Abstract
Magnetic vortex chirality in patterned square dots has been investigated by means of a field-dependent magnetic force microscopy technique that allows to measure local hysteresis loops. The chirality affects the two loop branches independently, giving rise to curves that have different shapes and symmetries as a function of the details of the magnetisation reversal process in the square dot, that is studied both experimentally and through micromagnetic simulations. The tip-sample interaction is taken into account numerically, and exploited experimentally, to influence the side of the square where nucleation of the vortex preferably occurs, therefore providing a way to both measure and drive chirality with the present technique.
Highlights
Magnetic vortex chirality in patterned square dots has been investigated by means of a field-dependent magnetic force microscopy technique that allows to measure local hysteresis loops
We demonstrate the possibility of determining vortex chirality from “local hysteresis loop” measurements, by means of a recent magnetic force microscopy (MFM) technique based on a local characterisation of sub-micrometric structures[21]
It is important to point out that the actual chirality states displayed by the square dots of Fig. 1 at their remanence do not remain the same when they are brought to remanence from saturation in subsequent times; even though imperfections in dots shape or on their surface may induce preferred chirality states[37], in our samples they do not seem to univocally determine the vortex chirality
Summary
Magnetic vortex chirality in patterned square dots has been investigated by means of a field-dependent magnetic force microscopy technique that allows to measure local hysteresis loops. As important as the control of the vortex chirality is the possibility to retrieve the stored information by measuring (i.e. reading) the chirality; to this purpose, experiments exploiting techniques like X-ray magnetic circular dichroism (XMCD)[13], SEM with polarisation analysis (SEMPA)[9], planar Hall effect[14], magneto-resistance[3,15] and magnetic force microscopy (MFM)[10] have been proposed, as well as nanopatterned devices like lateral spin valves[16] Some of these techniques do not provide the evolution of the magnetisation with the applied field, or are rather complex or not generally available in laboratories. With the technique exploited here, information on vortex chirality is fully available for both loop branches, together with a complete field dependence of the magnetisation behaviour in the square dot
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