Size limits of magnetic-domain engineering in continuous in-plane exchange-bias prototype films

Background: The application of superparamagnetic particles as biomolecular transporters in microfluidic systems for lab-on-a-chipapplications crucially depends on the ability to control their motion. One approach for magnetic-particle motion control is thesuperposition of static magnetic stray field landscapes (MFLs) with dynamically varying external fields. These MFLs may emergefrom magnetic domains engineered both in shape and in their local anisotropies. Motion control of smaller beads does necessarilyneed smaller magnetic patterns, i.e., MFLs varying on smaller lateral scales. The achievable size limit of engineered magneticdomains depends on the magnetic patterning method and on the magnetic anisotropies of the material system. Smallest patterns areexpected to be in the range of the domain wall width of the particular material system. To explore these limits a patterning technol-ogy is needed with a spatial resolution significantly smaller than the domain wall width.Results: We demonstrate the application of a helium ion microscope with a beam diameter of 8 nm as a mask-less method for localdomain patterning of magnetic thin-film systems. For a prototypical in-plane exchange-bias system the domain wall width has beeninvestigated as a function of the angle between unidirectional anisotropy and domain wall. By shrinking the domain size of periodic domain stripes, we analyzed the influence of domain wall overlap on the domain stability. Finally, by changing the geometryof artificial two-dimensional domains, the influence of domain wall overlap and domain wall geometry on the ultimate domain sizein the chosen system was analyzed.Conclusion: The application of a helium ion microscope for magnetic patterning has been shown. It allowed for exploring thefundamental limits of domain engineering in an in-plane exchange-bias thin film as a prototypical system. For two-dimensionaldomains the limit depends on the domain geometry. The relative orientation between domain wall and anisotropy axes is a crucialparameter and therefore influences the achievable minimum domain size dramatically