Nanoscale magnetic imaging of ferromagnetic nanostructures

In the first part of this thesis, ferromagnetic nanotubes (FNTs) consisting of a non-magnetic GaAs core and a ferromagnetic shell of Py or CoFeB with a hexagonal cross-section are investigated using two different magnetic imaging techniques. These techniques allow the investigation of equilibrium magnetic configurations of FNTs depending on the length and diameter. First, x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) is used to image the local magnetization of the FNTs at their surface. For such three-dimensional structures, the technique also yields information about the average magnetization of the volume traversed by the x-rays. Second, a nanometer-scale superconducting quantum interference device sensor located at the end of a hollow quartz tip (SQUID-on-tip) is fabricated and used to image the FNTs’ magnetic stray field distribution. The obtained magnetic images of the FNTs from both magnetic imaging tools provide direct evidence for flux-closure configurations, including a global vortex state, in which the magnetization points circumferentially around the tube axis. Consistent with an analytical theory by Landeros et al. and our own numerical simulations, the FNT length-to-diameter ratio is found to play a crucial role in stabilizing the global vortex state. The XMCD-PEEM images of the equilibrium magnetization configurations show that the relative circulation sense of vortex ends in real FNTs does not always match the lowest energy configuration calculated in simulations. Short FNTs are found not only in remnant global vortex states, but also in opposing vortex states, which include a Néel wall between two opposing vortices. Additional simulations suggest that sample imperfections including variations in thickness and deviation from a perfect geometry are responsible for this discrepancy. Our results show the promise of using geometry to program both the overall equilibrium magnetization configurations and the reversal process in nanomagnets.
In the second part, an experiment is performed using scanning SQUID-on-tip to image the stray field of an artificial spin ice system, which displays structural chirality. Experiments are carried out in series of magnetic fields at 4.2 K. The “chiral ice” is a two-dimensional arrangement of lithographically patterned Py nanomagnets. Each nanomagnet is much thinner than its in-plane dimensions, producing a strong shape anisotropy that favors a single-domain magnetization configuration. The measurements, backed by micromagnetic simulations, reveal that the magnetization in the nanomagnets is not uniform close to zero-field, displaying a bending at the edges of the nanostructures. The results show that the number of degrees of freedom in an artificial spin ice can be much larger than typically captured in dipolar models. These additional degrees of freedom contribute to the field-induced dynamics and may be used to create reprogrammable magnonic crystals.
The final part of the thesis deals with a further development of the SQUID-on-tip technology: the realization of a SQUID at the tip of a conventional atomic force microscopy (AFM) cantilever. To realize such a probe, a focused ion beam (FIB) is used to mill the apex of the Si-cantilever into a suitable shape, which later serves as a template for the nanoSQUID. A SQUID-on-tip, located at the cantilever apex, is obtained through the directional evaporation of a thin Pb film. With this new technology we expect to retain the favorable properties of the SQUID-on-tip technique, while also adding sensitivity to tip-sample forces through standard non-contact AFM techniques. Such a hybrid system would allow the simultaneous imaging of topography, magnetic stray field, and temperature on the nanometer-scale. Thereby it would be possible to directly correlate these quantities, which is crucial for many applications in basic research.