Nowak, Ulrich
0000-0003-2925-6774 Berufsbeschreibung
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Training effect of exchange-bias bilayers within the domain state model
An investigation of the temperature dependence of the training effect of various exchange coupled bilayers with different types of anisotropy is presented. We use an atomistic model for the magnetic interactions within a classical Heisenberg spin Hamiltonian. In general, the behavior of the exchange-bias field is separated into low- and high-temperature regions. This separation is made according to the trend of exchange-bias field after the second hysteresis loop and the parameters of the power-law fit for these fields. It is found that with increasing antiferromagnetic thickness, systems follow the same temperature trend but with lower values of the exchange-bias field and a weaker training effect. This is due to the fact that thicker antiferromagnetic layers lead to increased stability of the antiferromagnetic domains. Also, the behavior of the coercive fields is investigated, concluding that the training effect occurs predominantly in the first half of the hysteresis loop.
Exchange bias in ferromagnetic/antiferromagnetic bilayers with imperfect interfaces
The influence of an imperfect interface on exchange bias (EB) properties is investigated. Within the framework of the domain state model, the EB field HEB and the coercive field HC are determined using computer simulations, and they are found to depend strongly on the details of the interface structure. This dependence is sensitive to the dilution of the antiferromagnet (AFM) with non-magnetic defects in the bulk. For the optimal interface structure, giving greatest EB, the optimal dilution is found to be much less than that for an ideal-interface system, taking a value in better agreement with experimental results. Even without any defects in the bulk of the AFM the interface roughness leads to EB for thin antiferromagnetic layers, in accordance with the model by Malozemoff. Finally, the thickness dependence of rough-interface systems is found to differ significantly from that of ideal-interface systems.
Asymmetric Reversal Modes in Ferromagnetic/Antiferromagnetic Multilayers
Experimentally an asymmetry of the reversal modes has been found in certain exchange bias systems. From a numerical investigation of the domain state model evidence is gained that this effect depends on the angle between the easy axis of the antiferromagnet and the applied magnetic field. Depending on this angle the ferromagnet reverses either symmetrically, e.g., by a coherent rotation on both sides of the loop, or the reversal is asymmetric with a nonuniform reversal mode for the ascending branch, which may even yield a zero perpendicular magnetization.
Spin configuration of ferromagnetic/antiferromagnetic nano-composite particles
A systematic study of the spin configuration of ferromagneti (core)/antiferromagnetic(shell) nanocomposite particles was carried out by two approaches: micromagnetic and atomistic simulations. In the case of strong interlayer exchange coupling between ferromagnet and antiferromagnet, the micromagnetic simulation result is consistent with the atomistic simulation result. However, in the case of weak interlayer exchange coupling the micromagnetic simulation predicts a multi-domain structure of the ferromagnet, which is inconsistent with the atomistic simulation. The failure of micromagnetic simulation in describing the spin structure of the nano-composite particles might be interpreted in terms of the under-estimation of the exchange energy for rapid spatial fluctuations of the magnetization by the continuum exchange formalism or might be due to the cooling process procedure. Also, the variation of spin structure as a function of the uniaxial anisotropy of the core is presented and discussed.
Coercivity and exchange bias of ferromagnetic/antiferromagnetic multilayers
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 Néel temperature T N 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 T N also found experimentally and the occurrence of exchange bias are of different origin.
Domain state model for exchange bias : II. Experiments
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 (Co1-xMgxO) or (ii) Co deficiencies (Co1-yO). 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.
Cooling-field dependence of asymmetric reversal modes for ferromagnetic/antiferromagnetic multilayers
A numerical investigation of exchange coupled ferromagnetic/antiferromagnetic multilayers with a twinned crystal structure for the antiferromagnet is presented. Motivated by recent experimental findings we focus on the influence of the directions of the magnetic field during the initial cooling procedure and during the hysteresis. Upon variation of these directions the ferromagnet displays different reversal modes or even an asymmetric reversal with different kinds of reversal mechanisms for the decreasing and increasing branch of a single hysteresis loop. These findings can be explained within the context of the domain state model for exchange bias.
Antiferromagnetic layer thickness dependence of the IrMn/Co exchange-bias system
A study of exchange bias in IrMn/Co systems is presented. Temperature and thickness dependence studies have revealed nonmonotonic behavior in both exchange bias field and coercivity with both variables. In particular the exchange bias field shows a peak for low IrMn thicknesses that is suppressed at temperatures higher than about 200 K. Calculations using the domain state model of exchange biasing are able to describe all the features seen in the experimental data.
Diluted Antiferromagnets in Exchange Bias : Proof of the Domain State Model
The exchange bias coupling at ferromagnetic/antiferromagnetic interfaces in epitaxially grown Co/CoO layers can intentionally be increased by a factor of up to 3 if the antiferromagnetic CoO layer is diluted by nonmagnetic defects in its volume part away from the interface. Monte Carlo simulations of a simple model of a ferromagnetic layer on a diluted antiferromagnet show exchange bias and explain qualitatively its dilution and temperature dependence. These investigations reveal that diluting the antiferromagnet leads to the formation of volume domains, which cause and control exchange bias.