@article{3488, abstract = { Exchange bias (EB) is a unidirectional anisotropy occurring in exchange-coupled ferromagnetic/antiferromagnetic systems, such as thin films, core–shell particles, or nanostructures. In addition to a horizontal shift of the hysteresis loop, defining the exchange bias, asymmetric loops and even vertical shifts can often be found. While the effect is used in hard disk read heads and several spintronics applications, its origin is still not fully understood. Especially in nanostructures with their additional shape anisotropies, interesting and often unexpected effects can occur. Here, we provide an overview of the most recent experimental findings and theoretical models of exchange bias in nanostructures from different materials.}, author = {Blachowicz, Tomasz and Ehrmann, Andrea and Wortmann, Martin}, issn = {2079-4991}, journal = {Nanomaterials}, keywords = {exchange bias (EB), hysteresis loop shift, coercivity, ferromagnet, antiferromagnet, coercive field, asymmetric hysteresis loop}, number = {17}, publisher = {MDPI AG}, title = {{Exchange Bias in Nanostructures: An Update}}, doi = {10.3390/nano13172418}, volume = {13}, 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{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{1627, abstract = { The exchange bias (EB) is an effect occurring in coupled ferromagnetic/antiferromagnetic materials of diverse shapes, from core–shell nanoparticles to stacked nanostructures and thin films. The interface coupling typically results in a horizontal—often also vertical—shift of the hysteresis loop, combined with an increased coercivity, as compared to the pure ferromagnet, and the possibility of asymmetric hysteresis loops. Several models have been developed since its discovery in 1956 which still have some drawbacks and some unexplained points, while exchange bias systems are at the same time being used in hard drive read heads and are part of highly important elements for spintronics applications. Here, we give an update of new theoretical models and experimental findings regarding exchange bias phenomena in thin films during the last years, including new material combinations in which an exchange bias was found. }, author = {Blachowicz, Tomasz and Ehrmann, Andrea}, issn = {2079-6412}, journal = {Coatings}, keywords = {exchange bias (EB), ferromagnet, antiferromagnet, coercivity, coercive field, hysteresis loop shift, asymmetric hysteresis loop, domain state model}, number = {2}, publisher = {MDPI AG}, title = {{Exchange Bias in Thin Films—An Update}}, doi = {10.3390/coatings11020122}, volume = {11}, year = {2021}, } @article{678, abstract = {Asymmetric magnetic hysteresis loops are usually found in exchange bias (EB) systems, typically after field cooling a system below the Néel temperature of an antiferromagnet exchange coupled to a ferromagnet. Alternatively, asymmetric hysteresis loops may occur due to undetected minor loops or in systems with a rotational anisotropy. Here, we report on an exchange bias thin film system MgO(100)/Co/CoO, examined at room temperature, which is far above the blocking temperature, by the magneto-optical Kerr effect (MOKE). While the longitudinal hysteresis loops partly show steps which are well-known from diverse purely ferromagnetic systems, the transverse hysteresis loops exhibit clear asymmetries, similar to exchange biased systems at low temperatures, and unusual transverse magnetization values at saturation. Since minor loops and a rotational anisotropy can be excluded in this case, this asymmetry can possibly be a residue of the exchange bias coupling at lower temperatures.}, author = {Ehrmann, Andrea and Blachowicz, Tomasz}, issn = {2410-3896}, journal = {Condensed Matter}, keywords = {exchange bias, rotatable anisotropy, blocking temperature, minor loops, Co/CoO}, number = {4}, title = {{Asymmetric Hysteresis Loops in Co Thin Films}}, doi = {10.3390/condmat5040071}, volume = {5}, 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}, }