High-Resolution Imaging of Magnetic Structure

To design spintronics devices, an in-depth knowledge of the spin structure is crucial. Magnetic imaging techniques allow for the required direct view on local spin structure on the nanoscale. The magnetic properties of ferromagnetic elements start to be governed very much by the element geometry and not only by the intrinsic materials properties when the geometry changes from the bulk to the nanoscale. This allows one to tailor the magnetization configuration and thus to open an enormous playground for research. For instance, magnetic rings offer a good control of the position and type of domain walls [1] and by changing the thickness and width of the structure different types of domain wall spin configuration can be generated.

We use a variety of imaging techniques including Magnetic Force Microscopy (MFM), which is sensitive to magnetic stray fields, Kerr microscopy based on changes to the polarization state of reflected light and synchrotron-based techniques including Photo-Emission Electron Microscopy (PEEM) [3] and dynamic Scanning Transmission X-ray Microscopy (STXM). The highest resolution is obtained using our laboratory-based Scanning Electron Microscopy with Polarization Analysis (SEMPA) [2]. This technique enables very high resolution direct imaging of the spin configurations down to < 20 nm and allows the determination of the stability of distinct domain wall spin structures in different systems. Furthermore with these techniques it is possible to image the domain structures of thin-films of novel materials such as magnetic oxides at variable temperature to learn about their fundamental magnetic properties [4].

Modern spintronics devices rely on the interaction of magnetization with spin currents, and direct imaging [5] can also be employed to distinguish between the influence of the spin-torque and the Oersted-field torque on a charge current-driven magnetic vortex state displacement (see Fig. 1). It has been used to extract the non-adiabaticity of systems [6, 7].

high res
Figure 1: SEM-image of a 25nm thick Ni(80)Fe(20)-disk (left panel) showing a magnetic vortex state with an off-center vortex core (right panel).

 

References

[1] Rothman et al., Phys. Rev. Lett. 86, 1098 (2001).

[2] P. Oepen, J. Vac. Sci. Technol. B 20, 2535 (2005).

[3] Finizio et al. (2015) New J. Phys. 17, 083030 (2015).

[4] M. Reeve et al., Appl. Phys. Lett. 102, 122407 (2013).

[5] Krüger et al., Phys. Rev. Lett. 104, 077201 (2010).

[6] Heyne et al., Phys. Rev. Lett. 105, 187203 (2010).

[7] Rößler et al., Phys. Rev. B 89, 174426 (2014).