High Resolution Imaging of Nanoscale Structures using SEMPA
Pascal Krautscheid (Institut für Physik)

For proposed spintronic devices such as memory, logic and sensors a control over the static spin configuration and an ability to understand and manipulate the dynamics is required. The initial configuration, such as the domain wall spin structure can be tailored through the geometry and the use of specific materials. For instance, magnetic rings offer a good control of the domain wall position and the domain wall type. Changing the thickness and width of the structure and applying an external magnetic field results in the onion state [1] with different types of domain wall spin configuration at opposing sides. The occurrence of a specific kind of domain wall critically depends on the dimensions of the structure [2]. The type and size of the domain wall limits achievable data storage densities and influences the dynamics. Previous measurements of Co and Ni_{80}Fe_{20} based rings showed a transition from a transverse domain wall to a vortex domain wall structure with increasing ring width and thickness [2]. Here, we study domain walls in Fe rings with various width and thicknesses using micromagnetic simulations [3] mimicking the experiment by applying an external field and relaxing the domain wall while reducing the field stepwise. Due to the nucleation process transverse domain walls are expected at higher widths than the scaling behaviour of the competing energy terms would suggest [4]. We compare the results with experimental data obtained using a scanning electron microscope with polarization analysis (SEMPA) [5], which offers the necessary high-resolution magnetic imaging to gain vectorial magnetization information within the plane of our nanoscale structures. The observed vortex wall configuration is accessible in a large range of ring sizes in iron and the transverse wall phase boundary is found in smaller rings for iron as compared to Ni_{80}Fe_{20}. The data suggest that the experimental phase boundary follows the absolute boundary closely.
[1] J. Rothman et al., Phys. Rev. Lett. 86, 1098 (2001).
[2] M. Kl”aui, J. Phys.: Condens. Matter 20 (2008).
[3] Mikromagnum.informatik.uni-hamburg.de.
[4] R. D. McMichael et al. IEEE Trans. Magn. 33, 4167 (1997).
[5] R. Allensbach, IBM J. Res. Develop. 44, 553 (2000).