Observation and Electrical Detection of Magnetic Skyrmions in Iron Germanide Nanostructures

Book excerpt: Magnetic skyrmions are topologically protected spin textures that are being heavily investigated for their potential use in next generation information storage. However, transport studies of skyrmions is limited due to the difficulty of their detection especially in nanostructures. Among cubic B20 helimagnets, FeGe has the highest helimagnetic transition temperature of ~280 K. Nanostructures of skyrmion materials, such as B20 FeGe, are highly desired due to the stabilization of the magnetic skyrmion phase brought on by geometrical confinement. In this thesis, I studied the magnetic skyrmion phase in bulk powders and nanostructures of both cubic B20 FeGe and Fe1-xCoxGe alloys via a combination of Lorentz transmission electron microscopy (LTEM), AC magnetic susceptibility (ACMS), and magnetotransport measurements. Prior to this work, powders of cubic B20 Fe1-xCoxGe alloys were only known to form under high pressures (~4 GPa) and high temperatures (~ 1000 °C), however it was found that Fe1-xCoxGe alloys can form in the B20 structure using elemental precursors reacted at moderate temperatures (~550 °C). During the course of this study, a new skyrmion material, Fe0.95Co0.05Ge, was discovered and its entire magnetic phase diagram was mapped out using ACMS. The powders were then used as seeds in the chemical vapor deposition reactions used to grow both FeGe nanowires (NWs) and Fe1-xCoxGe nanoplates (NPLs). LTEM detected the different magnetic states (helimagnetic, magnetic skyrmion, conical, and field polarized) present in the NWs and NPLs. In both nanostructures, their magnetic skyrmion phases were observed to be far more stable than their bulk counterparts. Magnetoresistance measurements were used to electrically detect the different magnetic phase transitions in both the FeGe NWs and Fe1-xCoxGe NPLs as well as probe their anisotropic electrical signals. The Fe1-xCoxGe NPLs 2D geometry allowed for facile fabrication and detection of the magnetic skyrmion phase using Hall effect measurements. The topological Hall effect, which has been established as the electrical signature of the magnetic skyrmion phase, was detected in the NPLs and used study the stability region of the magnetic skyrmion lattice. The use of the LTEM imaging of these FeGe nanostructures in conjunction with the magnetotransport detection methods lays the solid foundation for future studies of skyrmion-based devices to realize their full potential in information storage technologies.