Andrea Tillmanns

Magneto-optical study of magnetization reversal asymmetry in exchange bias

The asymmetric magnetization reversal in exchange biased Fe∕MnF2 involves coherent (Stoner-Wohlfarth) magnetization rotation into an intermediate, stable state perpendicular to the applied field. We provide here the experimentally tested analytical conditions for the unambiguous observation of both longitudinal and transverse magnetization components using the magneto-optical Kerr effect. This provides a fast and powerful probe of coherent magnetization reversal as well as its chirality. Surprisingly, the sign and asymmetry of the transverse magnetization component of exchange biased, low-anisotropy MnF2 and high-anisotropy FeF2 change with the angle between cooling and measurement fields.

Angular dependence and origin of asymmetric magnetization reversal in exchange-biased Fe / FeF 2 ( 110 )

The asymmetry of the magnetization reversal process in exchange biased Fe/FeF

Domain State Model for Exchange Bias: Influence of Structural Defects on Exchange Bias in Co/CoO

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Structural and magnetic properties of Ni/NiOxide- and Co/CoOxide core/shell nanoparticles and their possible use for ferrofluids

has been studied by magneto-optical Kerr effect. Qualitatively different transverse magnetization loops are observed for different directions of the cooling and the measuring field. These loops can be simulated by a simple calculation of the total energy density which includes the relevant magnetic anisotropies and coherent magnetization rotation only. Asymmetric magnetization reversal is shown to originate from the unidirectional anisotropy and may be observed if the external measuring field is not collinear with either the exchange bias or the easy axis of the antiferromagnetic epitaxial FeF

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

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Exchange bias in epitaxial Co O ∕ Co bilayers with different crystallographic symmetries

(110) layer.

Anomalous magnetization reversal mechanism in unbiased Fe/FeF2 investigated by means of the magneto-optic Kerr effect

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.

Magnetisierungsumkehr und -dynamik in Exchange-Bias-Systemen

Summary Ni- and Co-nanoparticles of average sizes of about 4 to 10 nm were produced by “physical” means with the cluster source LUCAS based upon laser evaporation into seeding gas and adiabatic expansion into an UHV experimentation chamber. They were deposited and subsequently oxidized in situ at about 400 °C. The structures of the nanos were characterized by HRTEM and chemically analyzed by EFTEM. Clear core/shell structures were established with crystalline metallic core and crystalline oxide shell of final thickness of about 2 nm. In one sample it might be that the shell is amorphous. An analysis of magnetic properties by SQUID was performed at temperatures between 5 and 300 K and for three different states of oxidation. Surprisingly strong influences of magnetic defects in the nominally (but probably incompletely saturated) antiferromagnetic oxide layers were observed which result in additional exchange anisotropy, in drastic exchange bias shifts of the hystereses and, in one sample, in a second hysteresis loop contribution. In all samples we found suppression of the superparamagnetic state. The special sample with two loop contributions was numerically evaluated (by co-author H. Krenn). In the final chapter a method is proposed how to transform these “physically” produced magnetic nanoparticles into a ferrofluid of well known composition.

Polarisation auf dem Schreibtisch. Experimente mit TFT‐Flachbildschirmen

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.