer. The magnetic switching field of EuS can be adjusted by making use of magnetic interlayer coupling. The magnetic coupling between two EuS layers, separated by a nonmagnetic PbS layer, is discussed in chapter 6. In contrast to what is commonly observed in metallic structures, the coupling is always of antiferromagnetic nature in this semiconductor system. The magnetization of EuS was varied by performing measurements in a range of temperatures up to its Curie temperature. The interlayer coupling energy was found to depend strongly (as a fourth power) on the EuS magnetization. This is explained by the influence of the EuS magnetization on the band structure in the PbS spacer layer. Finally, chapter 7 discusses the effect of the addition of charge carriers on the magnetic properties of a magnetic semiconductor. Europium telluride (EuTe), a material that is chemically comparable to EuS, was n-doped with Gd (substitutional at Eu sites in the lattice). The originally antiferromagnetic EuTe becomes ferromagnetic upon the substitution of 1% of the Eu atoms with Gd, and can reach a Curie temperature of 15 K. This effect can be explained by magnetic coupling of the localized magnetic moments in the material to the free carriers that were added by the Gd doping. The investigated Gd-doped EuTe layers showed, apart from a ferromagnetic phase, also either a paramagnetic or an antiferromagnetic phase, probably due to a nonuniform distribution of the charge carriers in the layer, which could be an interesting subject for further research.

The revolution in (semi)conducting organic materials has been one of the highlights in physics over the past decade. Molecular and polymeric thin films are projected to be used as active elements in a wide range of electronic and optoelectronic applications. Among the main driving forces for such plastic electronics are the low-cost processing and the chemical tunability. Potential applications include ultrathin organic light emitting diodes for (flexible) flat displays, field-effect transistors, sensors and many other. Although intensively studied both in industrial and academic environments, the intrinsic limits of molecular materials is an open issue. Pushing the limits of these materials and devices is a major scientific challenge with enormous implications for the electronics industry. In this thesis we explored novel hybrid molecular - metallic structures with a magnetic functionality for "spintronic" applications. Spintronics is a new branch of electronics in which electron spin, in addition to charge, is manipulated to yield a desired outcome. The spintronic devices are particularly attractive for memory devices (MRAM’s) and magnetic sensors applications. It has been suggested that molecular materials would provide an attractive alternative, not only in view of the general advantages of plastic electronics, but particularly, also because of the intrinsically low spin-orbit scattering due to the low mass of atoms involved. Despite these challenging opportunities, from materials and preparation point of view a large number of issues still have to be solved. Some of them have been addressed in this thesis. One of the crucial requirements for the realization of molecular spintronics is to obtain control over the ordering and morphology of molecular layers. This aspect is generally considered as one of the decisive parameters for achieving molecular electronics with high carrier mobilities. Since spintronic devices are extremely sensitive to the magnetic properties of the outermost atomic layers, our choice was to work with deposition of molecules under ultra-high-vacuum environment, rather than using "wet" techniques under ambient atmosphere. Obtaining ordered organic molecules on ferromagnetic materials represents a challenge, since the high reactivity of these type of substrates tends to decompose molecules, such as happening for thiols, or lowers surface mobility as to hinder molecular ordering. In Chapter 3 of this thesis we investigated the structural properties of two novel molecular - ferromagnetic systems. We demonstrated that long-range ordering of these molecules can be obtained when the substrate is exposed to small amounts of oxygen (Perylene- tetracarboxylic- dianhydride (PTCDA) - Ni(111)) or when a proper molecule-substrate combination is chosen (PTCDA - Co, pentacene - Co, and pentacene - Ni(111)). Very promising is our finding that pentacene tends to grow in an almost layer-by-layer fashion, producing ordered terraces of few ¹m in lateral size even on polycrystalline Co. Another aspect addressed in this thesis is the electronic properties of thin molecular films in bulk and at interfaces with ferromagnetic metals. Proper functionality of the molecular spintronic devices requires appropriate electronic properties. These are determined, on the one hand, by intra-molecular properties such as transport gap, electron affinity, as well as inter-molecular overlap of molecular orbitals. Also the alignment of the energy levels of the molecular systems with respect to the Fermi level of the metal and the nature of interaction at these interfaces play an important role for the charge injection into the molecular films. We analyzed the electronic properties of thin pentacene films and of its interfaces with Co and Ni(111) by means of ultraviolet photoelectron spectroscopy (UPS). We found a difference of 1.4 eV between the ionization potential of the gas phase and the solid state, which we attribute to a change in the local environment and charge redistribution in pentacene. Despite the fact that the ionization potential of pentacene is very close to the work function of the two studied metals, an increased barrier for the hole injection at these two interfaces was found. We attributed these observations to hybridization between molecules and substrate. Besides the production of large area, pinhole-free and well-ordered layers, a strict requirement consists of preventing interdiffusion when depositing top electrodes on a organic film. While this process has been studied extensively for polymer LEDs, the requirements may be even more stringent in the present case, since diffused atoms may act as spin scattering centers. We studied the magnetic properties of Co layers deposited by two different deposition methods (magnetron sputtering and evaporation) on PTCDA. We demonstrated that the presence of the molecular film influences the magnetic properties of Co (such as magnetic moment and switching behavior). This might provide an attractive way of establishing different switching fields for top and bottom electrodes. Moreover, we have evidence that Co particles interdiffuse more strongly into the molecular film when sputter deposition is used instead of evaporation. As a potential application of organic materials in spintronics, we investigated the so-called magnetic tunnel junctions, with a barrier made out of molecular constituents. We have been able to produce a promising magnetoresistance (MR) of 7% at 4 K with junctions based on [2,2’; 6’,2"] terpyridine-4-yloxy-hexanoic acid (TERPY) deposited in UHV conditions, although still hampered by a poor reproducibility, severe interdiffusion and a full quenching of MR above 30 K.

The topic of this thesis is the unbalance of the electron spin in currents, a parameter known as spin polarization. Spin polarization is the most important parameter within the field currently known as spintronics, and is responsible for the high magnetic field-sensitivity of modern magnetoresistive devices. In this thesis two different methods are used to measure and study spin polarization. These two methods are point contact Andreev reflection spectroscopy and spin-polarized tunneling. In both methods a superconductor serves as a detector for the spin polarization. When a bias voltage forces electrons to flow from a normal metal (N) into a superconductor (S), the magnitude of the current is dependent on the spin polarization. This offers the opportunity to deduce the degree of spin polarization directly from the measurement of the current, or, more precisely, from the measurement of the conductance. N and S can be in metallic contact, as is the case in point contact Andreev reflection spectroscopy, or they can be separated by an ultra-thin insulator that serves as a tunnel barrier, as is the case in spin-polarized tunneling. Since the electron transport in N/S point contacts and N/barrier/S tunnel junctions have a different nature, different physical mechanisms are responsible for the spin-dependence of the conductance. In point contacts, the spin-dependence originates from the Andreev reflection process. This is a conduction process in which two electrons with opposite spin are paired into a Cooper pair. In tunnel junctions, the spin-dependence is due to the Zeeman splitting of the spin-up and spin-down state energy in the superconductor induced by an externally applied magnetic field. Point contacts Point contact Andreev reflection spectroscopy is the topic of part I of this thesis. The point contacts are obtained at low temperature by pressing a superconducting tip onto a sample of the normal metal. The conductance-voltage relation measured in case of a highly transparent N/S interface and a high polarization of the current, looks very similar to the one measured when the interface has a relatively low transparency. In chapter 3 it is pointed out that this similarity can potentially lead to a misinterpretation of the experimental data. Specifically, the suppression of the zero-bias conductance due to a low interface transparency may be incorrectly interpreted as proof for a highly spin-polarized current. Anomalous conductance minima at finite bias voltage observed for some contacts can be explained by a phase transition of the contact from the N/S state to the N/N state induced by the current-generated field and heating. For a series of contacts it is observed that the bias voltage at which the phase transition occurs, is systematically dependent on the contact resistance. In chapter 5 it is shown that a simple analysis of such a series can give insight in the microscopic contact geometry. It is shown in chapter 6 that the spin polarization measured with point contact Andreev reflection spectroscopy, decays exponentially with the amount of scattering at the N/S interface. This decay is fully explained by a simple model involving spin-flip scattering in an extended interface region. Tunnel junctions The topic of part II of this thesis is the spin polarization of the tunnel current in S/barrier/N tunnel junctions. The polarization is measured by the well-established spinpolarized tunneling technique, also known as superconducting tunneling spectroscopy. The junctions are prepared by a combination of room-temperature sputter-deposition and plasma oxidation. From the results presented in chapter 10, it is apparent that it is relatively difficult to prepare high-quality junctions with aluminum or vanadium superconducting electrodes deposited on top of the barrier. On the other hand, by depositing a magnesium seed prior to the deposition of aluminium, high-quality junctions with a superconducting aluminum bottom electrode are prepared reproducibly with satisfactory large transition temperature and critical field. The topic of chapter 11 is a recently recognized aspect of spin-polarized tunneling transport, namely the dependence of the spin polarization on the insulator used for the tunnel barrier. To study this dependence, Al/barrier/Co(Fe) junctions are prepared with various barrier materials. A significant difference between the tunneling spin polarization in junctions with AlOx and MgO barriers is observed. With cobalt and iron electrodes, a tunneling spin polarization of roughly +40 % is found with an AlOx barrier and one of roughly +30 % with a MgO barrier. Observations support that this difference is intrinsically determined by the barrier material. With HfOx and TaOx barriers, the Zeeman splitting in the conductance-voltage characteristic is quenched. This is caused by the large spin-orbit scattering rate induced by the heavy hafnium and tantalum atoms that are probably located at the Al/barrier interface. Chapter 12 describes a unique investigation of the role of manganese diffusion in the thermal stability of the tunneling spin polarization. Thermal stability is crucial for successful incorporation of tunnel junction devices into existing semiconductor technology. The diffusion of manganese is widely believed to be responsible for the collapse of the magnetoresistance effect of magnetic tunnel junctions observed above 250 ±C. Tunneling spin polarization is the parameter responsible for the magnetoresistance effect. The thermal stability of this parameter is directly investigated using the spin-polarized tunneling technique. To this end Al/AlOx/Co and Al/AlOx/Co90Fe10 junctions are used both with and without an additional FeMn layer deposited on top of the ferromagnetic electrode. The diffusion of Mn atoms from the FeMn layer to the AlOx barrier interface is confirmed independently with X-ray photoelectron spectroscopy. All the junctions, including their tunneling spin polarization, are found to be intrinsically stable up to 500 ±C, showing that the diffusion of manganese has no observable influence. The tunneling spin polarization, however, is preserved only when annealing is performed under sufficiently clean conditions. Under less clean conditions, it degrades severely above 250 ±C, similar as observed for the tunnel magnetoresistance effect. The degradation of the polarization is attributable to the diffusion of impurities from the environment into the junctions.