Magnetization reversal was studied in square arrays of square Co/CoO dots with lateral size varying between 200 and 900 nm. While reference nonpatterned Co/CoO films show the typical shift and increased width of the hysteresis loop due to exchange bias, the patterned samples reveal a pronounced size dependence. In particular, an anomaly appears in the upper branch of the magnetization cycle and becomes stronger as the dot size decreases. This anomaly, which is absent at room temperature in the patterned samples, can be understood in terms of a competition between magnetostatic interdot interaction and exchange anisotropy during the magnetic switching process.

We have studied the effect of magnetic ordering on the electrical transport in magnetic GaAs diodes. The fabricated pn‐junctions consist of Mn‐doped ferromagnetic layers on top of non‐magnetic semiconductor substrates. First the electrical and magnetic properties of the magnetic layers are investigated, which shows that the exchange interaction between the charge carriers and the localized magnetic moments of the Mn‐atoms dominates the charge transport at temperatures close to the Curie temperature. In the Gax Mn1–x As/GaAs‐p++n++‐junctions having heavily doped p‐ and n‐regions, the inter‐band tunnelling current shows large dependence on the magnetic ordering. The spin‐dependent tunnelling can be explained by a model, where the density of valence band states decreases due to the band splitting caused by the exchange interaction. From the measured decrease of the tunnelling current we estimated the value for the exchange parameter $ J^{\rm pd}_{\rm exch} $ = 2.9 eV. Various explanations for the absence of magnetoresistance in the magnetic diodes with lightly doped non‐magnetic regions are discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We report experimental studies of crystals of Mn 12 molecular magnetic clusters in pulsed magnetic fields with sweep rates up to 4 ϫ 10 3 T / s. The steps in the magnetization curve are observed at fields that are shifted with respect to the resonant field values. The shift systematically increases as the rate of the field sweep goes up. These data are consistent with the theory of the collective dipolar relaxation in molecular magnets.

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