Vortex states in patterned exchange biased NiO/Ni samples

We investigated the magnetization reversal of arrays of exchange biased NiO/Ni squares with superconducting quantum interference device magnetometry and micromagnetic simulations. The edges of the squares were 0.5, 1.5, and 3.0 $\mu$m long. The NiO/Ni structures exhibit vortexlike hysteresis loops and micromagnetic calculations show that this feature is due to several vortices nucleating in the islands. Furthermore, for the arrays with squares of 1.5 $\mu$m edge length, the sign of the exchange bias field changes, as compared to the same continuous NiO/Ni layer. We attribute the vortex nucleation and the change of the exchange bias field to the interplay between shape and unidirectional anisotropy.Comment: 6 pages, 7 figure

Asymmetric Reversal in Inhomogeneous Magnetic Heterostructures

Asymmetric magnetization reversal is an unusual phenomenon in antiferromagnet / ferromagnet (AF/FM) exchange biased bilayers. We investigated this phenomenon in a simple model system experimentally and by simulation assuming inhomogeneously distributed interfacial AF moments. The results suggest that the observed asymmetry originates from the intrinsic broken symmetry of the system, which results in local incomplete domain walls parallel to the interface in reversal to negative saturation of the FM. Magneto-optic Kerr effect unambiguously confirms such an asymmetric reversal and a depth-dependent FM domain wall in accord with the magnetometry and simulations.Comment: 4 pages, 4 figure

Phase composition of iron oxide nanoparticles studied using hard X-ray absorption spectroscopy

At the surface of iron oxide nanoparticles, an oxidized or disordered layer is often found. Due to the large surface-to-volume ratio of nanomaterials, such a surface layer plays an important role in the overall magnetic properties of the particles. Consequently, it is important to characterize the surface layer if applications of iron oxide nanoparticles, e.g., for magnetic hyperthermia, magnetic particle imaging, or ferrofluidics, are envisaged. In this work, we tuned the phase of the surface layer of 14 nm iron oxide nanoparticles via annealing procedures. The phase composition of the particles is systematically studied using hard X-ray absorption spectroscopy

Loop Bifurcation and Magnetization Rotation in Exchange Biased Ni/FeF2

Exchange biased Ni/ FeF2 films have been investigated using vector coil vibrating sample magnetometry as a function of the cooling field strength H_FC. In films with epitaxial FeF2, a loop bifurcation develops with increasing H_FC as it divides into two sub-loops shifted oppositely from zero field by the same amount. The positively biased sub-loop grows in size with H_FC until only a single positively shifted loop is found. Throughout this process, the negative/positive (sub)loop shift has maintained the same discrete value. This is in sharp contrast to films with twinned FeF2 where the exchange field gradually changes with increasing H_FC. The transverse magnetization shows clear correlations with the longitudinal sub-loops. Interestingly, over 85% of the Ni reverses its magnetization by rotation, either in one step or through two successive rotations. These results are due to the single crystal nature of the antiferromagnetic FeF2, which breaks down into two opposite regions of large domains.Comment: 16 pages, 3 figures, to appear in PR

Novel concepts for University teaching

This talk intends to summarize a number of novel concepts for University teaching, especially but not exclusively for virtual frameworks. Since I am not an expert in the science of teaching, but rather a typical teacher who just wishes to increase the efficiency, this talk primarily aims at inducing an open discussion and the sharing of examples from the audience. I will present several interesting and relevant concepts including "inverted classroom", "live feedback" etc. The talk isrelevant for senior scientists, post-docs, Master and PhD students. Since most of us are in some role of teaching, whether it is in a University course, a tutorial, a spring school or similar, the necessity for efficient teaching is the same. In addition, many junior scientists might come into the role of a teacher in the future. Also, input and experiences from the student's perspective are very important