Ferrimagnetic rare-earth-transition-metal heterostructures: implications for future data storage, sensors, and unconventional computing

In this work, different ferrimagnetic rare-earth-transition-metal heterostructures are investigated. The findings provide implications for future data storage, sensor, and unconventional computing devices. In the first part, ferri- and ferromagnetic films are exchange-coupled and studied as potential composite media for magnetic recording technologies. For this, the underlying individual layers are examined, too. Within this study, the influence of Pd and Pt insertion layers in ferromagnetic Co/Ni multilayers is investigated. In these systems, the maximum effective magnetic anisotropy is more than doubled by the introduced insertion layers, while the initial saturation magnetization and Curie temperature are reduced. Further, amorphous Tb-FeCo alloys and multilayers are studied as the second building block of the desired composite medium. In particular, the structural and magnetic properties are analyzed upon post-annealing. At temperatures above 400 K, irreversible effects on the structural properties are found, which also influence the magnetic properties. It is shown that these changes in properties cannot be prevented by tuning the composition or by a multilayer structure of the film. However, key insights on the structural and magnetic properties upon annealing are provided for future high-temperature devices. Afterward, the exchange-coupled ferrimagnetic/ferromagnetic bilayer is studied. Measurements on the dependency on temperature, the ferrimagnetic composition, and the thickness of the ferromagnet are carried out. Two distinct magnetic reversal mechanisms are revealed. The reversal characteristics depend critically on the thickness of the ferromagnetic layer. The underlying microscopic origin is revealed by high-resolution magnetic force microscopy. Above a certain thickness of the ferromagnet, the switching process is driven by in-plane domain wall propagation. In contrast, thinner ferromagnetic layers exhibit a nucleation-dominated reversal due to grain-to-grain variations in magnetic anisotropy. Although the realization of an exchange-coupled composite medium for magnetic recording can not be achieved, insights for the future realization of sub micron high energy density permanent magnets and spintronic devices are gained. In the second part of this work, topologically protected spin structures, including skyrmions and antiskyrmions, are investigated in Fe/Gd-based multilayers. Particularly in coexisting phases, different topologically protected magnetic quasi-particles may show fascinating physics and potential for spintronic devices. While skyrmions are observed in a wide range of materials, until now, antiskyrmions have been exclusive to materials with D2d or S4 symmetry. In this work, first and second-order antiskyrmions are stabilized for the first time by magnetic dipole-dipole interaction. Using Lorentz transmission electron microscopy imaging, coexisting first- and second-order antiskyrmions, Bloch skyrmions, and type-2 bubbles are observed, and the range of material properties and magnetic fields where the different spin objects form and dissipate is determined. The discovered phase pocket of metastable antiskyrmions for low saturation magnetization and uniaxial magnetic anisotropy values is confirmed by micromagnetic simulations and represents a recipe, which has to be satisfied for the stabilization of antiskyrmions by dipole-dipole interaction in other material systems. Furthermore, the nucleation process of the spin objects and the influence of an in-plane magnetic field are studied. Additionally, post-deposition techniques are employed to locally change the anisotropy of the samples and influence the nucleation and stability range of the spin objects. The gained knowledge significantly simplifies future investigations of antiskyrmions. Moreover, the coexisting phases of different topologically protected spin objects and their controlled nucleation provide great potential for further studies on magnetic quasi-particle interactions, spin dynamics, as well as for possible future applications in spintronics, namely the racetrack memory, skyrmionic interconnections, skyrmion-based unconventional computing, and sensor devices

Enhancing Computer Network Security through Improved Outlier Detection for Data Streams

V několika posledních letech se metody strojového učení (zvláště ty zabývající se detekcí odlehlých hodnot - OD) v oblasti kyberbezpečnosti opíraly o zjišťování anomálií síťového provozu spočívajících v nových schématech útoků. Detekce anomálií v počítačových sítích reálného světa se ale stala stále obtížnější kvůli trvalému nárůstu vysoce objemných, rychlých a dimenzionálních průběžně přicházejících dat (SD), pro která nejsou k dispozici obecně uznané a pravdivé informace o anomalitě. Účinná detekční schémata pro vestavěná síťová zařízení musejí být rychlá a paměťově nenáročná a musejí být schopna se potýkat se změnami konceptu, když se vyskytnou. Cílem této disertace je zlepšit bezpečnost počítačových sítí zesílenou detekcí odlehlých hodnot v datových proudech, obzvláště SD, a dosáhnout kyberodolnosti, která zahrnuje jak detekci a analýzu, tak reakci na bezpečnostní incidenty jako jsou např. nové zlovolné aktivity. Za tímto účelem jsou v práci navrženy čtyři hlavní příspěvky, jež byly publikovány nebo se nacházejí v recenzním řízení časopisů. Zaprvé, mezera ve volbě vlastností (FS) bez učitele pro zlepšování již hotových metod OD v datových tocích byla zaplněna navržením volby vlastností bez učitele pro detekci odlehlých průběžně přicházejících dat označované jako UFSSOD. Následně odvozujeme generický koncept, který ukazuje dva aplikační scénáře UFSSOD ve spojení s online algoritmy OD. Rozsáhlé experimenty ukázaly, že UFSSOD coby algoritmus schopný online zpracování vykazuje srovnatelné výsledky jako konkurenční metoda upravená pro OD. Zadruhé představujeme nový aplikační rámec nazvaný izolovaný les založený na počítání výkonu (PCB-iForest), jenž je obecně schopen využít jakoukoliv online OD metodu založenou na množinách dat tak, aby fungovala na SD. Do tohoto algoritmu integrujeme dvě varianty založené na klasickém izolovaném lese. Rozsáhlé experimenty provedené na 23 multidisciplinárních datových sadách týkajících se bezpečnostní problematiky reálného světa ukázaly, že PCB-iForest jasně překonává už zavedené konkurenční metody v 61 % případů a dokonce dosahuje ještě slibnějších výsledků co do vyváženosti mezi výpočetními náklady na klasifikaci a její úspěšností. Zatřetí zavádíme nový pracovní rámec nazvaný detekce odlehlých hodnot a rozpoznávání schémat útoku proudovým způsobem (SOAAPR), jenž je na rozdíl od současných metod schopen zpracovat výstup z různých online OD metod bez učitele proudovým způsobem, aby získal informace o nových schématech útoku. Ze seshlukované množiny korelovaných poplachů jsou metodou SOAAPR vypočítány tři různé soukromí zachovávající podpisy podobné otiskům prstů, které charakterizují a reprezentují potenciální scénáře útoku s ohledem na jejich komunikační vztahy, projevy ve vlastnostech dat a chování v čase. Evaluace na dvou oblíbených datových sadách odhalila, že SOAAPR může soupeřit s konkurenční offline metodou ve schopnosti korelace poplachů a významně ji překonává z hlediska výpočetního času . Navíc se všechny tři typy podpisů ve většině případů zdají spolehlivě charakterizovat scénáře útoků tím, že podobné seskupují k sobě. Začtvrté představujeme algoritmus nepárového kódu autentizace zpráv (Uncoupled MAC), který propojuje oblasti kryptografického zabezpečení a detekce vniknutí (IDS) pro síťovou bezpečnost. Zabezpečuje síťovou komunikaci (autenticitu a integritu) kryptografickým schématem s podporou druhé vrstvy kódy autentizace zpráv, ale také jako vedlejší efekt poskytuje funkcionalitu IDS tak, že vyvolává poplach na základě porušení hodnot nepárového MACu. Díky novému samoregulačnímu rozšíření algoritmus adaptuje svoje vzorkovací parametry na základě zjištění škodlivých aktivit. Evaluace ve virtuálním prostředí jasně ukazuje, že schopnost detekce se za běhu zvyšuje pro různé scénáře útoku. Ty zahrnují dokonce i situace, kdy se inteligentní útočníci snaží využít slabá místa vzorkování.ObhájenoOver the past couple of years, machine learning methods - especially the Outlier Detection (OD) ones - have become anchored to the cyber security field to detect network-based anomalies rooted in novel attack patterns. Due to the steady increase of high-volume, high-speed and high-dimensional Streaming Data (SD), for which ground truth information is not available, detecting anomalies in real-world computer networks has become a more and more challenging task. Efficient detection schemes applied to networked, embedded devices need to be fast and memory-constrained, and must be capable of dealing with concept drifts when they occur. The aim of this thesis is to enhance computer network security through improved OD for data streams, in particular SD, to achieve cyber resilience, which ranges from the detection, over the analysis of security-relevant incidents, e.g., novel malicious activity, to the reaction to them. Therefore, four major contributions are proposed, which have been published or are submitted journal articles. First, a research gap in unsupervised Feature Selection (FS) for the improvement of off-the-shell OD methods in data streams is filled by proposing Unsupervised Feature Selection for Streaming Outlier Detection, denoted as UFSSOD. A generic concept is retrieved that shows two application scenarios of UFSSOD in conjunction with online OD algorithms. Extensive experiments have shown that UFSSOD, as an online-capable algorithm, achieves comparable results with a competitor trimmed for OD. Second, a novel unsupervised online OD framework called Performance Counter-Based iForest (PCB-iForest) is being introduced, which generalized, is able to incorporate any ensemble-based online OD method to function on SD. Two variants based on classic iForest are integrated. Extensive experiments, performed on 23 different multi-disciplinary and security-related real-world data sets, revealed that PCB-iForest clearly outperformed state-of-the-art competitors in 61 % of cases and even achieved more promising results in terms of the tradeoff between classification and computational costs. Third, a framework called Streaming Outlier Analysis and Attack Pattern Recognition, denoted as SOAAPR is being introduced that, in contrast to the state-of-the-art, is able to process the output of various online unsupervised OD methods in a streaming fashion to extract information about novel attack patterns. Three different privacy-preserving, fingerprint-like signatures are computed from the clustered set of correlated alerts by SOAAPR, which characterize and represent the potential attack scenarios with respect to their communication relations, their manifestation in the data's features and their temporal behavior. The evaluation on two popular data sets shows that SOAAPR can compete with an offline competitor in terms of alert correlation and outperforms it significantly in terms of processing time. Moreover, in most cases all three types of signatures seem to reliably characterize attack scenarios to the effect that similar ones are grouped together. Fourth, an Uncoupled Message Authentication Code algorithm - Uncoupled MAC - is presented which builds a bridge between cryptographic protection and Intrusion Detection Systems (IDSs) for network security. It secures network communication (authenticity and integrity) through a cryptographic scheme with layer-2 support via uncoupled message authentication codes but, as a side effect, also provides IDS-functionality producing alarms based on the violation of Uncoupled MAC values. Through a novel self-regulation extension, the algorithm adapts its sampling parameters based on the detection of malicious actions on SD. The evaluation in a virtualized environment clearly shows that the detection rate increases over runtime for different attack scenarios. Those even cover scenarios in which intelligent attackers try to exploit the downsides of sampling

Magnetic properties of Co/Ni-based multilayers with Pd and Pt insertion layers

In this study, the influence of Pd and Pt insertion layers in Co/Ni multilayers (MLs) on their magnetic properties, e.g. magnetic anisotropies, saturation magnetization, coercivity, magnetic domain size, and Curie temperature, is investigated. We compare three series of [Co/Ni/X]N ML systems (X = Pd, Pt, no insertion layer), varying the individual Co layer thickness as well as the repetition number N. All three systems behave very similarly for the different Co layer thicknesses. For all systems, a maximum effective magnetic anisotropy was achieved for MLs with a Co layer thickness between 0.15 nm and 0.25 nm. The transition from an out-of-plane to an in-plane system occurs at about 0.4 nm of Co. While [Co(0.2 nm)/Ni(0.4 nm)]N MLs change their preferred easy magnetization axis from out-of-plane to in-plane after 6 bilayer repetitions, insertion of Pd and Pt results in an extension of this transition beyond 15 repetitions. The maximum effective magnetic anisotropy was more than doubled from 105 kJ/m3 for [Co/Ni]3 to 275 and 186 kJ/m3 for Pt and Pd, respectively. Furthermore, the insertion layers strongly reduce the initial saturation magnetization of 1100 kA/m of Co/Ni MLs and lower the Curie temperature from 720 to around 500

Ultrafast high-harmonic nanoscopy of magnetization dynamics

Light-induced magnetization changes, such as all-optical switching, skyrmion nucleation, and intersite spin transfer, unfold on temporal and spatial scales down to femtoseconds and nanometers, respectively. Pump-probe spectroscopy and diffraction studies indicate that spatio-temporal dynamics may drastically affect the non-equilibrium magnetic evolution. Yet, direct real-space magnetic imaging on the relevant timescale has remained challenging. Here, we demonstrate ultrafast high-harmonic nanoscopy employing circularly polarized high-harmonic radiation for real-space imaging of femtosecond magnetization dynamics. We observe the reversible and irreversible evolution of nanoscale spin textures following femtosecond laser excitation. Specifically, we map quenched magnetic domains and localized spin structures in Co/Pd multilayers with a sub-wavelength spatial resolution down to 16 nm, and strobosocopically trace the local magnetization dynamics with 40 fs temporal resolution. Our approach enables the highest spatio-temporal resolution of magneto-optical imaging to date. Facilitating ultrafast imaging with an extreme sensitivity to various microscopic degrees of freedom expressed in chiral and linear dichroism, we envisage a wide range of applications spanning magnetism, phase transitions, and carrier dynamics.Comment: 14 pages, 4 figure

Generation and annihilation of skyrmions and antiskyrmions in magnetic heterostructures

We demonstrate the controlled generation and annihilation of (anti)skyrmions with tunable chirality in magnetic heterostructures by means of micromagnetic simulations. By making use of magnetic (anti)vortices in a patterned ferromagnetic layer, we stabilize (anti)skyrmions in an underlying skyrmionic thin film in a reproducible manner. The stability of the (anti)skyrmion depends on the polarization of the (anti)vortex, whereas their chirality is given by those of the (anti)vortices. We investigate the influence of geometric parameters such as nanodisk radius and film thickness on the stability of the (anti)skyrmions. By introducing the interlayer Dzyaloshinskii-Moriya interaction into our modeling, we predict that the same coupling mechanism works also for chiral skyrmions. Furthermore, we demonstrate that the core coupling between the (anti)vortices and (anti)skyrmions allows deleting and writing of spin objects in a controlled fashion by applying short pulses of in-plane external magnetic fields or charge currents, representing a new key paradigm in skyrmionic devices

Anonymization procedures for tabular data: an explanatory technical and legal synthesis

In the European Union, Data Controllers and Data Processors, who work with personal data, have to comply with the General Data Protection Regulation and other applicable laws. This affects the storing and processing of personal data. But some data processing in data mining or statistical analyses does not require any personal reference to the data. Thus, personal context can be removed. For these use cases, to comply with applicable laws, any existing personal information has to be removed by applying the so-called anonymization. However, anonymization should maintain data utility. Therefore, the concept of anonymization is a double-edged sword with an intrinsic trade-off: privacy enforcement vs. utility preservation. The former might not be entirely guaranteed when anonymized data are published as Open Data. In theory and practice, there exist diverse approaches to conduct and score anonymization. This explanatory synthesis discusses the technical perspectives on the anonymization of tabular data with a special emphasis on the European Union’s legal base. The studied methods for conducting anonymization, and scoring the anonymization procedure and the resulting anonymity are explained in unifying terminology. The examined methods and scores cover both categorical and numerical data. The examined scores involve data utility, information preservation, and privacy models. In practice-relevant examples, methods and scores are experimentally tested on records from the UCI Machine Learning Repository’s “Census Income (Adult)” dataset

Hysteresis-free magnetization reversal of exchange-coupled bilayers with finite magnetic anisotropy

Exchange-coupled structures consisting of ferromagnetic and ferrimagnetic layers become technologically more and more important. We show experimentally the occurrence of completely reversible, hysteresis-free minor loops of [Co(0.2 nm)/Ni(0.4 nm)/Pt(0.6 nm)]$_N$ multilayers exchange-coupled to a 20 nm thick ferrimagnetic Tb$_{28}$Co$_{14}$Fe$_{58}$ layer, acting as hard magnetic pinning layer. Furthermore, we present detailed theoretical investigations by means of micromagnetic simulations and most important a purely analytical derivation for the condition of the occurrence of full reversibility in magnetization reversal. Hysteresis-free loops always occur if a domain wall is formed during the reversal of the ferromagnetic layer and generates an intrinsic hard-axis bias field that overcomes the magnetic anisotropy field of the ferromagnetic layer. The derived condition further reveals that the magnetic anisotropy and the bulk exchange of both layers, as well as the exchange coupling strength and the thickness of the ferromagnetic layer play an important role for its reversibility.Comment: 9 pages, 7 figure