@article{2333, abstract = { Magnetic nanofibers belong to the geometries which are intensively investigated in simulations and experiments due to their unique magnetic properties, varying in their lengths, cross-sections, and bending radii. Besides basic research of different magnetization reversal processes and magnetization dynamics in bent nanofibers, these structures are of potential interest for data storage applications, data transport, or other tasks in spintronics devices. While previous simulations concentrated on the domain wall transport through coupled bent nanofibers, creating networks with many in- and outputs to establish nanofiber-based domain wall logics, here we show the influence of the constricted area, in which a rotating magnetic field is applied in the middle of bent or straight magnetic nanofibers, on the magnetization dynamics. Our micromagnetic simulations, performed by Magpar, reveal a strong impact not only of this area, but also of the curvature of the nanofiber as well as of an additional Dzyaloshinskii–Moriya interaction (DMI). }, author = {Blachowicz, Tomasz and Steblinski, Pawel and Ehrmann, Andrea}, issn = {2073-8994}, journal = {Symmetry}, keywords = {micromagnetic simulation, Parallel Finite Element Micromagnetics Package (Magpar), magnetocrystalline anisotropy, exchange energy, demagnetization energy}, number = {1}, publisher = {MDPI AG}, title = {{Influence of Physical Symmetries on the Magnetization Dynamics in Magnetic Fibers}}, doi = {10.3390/sym15010234}, volume = {15}, year = {2023}, } @article{1579, abstract = {Asymmetric hysteresis loops can be found in exchange-bias systems in which a ferromagnet is exchange-coupled with an antiferromagnet. In purely ferromagnetic samples, such effects can occur due to undetected minor loops or thermal effects. While the exchange bias is long established in hard-disk read/write heads and diverse spintronics applications, minor loops are sometimes used for the calculation of first order reversal curves (FORCs). Reports about their technological relevance, however, are scarce. Here we report on micromagnetic simulations of a nanoparticle with areas of varying height, consisting of tessellations of a defined area, in which the shape anisotropy in narrow higher lines opposes magnetization reversal stronger than in the larger, lower areas between, thus interacting similar to hard/soft magnetic materials although consisting of the same material and thus having identical magneto-crystalline anisotropy. After saturating this nanostructure by a strong magnetic field pulse, distinctly asymmetric, horizontally shifted hysteresis loops can be recognized, especially in the transverse magnetization component. We show the influence of the external magnetic field orientation on this asymmetry.}, author = {Ehrmann, Andrea and Blachowicz, T.}, issn = {03048853}, journal = {Journal of Magnetism and Magnetic Materials}, keywords = {Pseudo-exchange bias Micromagnetic simulation Hysteresis loops Magnetic nanostructure Tessellation}, publisher = {Elsevier BV}, title = {{Magnetization reversal asymmetry in a structured ferromagnetic nanoparticle with varying shape anisotropy}}, doi = {10.1016/j.jmmm.2021.168929}, volume = {546}, year = {2022}, } @article{1426, abstract = {Round magnetic nano-dots belong to the frequently investigated magnetic nanostructures since they can often reverse magnetization via a vortex state which is characterized by low stray fields, making them useful for data storage applications. Nano-rings, with an open aperture in the middle, regularly show flux-closed vortex states without a vortex core, again reducing stray fields. Here we theoretically investigate “Pac-Man” shaped iron nanostructures with varying aperture, i.e. structures ranging from a thin ring to a solid cylinder with a triangular cut. Our results show that in most cases, a vortex-like state without vortex core along the whole nanostructure occurs during magnetization reversal. Depending on the angle of the external magnetic field and the nanostructure thickness, different magnetization reversal processes have been found, including domain wall nucleation and propagation as well as small vortex states with cores. The occurrence of such special magnetic states can be attributed to the interaction of the shape anisotropy with the relatively high magneto-crystalline anisotropy of iron. Our simulations reveal the possibilities to use such structures for data storage applications as well as their importance in basic research, enabling formation of asymmetric magnetic structures beyond common onion and symmetric vortex states.}, author = {Bachar, Fatima-Zohra and Schröder, Christian and Ehrmann, Andrea}, issn = {03048853}, journal = {Journal of Magnetism and Magnetic Materials}, keywords = {Magnetic nanostructures Micromagnetic simulation Magnetization reversal Iron Vortex state Domain walls Shape anisotropy}, publisher = {Elsevier BV}, title = {{Magnetization reversal in Pac-Man shaped Fe nanostructures with varying aperture}}, doi = {10.1016/j.jmmm.2021.168205}, volume = {537}, year = {2021}, } @article{1601, abstract = { Combining soft and hard magnetic materials is not only of technological importance in diverse spintronics elements, but also of high interest in basic research. Here, we report on different arrays combining iron and nickel, e.g., by embedding circular nanodots of one material in a matrix of the other. Micromagnetic simulations were performed using OOMMF. Our results show that magnetization reversal processes are strongly influenced by neighboring nanodots and the magnetic matrix in which the nanodots are embedded, respectively, which becomes macroscopically visible by several steps along the slopes of the hysteresis loops. Such material combinations allow for preparing quaternary memories and are thus highly relevant for applications in data storage and processing. }, author = {Sudsom, Devika and Ehrmann, Andrea}, issn = {2673-4605}, journal = {Materials Proceedings}, keywords = {micromagnetic simulation, OOMMF, nanodots, antidots, array, spintronics}, number = {1}, publisher = {MDPI AG}, title = {{Micromagnetic Simulations of Magnetic Particles Embedded in Magnetic or Non-Magnetic Matrices}}, doi = {10.3390/IOCN2020-07940}, volume = {4}, year = {2021}, } @article{1611, abstract = { Horizontally shifted and asymmetric hysteresis loops are often associated with exchange-biased samples, consisting of a ferromagnet exchange-coupled with an antiferromagnet. In purely ferromagnetic samples, such effects can occur due to undetected minor loops or thermal effects. Simulations of ferromagnetic nanostructures at zero temperature with sufficiently large saturation fields should not lead to such asymmetries. Here we report on micromagnetic simulations at zero temperature, performed on sputtered nanoparticles with different shapes. The small deviations of the systems due to random anisotropy orientations in the different grains can not only result in strong deviations of magnetization reversal processes and hysteresis loops, but also to distinctly asymmetric, horizontally shifted hysteresis loops in purely ferromagnetic nanoparticles. }, author = {Detzmeier, Joscha and Königer, Kevin and Ehrmann, Andrea}, issn = {2673-4605}, journal = {Materials Proceedings}, keywords = {pseudo-exchange bias, minor loop, micromagnetic simulation, OOMMF, spintronics}, number = {1}, publisher = {MDPI AG}, title = {{Asymmetric Hysteresis Loops and Horizontal Loop Shifts in Purely Ferromagnetic Nanoparticles}}, doi = {10.3390/IOCN2020-07836}, volume = {4}, year = {2021}, } @article{1614, abstract = { Horizontally shifted and asymmetric hysteresis loops are often associated with exchange-biased samples, consisting of a ferromagnet exchange coupled with an antiferromagnet. In purely ferromagnetic samples, such effects can occur due to undetected minor loops or thermal effects. Simulations of ferromagnetic nanostructures at zero temperature with sufficiently large saturation fields should not lead to such asymmetries. Here we report on micromagnetic simulations at zero temperature, performed on sputtered nanoparticles with different structures. The small deviations of the systems due to random anisotropy orientations in the different grains can not only result in strong deviations of magnetization reversal processes and hysteresis loops, but also lead to distinctly asymmetric, horizontally shifted hysteresis loops in purely ferromagnetic nanoparticles. }, author = {Detzmeier, Joscha and Königer, Kevin and Blachowicz, Tomasz and Ehrmann, Andrea}, issn = {2079-4991}, journal = {Nanomaterials}, keywords = {pseudo-exchange bias, minor loop, micromagnetic simulation, OOMMF, spintronics}, number = {3}, publisher = {MDPI AG}, title = {{Asymmetric Hysteresis Loops in Structured Ferromagnetic Nanoparticles with Hard/Soft Areas}}, doi = {10.3390/nano11030800}, volume = {11}, year = {2021}, } @article{1616, abstract = {Electrospun nanofibers produced from magnetic materials form a magnetic network which may be used for neuromorphic computing and other novel applications. While the influence of bending radii on the magnetization reversal in such nanofibers was already discussed in the literature, the often occurring beads along the fibers have not yet been investigated in detail. It can be assumed that such beads will support domain wall formation due to a reduction of the relative impact of the shape anisotropy, in this way influencing magnetization reversal along the fiber. Here, we simulate magnetization reversal processes in iron, nickel, cobalt and magnetite fibers with a bead in the middle, produced in three typical dimensions gained by electrospinning. In most cases a vortex state occurs during magnetization reversal, independent from the material and the dimensions. For some angular orientations of the external magnetic field, double-vortex or meander states are visible, usually also followed by vortex states. These simulations underline the strong and highly reliable impact of beads along electrospun fibers, making these structures useful for data storage, transport and other applications.}, author = {Amini, Fedi and Blachowicz, Tomasz and Ehrmann, Andrea}, issn = {03048853}, journal = {Journal of Magnetism and Magnetic Materials}, keywords = {Magnetic nanostructures Micromagnetic simulation Magnetization reversal Iron Nickel Cobalt Magnetite Vortex state Domain walls Shape anisotropy}, publisher = {Elsevier BV}, title = {{Systematic study of magnetization reversal in beaded fibers from different magnetic materials}}, doi = {10.1016/j.jmmm.2021.167855}, volume = {529}, year = {2021}, } @article{1624, abstract = { Combining clusters of magnetic materials with a matrix of other magnetic materials is very interesting for basic research because new, possibly technologically applicable magnetic properties or magnetization reversal processes may be found. Here we report on different arrays combining iron and nickel, for example, by surrounding circular nanodots of one material with a matrix of the other or by combining iron and nickel nanodots in air. Micromagnetic simulations were performed using the OOMMF (Object Oriented MicroMagnetic Framework). Our results show that magnetization reversal processes are strongly influenced by neighboring nanodots and the magnetic matrix by which the nanodots are surrounded, respectively, which becomes macroscopically visible by several steps along the slopes of the hysteresis loops. Such material combinations allow for preparing quaternary memory systems, and are thus highly relevant for applications in data storage and processing. }, author = {Sudsom, Devika and Ehrmann, Andrea}, issn = {2079-4991}, journal = {Nanomaterials}, keywords = {micromagnetic simulation, OOMMF, nanodots, antidots, array, spintronics}, number = {2}, publisher = {MDPI AG}, title = {{Micromagnetic Simulations of Fe and Ni Nanodot Arrays Surrounded by Magnetic or Non-Magnetic Matrices}}, doi = {10.3390/nano11020349}, volume = {11}, year = {2021}, } @article{669, abstract = {Magnetic vortices belong to the possibilities to store information in magnetic structures. Understanding their nucleation and propagation is also of interest in basic research. While vortices are usually examined in particles of homogeneous material, here we give an overview of the impact of borders between two materials, i.e. iron and permalloy, on magnetization reversal and vortex formation in double-wedges as well as adjacent rectangles of different thickness. While former investigations of pure iron nanoparticles of similar dimensions revealed different magnetization reversal magnetisms without vortex, with one or two vortices, the addition of a permalloy part stabilized the magnetization reversal so that in all situations under investigation, a single vortex was observed. Our simulations underline the technological importance of such double-material structures for the preparation of nanostructures for storage devices with reliable magnetization reversal processes, stable against erroneous modifications of nanoparticle dimensions and magnetic field orientations.}, author = {Sudsom, Devika and Blachowicz, Tomasz and Hahn, Lothar and Ehrmann, Andrea}, issn = {0304-8853}, journal = {Journal of Magnetism and Magnetic Materials}, keywords = {Magnetic nanostructures, Wedges, Micromagnetic simulation, Vortex, Magnetization reversal, Iron, Permalloy, Lithography}, title = {{Vortex nucleation and propagation in magnetic double-wedges and semi-squares for reliable quaternary storage systems}}, doi = {10.1016/j.jmmm.2020.167294}, volume = {514}, year = {2020}, } @article{635, abstract = {Magnetic vortex structures are of high technological relevance due to their possible application in magnetic memory. Moreover, investigating magnetization reversal via vortex formation is an important topic in basic research. Typically, such vortices are only investigated in homogeneous magnetic materials of diverse shapes. Here, we report for the first time on micromagnetic simulation of vortex formation in magnetic bow-tie nanostructures, comprising alternating parts from iron and permalloy, investigated for two different thicknesses and under different angles of the external magnetic field. While no vortex was found in pure permalloy square, nanoparticles of the dimensions investigated in this study and in case of iron only a relatively thick sample allowed for vortex formation, different numbers of vortices and antivortices were found in the bow-tie structures prepared from both materials, depending on the angular field orientation and the sample thickness. By stabilizing more than one vortex in a confined nanostructure, it is possible to store more than one bit of information in it. Our micromagnetic simulations reveal that such bi-material structures are highly relevant not only for basic research, but also for data storage applications.}, author = {Sudsom, Devika and Juhász Junger, Irén and Döpke, Christoph and Blachowicz, Tomasz and Hahn, Lothar and Ehrmann, Andrea}, issn = {2410-3896}, journal = {Condensed Matter}, keywords = {magnetic nanostructures, micromagnetic simulation, bow-tie structure, vortex, magnetization reversal, iron, permalloy, lithography}, number = {1}, title = {{Micromagnetic Simulation of Vortex Development in Magnetic Bi-Material Bow-Tie Structures}}, doi = {10.3390/condmat5010005}, volume = {5}, year = {2020}, } @article{656, abstract = {Electrospinning can be used to create nanofibers with diameters of typically a few tens to a few hundred nanometers. While pure polymers are often electrospun, it is also possible to use polymer blends or to include nanoparticles. In this way, e.g., magnetic nanofiber networks can be created with a certain diameter distribution, random fiber orientations, and random crossing positions and angles. Here we present for the first time micromagnetic simulations of small parts of stochastically oriented nanofiber networks. Magnetization reversal mechanisms are investigated for different local spatial distributions; mutual influences of neighboring magnetic fibers due to dipolar interactions are depicted. This study serves as a base for the possible use of such stochastic nanofiber networks in the research area of neuro-inspired materials.}, author = {Blachowicz, Tomasz and Döpke, Christoph and Ehrmann, Andrea}, issn = {2079-4991}, journal = {Nanomaterials}, keywords = {micromagnetic simulation, magnetic nanofiber networks, magnetization reversal, electrospinning, iron}, number = {4}, title = {{Micromagnetic Simulations of Chaotic Ferromagnetic Nanofiber Networks}}, doi = {10.3390/nano10040738}, volume = {10}, year = {2020}, } @inproceedings{684, abstract = {Horizontally shifted and asymmetric hysteresis loops are often associated with exchange-biased samples, consisting of a ferromagnet exchange-coupled with an antiferromagnet. In purely ferromagnetic samples, such effects can occur due to undetected minor loops or thermal effects. Simulations of ferromagnetic nanostructures at zero temperature with sufficiently large saturation fields should not lead to such asymmetries. Here we report on micromagnetic simulations at zero temperature, performed on sputtered nanoparticles with different shapes. The small deviations of the systems due to random anisotropy orientations in the different grains can not only result in strong deviations of magnetization reversal processes and hysteresis loops, but also to distinctly asymmetric, horizontally shifted hysteresis loops in purely ferromagnetic nanoparticles.}, author = {Detzmeier, Joscha and Königer, Kevin and Ehrmann, Andrea}, booktitle = {2nd International Online-Conference on Nanomaterials}, keywords = {pseudo-exchange bias, minor loop, micromagnetic simulation, OOMMF, spintronics}, location = {online}, publisher = {MDPI}, title = {{ Asymmetric hysteresis loops and horizontal loop shifts in purely ferromagnetic nanoparticles}}, year = {2020}, } @article{582, abstract = {Magnetic nanostructures are investigated nowadays for diverse applications as well as in basic research. They often show unexpected magnetization reversal processes which are not only of high interest for understanding magnetization reversal processes on the nano-scale, but also due to their possible use in magneto-electronic devices, spintronics, neuromorphic computing and other areas. Square nano-frames prepared from iron, e.g., belong to the magnetic nano-particles exhibiting additional stable states at remanence which can be used for quaternary storage devices. The reason for this study is that nano-particles prepared from permalloy or nickel with their very low magneto-crystalline anisotropy do usually not show steps in the hysteresis loop and corresponding stable intermediate state, while the magneto-crystalline anisotropy of cobalt often dominates over the shape-anisotropy, resulting in a large number of steps which are not always stable. Here we report on experimental investigations by magneto-optic Kerr effect (MOKE) and micromagnetic simulations of square Co nano-frames, exhibiting steps along the slopes of the hysteresis loops. The positions and numbers of these steps varied during measurements without intentionally changing the experimental setup. Micromagnetic simulations were carried out to explain the experimental findings which could be attributed to different Co crystallite orientations, resulting in a modified magneto-crystalline anisotropy and thus different hysteresis loops for smallest variations of the laser beam position on the samples during MOKE measurements.}, author = {Blachowicz, Tomasz and Kosmalska, Dorota and Döpke, Christoph and Leiste, Harald and Hahn, Lothar and Ehrmann, Andrea}, journal = {Journal of Magnetism and Magnetic Materials}, keywords = {Magnetic nanostructures Magneto-optical Kerr effect (MOKE) Micromagnetic simulation Magnetization reversal Cobalt Lithography}, title = {{Varying steps in hysteresis loops of Co square nano-frames}}, doi = {j.jmmm.2019.165619}, volume = {491}, year = {2019}, }