The need for diluted magnetic semiconductors has stimulated research on Mn-doped ZnO. However, the type of magnetic coupling (ferro/para) in ZnMnO remains an issue of debate. We have investigated the magnetic properties of Mn-doped ZnO layers grown by molecular beam epitaxy. Some samples showed a hysteresis with remnant magnetization on the order of 10−5emu, thus eventually suggesting ferromagnetism. We observed that the critical influence of the substrate substantially affects magnetic property measurements. This has to be taken into account in order to clearly confirm ferromagnetism. In our case, after subtraction of the substrate effect, there is no evidence of a ferromagnetic behavior for the ZnMnO samples.

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Ensembles consisting of Co nanoparticles fixed in space by freezing ferrofluids are investigated. In frozen ferrofluids a well defined low temperature magnetic state of the nanoparticle system can be achieved. We distinguish between textured samples (preferred orientation of easy axes) and non-textured with randomly oriented easy axes. Considering the magnetization curves of such systems bears in principle the possibility to distinguish between influences arising from particle anisotropy and particle-particle interaction. Our intention is to present new experimental strategies aiming to separate these influences from magnetization measurements versus temperature. In another approach we analysed the susceptibility derived from magnetization measurements and determined the mean magnetic moment of the particles and an additional field which may be caused by the anisotropy of the particles or the interaction between them. Transmission electron microscopy was used for the direct measurement of the particles' sizes and their size distribution.

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