Hysteresis behavior patterns in complex systems

Many complex systems such as magnets, shape memory alloys, as well as socioeconomic and biological systems are known to display hysteresis. This inherently irreversible process differs from the other irreversible processes most often addressed in literature by the memory that persists long after the external parameters stop changing. In general, hysteresis is a consequence of multi-scale system dynamics and the existence of many metastable states. Although hysteresis is typically illustrated by closed minor loops, other types of hysteretic trajectories are often observed where closed loops form gradually after several external parameter periods or not at all. The question arises: What in the structure of a system determines these qualitatively different behaviors of hysteretic trajectories?This thesis models complex hysteretic systems using a network of bistable binary elements and investigates network structure induced changes in hysteretic behavior. The main focus is on studying the minor loop formation processes for a single cyclically varying external parameter. Stable minor loops are observed to form at different rates as a function of the number of cycles, depending on the sign of the interactions, disorder level, and on the connectivity and topology of the interaction networks. For certain dense interaction networks, hysteretic trajectories that do not converge to a minor loop after an arbitrarily large number of external parameter periods are discovered. It is shown that their appearance is related to the presence of specific topological structures in the network. Thus, the thesis demonstrates several interesting links between hysteretic behavior and the underlying structure of complex systems.Ph.D., Electrical Engineering -- Drexel University, 200

The efficacy of surgical treatment of cerebral arteriovenous malformations in a single academic institution: a case series

Aim To report on patients who underwent surgical treatment of arteriovenous malformations (AVMs) at our institution. Methods This retrospective single-center case series enrolled the patients who underwent surgical treatment of pial AVM at the Department of Neurosurgery, University Hospital Brno, between 2005 and 2019. The data are summarized as descriptive statistics presenting basic characteristics in all the patients and in sex or age subgroups. Results Fifty patients were enrolled. The majority of AVMs were of Spetzler-Martin grade II (n=27; 54%), localized supratentorialy (n=43; 86%), and half of AVMs were ruptured. A total resection was performed in 48 patients (96%), and a good overall outcome was achieved in 44 patients (88%). Surgery-associated morbidity was 2%, and the mortality rate was 0% due to meticulous selection of patients for surgical treatment. Conclusion Microsurgery is an appropriate method of treatment for S-M grade I-III pial AVMs. Microsurgery may be used to treat the majority of small-nidus AVMs with a low mortality and morbidity, when precisely planned and performed by an expert vascular team. The meticulous selection of patients for surgical treatment is crucial

Center of mass method for exchange bias measurements

Applied Physics Letters, 89(14): pp. 142513-1--142513-3.Exchange bias measurement techniques are tested using an Ising model for exchange-coupled bilayer structures. In the presence of hysteresis loop asymmetry, the conventional exchange bias characterization method of measuring the sum of the coercive fields is found to be rather inaccurate if compared to the interface coupling energy. An alternative method based on the analysis of entire hysteresis loops is proposed, tested, and found to be substantially more robust

Estimation of exchange coupling distribution in all-ferromagnetic bilayers

IEEE Transactions on Magnetics, 43(6): pp. 2953-2955.Methodology for identification of interfacial exchange coupling distribution in all-ferromagnetic bilayers is proposed. The method is simple and requires only a basic set of hysteretic loop measurements as input data

Hysteresis of nanocylinders with Dzyaloshinskii-Moriya interaction

The potential for application of magnetic skyrmions in high density storage devices provides a strong drive to investigate and exploit their stability and manipulability. Through a three-dimensional micromagnetic hysteresis study, we investigate the question of existence of skyrmions in cylindrical nanostructures of variable thickness. We quantify the applied field and thickness dependence of skyrmion states, and show that these states can be accessed through relevant practical hysteresis loop measurement protocols. As skyrmionic states have yet to be observed experimentally in confined helimagnetic geometries, our work opens prospects for developing viable hysteresis process-based methodologies to access and observe skyrmionic states.Comment: 4 pages, 2 figure

Thermal stability and topological protection of skyrmions in nanotracks

Magnetic skyrmions are hailed as a potential technology for data storage and other data processing devices. However, their stability against thermal fluctuations is an open question that must be answered before skyrmion-based devices can be designed. In this work, we study paths in the energy landscape via which the transition between the skyrmion and the uniform state can occur in interfacial Dzyaloshinskii-Moriya finite-sized systems. We find three mechanisms the system can take in the process of skyrmion nucleation or destruction and identify that the transition facilitated by the boundary has a significantly lower energy barrier than the other energy paths. This clearly demonstrates the lack of the skyrmion topological protection in finite-sized magnetic systems. Overall, the energy barriers of the system under investigation are too small for storage applications at room temperature, but research into device materials, geometry and design may be able to address this