Domain State Model Research Articles

Structure of domains in an exchange-bias model

The structure of domains in the interface monolayer of the antiferromagnet in an exchange-bias system is investigated in the framework of the domain state model. These interface domains carrying remanent magnetization provide the bias field and are strongly influenced by the bulk. The stable part of the spin configurations at the interface, which is responsible for exchange bias, is identified. The stability analysis of the interface domains leads to an explanation of the nontrivial dependence of the bias field on thickness and anisotropy of the antiferromagnet.

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.

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.

Control of exchange bias by diluting the antiferromagnetic layer

The domain state model for exchange bias is used for an investigation of recent experiments where the magnitude and direction of the exchange bias was controlled by He ion irradiation of an FeNi/FeMn sample. The defects in the sample which result from the irradiation are modeled as diluting the antiferromagnet (AFM) after the initial cooling procedure. This late dilution, carried out in presence of a field, leads to a rearrangement of the original domain structure of the AFM resulting in an enhancement or reduction in the bias field.

Domain state model for exchange bias: training effect of diluted Co/sub 1-y/O on exchange bias in Co-CoO

Summary form only given. Summary only form given. Recently, there has been a strong interest in the exchange bias effect (EB) as its origin and its related phenomena are not fully understood. We present systematic investigations of a variety of EB phenomena for Co/Co/sub 1-y/O bilayers as a function of dilution y. These phenomena include besides EB and coercivity fields, the shape and the vertical shift of the hysteresis loops, the temperature dependence of the EB field, the training effect (TE), and the cooling field dependence. By comparing these results to Monte Carlo (MC) simulations, the DS model is shown to provide a consistent microscopic description of the origin of EB as well as of a variety of related phenomena.

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 $({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{O})$ 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.

Domain state model for exchange bias. I. Theory

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.

Simulations of the Domain State Model

ABSTRACTThe domain state model for exchange bias consists of a ferromagnetic layer exchange coupled to an antiferromagnetic layer. In order to model a certain degree of disorder within the bulk of the antiferromagnet, the latter is diluted throughout its volume. Extensive Monte Carlo simulations of the model were performed in the past. Exchange bias is observed as a result of a domain state in the antiferromagnetic layer which develops during the initial field cooling, carrying a remanent domains state magnetization which is partly irreversible during hysteresis. A variety of typical effects associated with exchange bias like, e. g., its dependence on dilution, positive bias, temperature and time dependences as well as the dependence on the thickness of the antiferromagnetic layer can be explained within this model.

Domain state model for exchange bias: thickness dependence of diluted antiferromagnetic Co 1− yO on exchange bias in Co/CoO

The thickness dependence of different diluted antiferromagnetic Co 1− y O layers on the exchange bias (EB) in ferro/antiferromagnetic Co/Co 1− y O bilayers is investigated. For undiluted samples the EB decreases above a layer thickness of 5 nm whereas it increases and saturates for AFM layers thicker than 20 nm for diluted samples. These findings support the domain state model for EB.

Domain state model for exchange bias

Monte Carlo simulations of a system consisting of a ferromagnetic layer exchange coupled to a diluted antiferromagnetic layer described by a classical spin model show a strong dependence of the exchange bias on the degree of dilution in agreement with recent experimental observations on Co/CoO bilayers. These simulations reveal that diluting the antiferromagnet leads to the formation of domains in the volume of the antiferromagnet carrying a remanent surplus magnetization which causes and controls exchange bias. To further support this domain state model for exchange bias we study, in the present article, the dependence of the bias field on the thickness of the antiferromagnetic layer. It is shown that the bias field strongly increases with increasing film thickness and eventually goes over a maximum before it levels out for large thicknesses. These findings are in full agreement with experiments.

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.

Digital simulation of power conditioner-machine interaction for electronically commutated DC permanent magnet machines

The electromagnetic interactions between a transistorized chopper-inverter power conditioner and its associated permanent magnet machine is analyzed digitally by an experimentally verified time domain state model. The model is applied to the design of a system based on a ferrite type machine. The impact on the overall system performance of skewing the stator core to reduce the harmonics in the induced machine EMF is determined using this model. The EMF waveforms for zero, half, and one slot pitch skewing are determined by finite element analysis of the fields inside the machine. Furthermore, the impact on the system performance of varying the commutation angle to overcome inductance caused performance limitations is examined for each of the three cases of stator skewing.