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K.d. Usadel - One of the best experts on this subject based on the ideXlab platform.
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Modeling Exchange Bias microscopically
Journal of Magnetism and Magnetic Materials, 2020Co-Authors: U. Nowak, A. Misra, K.d. UsadelAbstract:Exchange Bias is a horizontal shift of the hysteresis loop observed for a ferromagnetic layer in contact with an antiferromagnetic layer. Since Exchange Bias is related to the spin structure of the antiferromagnet, for its fundamental understanding a detailed knowledge of the physics of the antiferromagnetic layer is inevitable. A model is investigated where domains are formed in the volume of the AFM stabilized by dilution. These domains become frozen during the initial cooling procedure carrying a remanent net magnetization which causes and controls Exchange Bias. Varying the anisotropy of the antiferromagnet we find a nontrivial dependence of the Exchange Bias on the anisotropy of the antiferromagnet.Comment: 7 pages, 5 figure
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Domain State Model for Exchange Bias: Influence of Structural Defects on Exchange Bias in Co/CoO
Advances in Solid State Physics, 2020Co-Authors: Bernd Beschoten, U. Nowak, Andrea Tillmanns, Jan Keller, Gernot Güntherodt, K.d. UsadelAbstract:The Exchange Bias coupling at ferromagnetic/antiferromagnetic interfaces in epitaxially grown Co/CoO bilayers can be intentionally enhanced by a factor of up to 4 if the antiferromagnetic CoO layer is diluted by non-magnetic defects in its volume part away from the interface. Monte Carlo simulations of a simple model consisting of a ferromagnetic layer Exchange coupled to a diluted antiferromagnetic layer show Exchange Bias of the right order of magnitude and qualitatively reproduce the experimentally observed dependence of the Exchange Bias field on the number of defects. The Exchange Bias results from a domain state in the antiferromagnet, which is formed during field cooling and carries an irreversible domain state magnetization. Apart from intentionally introduced non-magnetic defects, also structural defects can enhance the Exchange Bias coupling. Twin boundaries in undiluted CoO increase the Exchange Bias coupling in Co/CoO by more than a factor of 2 compared to untwined samples. This observation indicates that structural defects in the antiferromagnet, such as twin or grain boundaries, might also stabilize a domain state, suggesting that the domain state model for Exchange Bias is more generally applicable to understand the origin of the Exchange Bias phenomenon.
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Coercivity and Exchange Bias of ferromagnetic/antiferromagnetic multilayers
Physical Review B, 2005Co-Authors: Gregor Scholten, K.d. Usadel, U. NowakAbstract:For a model system consisting of a ferromagnetic layer Exchange coupled to an antiferromagnetic layer with a compensated interface, detailed mean-field-type calculations are performed. Both the coercive field and the Exchange Bias field are calculated. For the coercive field, a rather broad enhancement around the Neel temperature TN of the antiferromagnetic layer is found irrespective of whether the antiferromagnetic layer is structurally disordered or not, while Exchange Bias is only found for disordered systems. We show that the observed enhancement of the coercivity around TN also found experimentally and the occurrence of Exchange Bias are of different origin.
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domain state model for Exchange Bias ii experiments
Physical Review B, 2002Co-Authors: Jan Keller, U. Nowak, B. Beschoten, G. Guntherodt, Peter Miltenyi, K.d. UsadelAbstract:The Exchange Bias coupling at ferro-/antiferromagnetic interfaces of epitaxially grown Co/CoO bilayers can be intentionally enhanced and controlled by diluting the antiferromagnetic CoO layer, i.e., by introducing (i) nonmagnetic substitutions $({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{O})$ or (ii) Co deficiencies $({\mathrm{Co}}_{1\ensuremath{-}y}\mathrm{O}).$ All intentional nonmagnetic cations or defects were placed away from the interface throughout the whole volume part of the antiferromagnetic layer. This way the roughness at the Co/CoO interface was kept practically the same. For both types of defects, the Exchange Bias field can be increased by a factor of 3 to 4. Hence, Exchange Bias is primarily not due to roughness at the interface but rather can be controlled by the defects in the volume part of the antiferromagnetic layer. We systematically investigate the dilution dependence of various phenomena of Exchange Bias, such as the vertical magnetization shift of the hysteresis loop, temperature dependence, training effect, cooling field dependence, and antiferromagnetic layer thickness dependence. All these phenomena are directly compared to results from Monte Carlo simulations and are shown to be consistently described by the domain state model for Exchange Bias. The combined experimental and theoretical findings suggest that the origin of Exchange Bias in Co/CoO results from a domain state in the volume part of the antiferromagnet stabilized by the defects.
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domain state model for Exchange Bias i theory
Physical Review B, 2002Co-Authors: U. Nowak, K.d. Usadel, Jan Keller, B. Beschoten, Peter Miltenyi, G. GuntherodtAbstract:For a model system consisting of a ferromagnetic layer coupled to a diluted, antiferromagnetic layer extensive Monte Carlo simulations are performed. Exchange Bias is observed as a result of a domain state in the antiferromagnetic layer which develops during field cooling, carrying an irreversible domain state's magnetization. In agreement with recent experimental observations on Co/CoO bilayers a strong dependence of the Exchange Bias field on dilution of the antiferromagnet is found and it is shown that a variety of typical effects associated with Exchange Bias, such as positive Bias, temperature, and time dependencies as well as the dependence on the thickness of the antiferromagnetic layer can be explained within our model.
U. Nowak - One of the best experts on this subject based on the ideXlab platform.
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Modeling Exchange Bias microscopically
Journal of Magnetism and Magnetic Materials, 2020Co-Authors: U. Nowak, A. Misra, K.d. UsadelAbstract:Exchange Bias is a horizontal shift of the hysteresis loop observed for a ferromagnetic layer in contact with an antiferromagnetic layer. Since Exchange Bias is related to the spin structure of the antiferromagnet, for its fundamental understanding a detailed knowledge of the physics of the antiferromagnetic layer is inevitable. A model is investigated where domains are formed in the volume of the AFM stabilized by dilution. These domains become frozen during the initial cooling procedure carrying a remanent net magnetization which causes and controls Exchange Bias. Varying the anisotropy of the antiferromagnet we find a nontrivial dependence of the Exchange Bias on the anisotropy of the antiferromagnet.Comment: 7 pages, 5 figure
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Domain State Model for Exchange Bias: Influence of Structural Defects on Exchange Bias in Co/CoO
Advances in Solid State Physics, 2020Co-Authors: Bernd Beschoten, U. Nowak, Andrea Tillmanns, Jan Keller, Gernot Güntherodt, K.d. UsadelAbstract:The Exchange Bias coupling at ferromagnetic/antiferromagnetic interfaces in epitaxially grown Co/CoO bilayers can be intentionally enhanced by a factor of up to 4 if the antiferromagnetic CoO layer is diluted by non-magnetic defects in its volume part away from the interface. Monte Carlo simulations of a simple model consisting of a ferromagnetic layer Exchange coupled to a diluted antiferromagnetic layer show Exchange Bias of the right order of magnitude and qualitatively reproduce the experimentally observed dependence of the Exchange Bias field on the number of defects. The Exchange Bias results from a domain state in the antiferromagnet, which is formed during field cooling and carries an irreversible domain state magnetization. Apart from intentionally introduced non-magnetic defects, also structural defects can enhance the Exchange Bias coupling. Twin boundaries in undiluted CoO increase the Exchange Bias coupling in Co/CoO by more than a factor of 2 compared to untwined samples. This observation indicates that structural defects in the antiferromagnet, such as twin or grain boundaries, might also stabilize a domain state, suggesting that the domain state model for Exchange Bias is more generally applicable to understand the origin of the Exchange Bias phenomenon.
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Exchange Bias driven by Dzyaloshinskii-Moriya interactions
Physical Review Letters, 2013Co-Authors: R. Yanes, Jerome Jackson, László Udvardi, Laszlo Szunyogh, U. NowakAbstract:The Exchange Bias effect in a compensated IrMn3=Coð111Þ system is studied using multiscale modeling from ab initio to atomistic spin model calculations. We evaluate numerically the out-of-plane hysteresis loops of the bilayer for different thicknesses of the ferromagnetic layer. The results show the existence of a perpendicular Exchange Bias and an enhancement of the coercivity of the system. To identify the origin of the Exchange Bias, we analyze the hysteresis loops of a selected bilayer by tuning the different contributions to the Exchange interaction across the interface. Our results indicate that the Exchange Bias is primarily induced by Dzyaloshinskii-Moriya interactions, while the coercivity is increased mainly due to a spin-flop mechanism. DOI: 10.1103/PhysRevLett.111.217202
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Coercivity and Exchange Bias of ferromagnetic/antiferromagnetic multilayers
Physical Review B, 2005Co-Authors: Gregor Scholten, K.d. Usadel, U. NowakAbstract:For a model system consisting of a ferromagnetic layer Exchange coupled to an antiferromagnetic layer with a compensated interface, detailed mean-field-type calculations are performed. Both the coercive field and the Exchange Bias field are calculated. For the coercive field, a rather broad enhancement around the Neel temperature TN of the antiferromagnetic layer is found irrespective of whether the antiferromagnetic layer is structurally disordered or not, while Exchange Bias is only found for disordered systems. We show that the observed enhancement of the coercivity around TN also found experimentally and the occurrence of Exchange Bias are of different origin.
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domain state model for Exchange Bias ii experiments
Physical Review B, 2002Co-Authors: Jan Keller, U. Nowak, B. Beschoten, G. Guntherodt, Peter Miltenyi, K.d. UsadelAbstract:The Exchange Bias coupling at ferro-/antiferromagnetic interfaces of epitaxially grown Co/CoO bilayers can be intentionally enhanced and controlled by diluting the antiferromagnetic CoO layer, i.e., by introducing (i) nonmagnetic substitutions $({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{O})$ or (ii) Co deficiencies $({\mathrm{Co}}_{1\ensuremath{-}y}\mathrm{O}).$ All intentional nonmagnetic cations or defects were placed away from the interface throughout the whole volume part of the antiferromagnetic layer. This way the roughness at the Co/CoO interface was kept practically the same. For both types of defects, the Exchange Bias field can be increased by a factor of 3 to 4. Hence, Exchange Bias is primarily not due to roughness at the interface but rather can be controlled by the defects in the volume part of the antiferromagnetic layer. We systematically investigate the dilution dependence of various phenomena of Exchange Bias, such as the vertical magnetization shift of the hysteresis loop, temperature dependence, training effect, cooling field dependence, and antiferromagnetic layer thickness dependence. All these phenomena are directly compared to results from Monte Carlo simulations and are shown to be consistently described by the domain state model for Exchange Bias. The combined experimental and theoretical findings suggest that the origin of Exchange Bias in Co/CoO results from a domain state in the volume part of the antiferromagnet stabilized by the defects.
Ivan K. Schuller - One of the best experts on this subject based on the ideXlab platform.
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Exchange Bias phenomenon the role of the ferromagnetic spin structure
Physical Review Letters, 2015Co-Authors: Ali C. Basaran, R. Morales, J E Villegas, D Navas, N Soriano, B Mora, C Redondo, X Batlle, Ivan K. SchullerAbstract:: The Exchange Bias of antiferromagnetic-ferromagnetic (AFM-FM) bilayers is found to be strongly dependent on the ferromagnetic spin configuration. The widely accepted inverse proportionality of the Exchange Bias field with the ferromagnetic thickness is broken in FM layers thinner than the FM correlation length. Moreover, an anomalous thermal dependence of both Exchange Bias field and coercivity is also found. A model based on springlike domain walls parallel to the AFM-FM interface quantitatively accounts for the experimental results and, in particular, for the deviation from the inverse proportionality law. These results reveal the active role the ferromagnetic spin structure plays in AFM-FM hybrids which leads to a new paradigm of the Exchange Bias phenomenon.
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Exchange Bias induced by the Fe3O4Verwey transition
Physical Review B, 2012Co-Authors: J. De La Venta, M. Erekhinsky, Siming Wang, Kevin G. West, Rafael Morales, Ivan K. SchullerAbstract:We present a study of the Exchange Bias in different configurations of V2O3 thin films with ferromagnetic layers. The Exchange Bias is accompanied by a large vertical shift in the magnetization. These effects are only observed when V2O3 is grown on top of Ni80Fe20 permalloy. The magnitude of the vertical shift is as large as 60% of the total magnetization which has never been reported in any system. X-Ray diffraction studies show that the growth conditions promote the formation of a ferrimagnetic Fe3O4 interlayer. The change in the easy magnetization axis of Fe3O4 across the Verwey transition at 120 K is correlated with the appearance of Exchange Bias and vertical shift in magnetization. Both phenomena disappear above 120 K, indicating for the first time a direct relationship between the magnetic signature of the Verwey transition and Exchange Bias.
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role of the antiferromagnetic bulk spin structure on Exchange Bias
Physical Review Letters, 2009Co-Authors: R. Morales, J. Olamit, Zhi-pan Li, J M Alameda, Ivan K. SchullerAbstract:The cooling field dependence of the Exchange Bias field in ferromagnet/antiferromagnet (FM/AF) multilayers demonstrates that the bulk AF spin structure plays a crucial role on the origin of Exchange Bias. FM/AF/FM trilayers were designed to eliminate any interlayer Exchange coupling between the FM slabs. By choosing the magnetic cooling field, the AF is ordered below its Neel temperature with the FM layers fully saturated either parallel or antiparallel to each other. The significant difference in the Exchange Bias field between these two cooling configurations confirms that Exchange Bias cannot be a purely interfacial effect and that the bulk AF moments play a significant role in pinning the uncompensated spins at the AF/FM interface. This experiment also demonstrates that the mechanism responsible for coercivity enhancement has a different origin and is independent of the process that gives rise to Exchange Bias.
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Antiferromagnetic domain size and Exchange Bias
Physical Review B, 2008Co-Authors: Michael R. Fitzsimmons, David Lederman, M. Cheon, J. Olamit, Igor V. Roshchin, Ivan K. SchullerAbstract:Using neutron diffraction, we measured the sizes of antiferromagnetic domains in three ferromagnet/antiferromagnet bilayer samples as a function of the magnitude and sign of Exchange Bias, temperature, and antiferromagnet composition. Neutron-scattering techniques were applied to thin films with masses less than $10\text{ }\ensuremath{\mu}\text{g}$. We found the antiferromagnetic domain size to be consistently small regardless of the Exchange Bias. For a Co/untwinned single crystalline antiferromagnet (AF)-fluoride bilayer, the antiferromagnetic domain size is comparable to the crystallographic domain size of the AF. For one sample the highest temperature at which the Exchange Bias was nonzero (i.e., the blocking temperature) was suppressed by $\ensuremath{\sim}3\text{ }\text{K}$ compared to the N\'eel temperature of the antiferromagnet.
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Lateral length scales in Exchange Bias
EPL, 2005Co-Authors: Igor V. Roshchin, Oleg Petracic, R. Morales, Zhi-pan Li, Xavier Batlle, Ivan K. SchullerAbstract:When a ferromagnet is in proximity to an antiferromagnet, lateral length scales such as the respective magnetic domain sizes drastically affect the Exchange Bias. Bilayers of FeF2 and either Ni, Co or Fe are studied using SQUID and spatially resolved MOKE. When the antiferromagnetic domains are larger than or comparable to the ferromagnetic domains, a local, non-averaging Exchange Bias is observed. This gives rise to unusual and tunable magnetic hysteresis curves.
G. Guntherodt - One of the best experts on this subject based on the ideXlab platform.
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domain state model for Exchange Bias i theory
Physical Review B, 2002Co-Authors: U. Nowak, K.d. Usadel, Jan Keller, B. Beschoten, Peter Miltenyi, G. GuntherodtAbstract:For a model system consisting of a ferromagnetic layer coupled to a diluted, antiferromagnetic layer extensive Monte Carlo simulations are performed. Exchange Bias is observed as a result of a domain state in the antiferromagnetic layer which develops during field cooling, carrying an irreversible domain state's magnetization. In agreement with recent experimental observations on Co/CoO bilayers a strong dependence of the Exchange Bias field on dilution of the antiferromagnet is found and it is shown that a variety of typical effects associated with Exchange Bias, such as positive Bias, temperature, and time dependencies as well as the dependence on the thickness of the antiferromagnetic layer can be explained within our model.
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domain state model for Exchange Bias ii experiments
Physical Review B, 2002Co-Authors: Jan Keller, U. Nowak, B. Beschoten, G. Guntherodt, Peter Miltenyi, K.d. UsadelAbstract:The Exchange Bias coupling at ferro-/antiferromagnetic interfaces of epitaxially grown Co/CoO bilayers can be intentionally enhanced and controlled by diluting the antiferromagnetic CoO layer, i.e., by introducing (i) nonmagnetic substitutions $({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{O})$ or (ii) Co deficiencies $({\mathrm{Co}}_{1\ensuremath{-}y}\mathrm{O}).$ All intentional nonmagnetic cations or defects were placed away from the interface throughout the whole volume part of the antiferromagnetic layer. This way the roughness at the Co/CoO interface was kept practically the same. For both types of defects, the Exchange Bias field can be increased by a factor of 3 to 4. Hence, Exchange Bias is primarily not due to roughness at the interface but rather can be controlled by the defects in the volume part of the antiferromagnetic layer. We systematically investigate the dilution dependence of various phenomena of Exchange Bias, such as the vertical magnetization shift of the hysteresis loop, temperature dependence, training effect, cooling field dependence, and antiferromagnetic layer thickness dependence. All these phenomena are directly compared to results from Monte Carlo simulations and are shown to be consistently described by the domain state model for Exchange Bias. The combined experimental and theoretical findings suggest that the origin of Exchange Bias in Co/CoO results from a domain state in the volume part of the antiferromagnet stabilized by the defects.
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Domain state model for Exchange Bias: training effect of diluted Co/sub 1-y/O on Exchange Bias in Co-CoO
IEEE Transactions on Magnetics, 2002Co-Authors: B. Beschoten, J. Keller, A. Tillmanns, G. GuntherodtAbstract:Exchange Bias in Co-CoO can be controlled and enhanced by diluting the antiferromagnetic (AFM) CoO layer with nonmagnetic defects. Here we present experiments on the dependence of Exchange Bias on subsequent magnetization reversals. The measurements were performed at different temperatures for samples with both low and optimum defect concentration in the AFM layer, yielding small and maximum Exchange Bias shift, respectively. The results are discussed within the "domain state" model for Exchange Bias.
Jan Keller - One of the best experts on this subject based on the ideXlab platform.
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Domain State Model for Exchange Bias: Influence of Structural Defects on Exchange Bias in Co/CoO
Advances in Solid State Physics, 2020Co-Authors: Bernd Beschoten, U. Nowak, Andrea Tillmanns, Jan Keller, Gernot Güntherodt, K.d. UsadelAbstract:The Exchange Bias coupling at ferromagnetic/antiferromagnetic interfaces in epitaxially grown Co/CoO bilayers can be intentionally enhanced by a factor of up to 4 if the antiferromagnetic CoO layer is diluted by non-magnetic defects in its volume part away from the interface. Monte Carlo simulations of a simple model consisting of a ferromagnetic layer Exchange coupled to a diluted antiferromagnetic layer show Exchange Bias of the right order of magnitude and qualitatively reproduce the experimentally observed dependence of the Exchange Bias field on the number of defects. The Exchange Bias results from a domain state in the antiferromagnet, which is formed during field cooling and carries an irreversible domain state magnetization. Apart from intentionally introduced non-magnetic defects, also structural defects can enhance the Exchange Bias coupling. Twin boundaries in undiluted CoO increase the Exchange Bias coupling in Co/CoO by more than a factor of 2 compared to untwined samples. This observation indicates that structural defects in the antiferromagnet, such as twin or grain boundaries, might also stabilize a domain state, suggesting that the domain state model for Exchange Bias is more generally applicable to understand the origin of the Exchange Bias phenomenon.
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domain state model for Exchange Bias ii experiments
Physical Review B, 2002Co-Authors: Jan Keller, U. Nowak, B. Beschoten, G. Guntherodt, Peter Miltenyi, K.d. UsadelAbstract:The Exchange Bias coupling at ferro-/antiferromagnetic interfaces of epitaxially grown Co/CoO bilayers can be intentionally enhanced and controlled by diluting the antiferromagnetic CoO layer, i.e., by introducing (i) nonmagnetic substitutions $({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{O})$ or (ii) Co deficiencies $({\mathrm{Co}}_{1\ensuremath{-}y}\mathrm{O}).$ All intentional nonmagnetic cations or defects were placed away from the interface throughout the whole volume part of the antiferromagnetic layer. This way the roughness at the Co/CoO interface was kept practically the same. For both types of defects, the Exchange Bias field can be increased by a factor of 3 to 4. Hence, Exchange Bias is primarily not due to roughness at the interface but rather can be controlled by the defects in the volume part of the antiferromagnetic layer. We systematically investigate the dilution dependence of various phenomena of Exchange Bias, such as the vertical magnetization shift of the hysteresis loop, temperature dependence, training effect, cooling field dependence, and antiferromagnetic layer thickness dependence. All these phenomena are directly compared to results from Monte Carlo simulations and are shown to be consistently described by the domain state model for Exchange Bias. The combined experimental and theoretical findings suggest that the origin of Exchange Bias in Co/CoO results from a domain state in the volume part of the antiferromagnet stabilized by the defects.
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domain state model for Exchange Bias i theory
Physical Review B, 2002Co-Authors: U. Nowak, K.d. Usadel, Jan Keller, B. Beschoten, Peter Miltenyi, G. GuntherodtAbstract:For a model system consisting of a ferromagnetic layer coupled to a diluted, antiferromagnetic layer extensive Monte Carlo simulations are performed. Exchange Bias is observed as a result of a domain state in the antiferromagnetic layer which develops during field cooling, carrying an irreversible domain state's magnetization. In agreement with recent experimental observations on Co/CoO bilayers a strong dependence of the Exchange Bias field on dilution of the antiferromagnet is found and it is shown that a variety of typical effects associated with Exchange Bias, such as positive Bias, temperature, and time dependencies as well as the dependence on the thickness of the antiferromagnetic layer can be explained within our model.
