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 of the local spin structure on the small length scales which are of interest for modern and future devices. As one moves from the bulk to the nanoscale, the magnetic properties of ferromagnetic elements become increasingly governed  by the element geometry and not only by the intrinsic materials. This allows one to tailor the magnetization configuration opens an enormous playground for research. For instance, magnetic rings offer a good control of the position and type of domain walls and by changing the thickness and width of the structure different types of domain wall spin configuration can be generated [1].

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) and dynamic Scanning Transmission X-ray Microscopy (STXM) [2]. Particularly high spatial resolution is obtained using our laboratory-based Scanning Electron Microscopy with Polarization Analysis (SEMPA) system [3]. 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 (see Figure) and the imaging of the domain structure of devices [4]. Recent advances also allow for enhanced detection sensitivity and dynamic imaging with a temporal resolution of a few ns [5]. With these techniques it is also possible to image the domain structures of thin-films and nanostructures of novel materials such as highly spin polarized magnetic oxides and Heusler alloys at variable temperature, to learn about their fundamental magnetic properties [6, 7].

Modern spintronics devices rely on the interaction of magnetization with spin currents, and direct imaging is also instrumental in revealing the details of this interaction. The imaging of current induced domain wall motion reveals domain wall spin structure changes which have a direct impact on device efficiencies [8], while imaging current-induced vortex core displacements (see Figure) allows the extraction of fundamental dynamic parameters of a system such as the debated non-adiabaticity parameter [9].

Left: SEM-image of a 25nm thick Ni(80)Fe(20)-disk showing a magnetic vortex state with an off-center displaced vortex core (color-code overlay, imaged via SEMPA) as a result of current excitation (black arrow).
Right: domain wall spin structure evolution in different width Ni(80)Fe(20) wires, measured with SEMPA.

 

References

[1] P. Krautscheid et al, J. Phys. D 49, 425005 (2016).
[2] H. Stoll et al., Frontiers in Physics 3, 26 (2015).
[3] R. Allenspach, IBM J. Res. Dev. 44, 553 (2000).
[4] R. Reeve et al., J. Phys.: Condens. Matter 26, 474207 (2014).
[5] R. Frömter et al., Appl. Phys. Lett. 108, 142401 (2016).
[6] R. M. Reeve et al., Appl. Phys. Lett. 102, 122407 (2013).
[7] Finizio et al., New J. Phys. 17, 083030 (2015).
[8] Finizio et al, J. Phys.: Condens. Matter 26, 456003 (2014).
[9] Heyne et al., Phys. Rev. Lett. 105, 187203 (2010).