No abstract

The size dependence of exchange bias field HE and coercivity Hc was studied by measuring exchange biased Fe-FeF2 dot arrays in comparison with an unstructured exchange biased Fe-FeF2 bilayer. The domain sizes in the ferromagnet (FM) and the antiferromagnet (AFM) play an important role for exchange bias (EB), and thus interesting phenomena may be expected when the size of an EB system becomes comparable to these sizes. We observe drastic changes of HE and Hc in nanostructured Fe-FeF2, which are unexpected because they appear even at a structure size which is too large for matching with AFM or FM domain size to play a role. We propose that under certain conditions the hysteresis loop is affected differently in the two branches of the reversal by shape anisotropy due to patterning. This is possible because the EB induces a reversal asymmetry already in the unpatterned bilayer system.

-Nanostructured Fe dots were prepared on antiferromagnetic FeF 2 thin films and investigated by magneto-optical Kerr effect (MOKE). We studied the influence of dot sizes on the magnetic hysteresis and compared the result with both continuous thin film bilayers and nanostructured Fe/FeF 2 pillars. Hysteresis loops were measured at temperatures below and above (10 and 90 K, respectively) the Néel temperature of the antiferromagnet. A vortex state is found for dots of 300 nm diameter, where the exchange bias field is reduced compared to larger dot system and the continuous bilayer.Micromagnetic simulations including the interaction with the antiferromagnet show qualitatively similar behavior.

The asymmetry of the magnetization reversal process in exchange biased Fe/FeF2 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 FeF2(110) layer.

Depth-dependent Fe spin structures of the remanent state in exchange-coupled Fe/MnF 2 films have been probed using 57 Fe conversion electron Mössbauer spectroscopy, both above and well below the MnF 2 Néel temperature. 57 Fe probe layers were embedded either at the Fe/MnF 2 interface or in the center of the Fe film. Remarkably, exchange bias induces a significant change of the in-plane angular distribution of the Fe magnetic moments at the interface and inside the Fe film, away from the saturation magnetization direction. Results from vector magnetometry support these conclusions.

The effect of annealing at 250 • C on the carrier depth profile, Mn distribution, electrical conductivity, and Curie temperature of (Ga,Mn)As layers with thicknesses ≥ 200 nm, grown by molecularbeam epitaxy at low temperatures, is studied by a variety of analytical methods. The vertical gradient in hole concentration, revealed by electrochemical capacitance-voltage profiling, is shown to play a key role in the understanding of conductivity and magnetization data. The gradient, basically already present in as-grown samples, is strongly influenced by post-growth annealing. From secondary ion mass spectroscopy it can be concluded that, at least in thick layers, the change in carrier depth profile and thus in conductivity is not primarily due to out-diffusion of Mn interstitials during annealing. Two alternative possible models are discussed.

We observe a thermally induced spontaneous magnetization reversal of epitaxial ferromagnet/antiferromagnet heterostructures under a constant applied magnetic field. Unlike any other magnetic system, the magnetization spontaneously reverses, aligning antiparallel to an applied field with decreasing temperature. We show that this unusual phenomenon is caused by the interfacial antiferromagnetic coupling overcoming the Zeeman energy of the ferromagnet. A significant temperature hysteresis exists, whose height and width can be tuned by the field applied during thermal cycling. The hysteresis originates from the intrinsic magnetic anisotropy in the system. The observation of this phenomenon leads to open questions in the general understanding of magnetic heterostructures. Moreover, this shows that in general heterogeneous nanostructured materials may exhibit unexpected phenomena absent in the bulk.

. Thcfilms had a thckness of 300 nm and a critlcal current plied. Magnetic flux penetrated into the superconducting film firs1 from the edges and from defects which were in conracr with the edges until a localequilibrium of the flux distribution due to tbe pinning lorce and the ~~iagneric force was reached. This induces a current distribution in the superconducting film. In order to disturb the equilibrium of tlus current distribution and to iuitiare a magnetic instability, a pulse of a frequency doubled Nd : YAG laser = 532 nm, halfwidrh T = 7 ns) was foc~ised onto the filn1 from the substrate side. The energy deas~ty i o the laser spot (diameter 30pm) was up to 30 mJ/m2. The san~ple temperature iu the locus could not be measured directly, but we estimate that the temperarure is well above the cnrical temperature.If the perfurbation is sufficiently strong, e.g. Cor energy densities of rhe laser pulse above 7 rnJ/cm2, this trigzers a maguetic instability, in which a ~nagnetic flux avalanche penetrates into the sin. Fig. 1 shows a mapnetooptical image of the resulting flux distribution aller the laser pulse. Bright redons correspond to high magnetic flux density. In contrast to tlie niore or less llomogeneous flux fronts which proplgate towards the sample center w h n thc external field is gradually increased, this instability develops in the form oC a dendritic pattern as already observed earlier [I]. The total area covered by the flux branches is found to increase linearly with the ex~ernal magnetic field B,,, above a threshold (B,,, = 7.5 mT), whereas the widrh o l the branches (0.1 mm) remains constant witbin our accuracy.