Novel asymmetric magnetization reversal in self-assembled cobalt nanodot arrays
Loedding, Thomas Bernhard
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Cobalt and MnAs dots in a scale less than 100 nm were fabricated using alumina templates for electron beam evaporation and molecular beam epitaxy. The magnetic properties of these close packed nanodot pattern were measured with a Vibrating Sample Magnetometer. The Co dots exhibit a negative exchange bias, which leads to the assumption that the surface of the Co dots oxidized in air. Furthermore an asymmetric step in the hysteresis loop is observed, appearing only in the second quadrant. The temperature and field dependence of this asymmetric step is analysed in detail. The reduction of the coercive field between the first and the second measurement at the same temperature are explained by the training effect. The two critical temperatures indicating the vanishing of the training effect and the negative exchange bias respectively are almost the same and far beneath the Néel temperature of CoO. The vortex state is discussed as a possible explanations for the appearance of the step that have been discovered the first time for a Co nanodot array. The Ni nanodot is absent of the step, suggesting a material dependence of this step. The hysteresis loop of MnAs dots, covered with a GaAs layer to prevent oxidization, exhibits none of these effects.