Investigation of Exotic Spin Textures in Chiral Magnetic Silicide Nanowires for Spintronics Applications

Chiral magnetic materials exhibit a wide array of exotic spin configurations that are incommensurate with the underlying crystal lattice and their formation results from the competition of ferromagnetic exchange interactions and weaker Dzyaloshinskii-Moriya spin-orbit interactions. One recently revealed configuration for the B20-structure type, known as a "skyrmion," is a topologically non-trivial vortex-like magnetic domain. Skyrmion domains have become of great interest for applications in future magnetic memory storage devices since they can be manipulated as particle-like entities with remarkably low electrical currents. A major question is how the formation and current-induced dynamics of skyrmion domains are affected by the nanoscale dimensions required for such applications, and we have developed several characterization techniques to address the electrical and magnetic properties of the two silicide nanowire systems, MnSi and Fe1-xCoxSi that both have the B20-structure type. We have first observed skyrmion domains in a focus ion-beam thinned MnSi nanowire specimen using Lorentz transmission electron microscopy, and we identified that the skyrmion lattice phase is stable under small applied magnetic fields between the temperatures of 6 K and 35 K. We then developed a novel Hall effect measurement method for nanowires that we used to identify the topological Hall effect which is the electrical signature of magnetic skyrmion domains. These topological Hall effect measurements reveal that the skyrmion phase is stable in pristine MnSi nanowires between 15 K and 35 K which is a significantly larger temperature window compared to the skyrmion lattice in bulk MnSi crystals (only observed between 27 K and 29.5 K). Finally, we have measured the spin polarization of conduction electrons (which can have a significant impact on the magnitude of the topological Hall effect) in Fe1-xCoxSi nanowires using Andreev reflection spectroscopy implemented with a superconducting heterojunction.