Quantum Phase Transition and Chiral Magnetism in Nanostructured Cobalt-Based Compounds

In this dissertation, structural, magnetic, and electron-transport properties of Co-based intermetallic compounds in the form of bulk and nanoparticles are studied and their quantum-phase transition (QPT), and high-temperature skyrmionic properties are presented. Co-based magnetic materials are produced by non-equilibrium fabrication processes in the form of bulk samples and nanoparticles, specifically Co1+xSn and Co1+xSi1-x in the bulk form, and CoSi in nanoparticles. The Co1+xSn alloys form a modified hexagonal NiAs-type crystal structure for 0.45 ≤ x ≤ 1. The excess of Co concentration (x) entering the interstitial 2d sites in the NiAs-ordered parent alloy CoSn yields a Griffiths phase and above a quantum critical point (xc = 0.65) a QPT to ferromagnetic order is observed. The magnetic phase transition is described in terms of magnetic cluster formation, percolation, and also as a QPT with critical exponents. B20-ordered Co1+xSi1-x alloys with a maximum excess Co solubility of x = 0.043 are produced and the effect of the QPT on the magnetic properties is studied. Above a critical Co content (xc = 0.028) the Co1+xSi1-x alloys exhibit a QPT and are magnetically ordered with a maximum Curie temperature of Tc = 328 K for x = 0.043, the highest among all B20-type magnets. In an attempt to address the challenge to realize a high magnetic ordering temperature (Tc ≥ 300 K) and a small skyrmion size of 10 nm or less for their potential applications, B20-type CoSi is fabricated in nanoparticle form. These nanoparticles have an average particle size of 11.6 nm and exhibit Tc = 330 K. The nanoparticles\u27 magnetization is controlled by surface atoms. The topological Hall effect (THE), which is a signature of skyrmions, and its temperature dependence is studied. A strong THE is observed at 320 K and lower temperatures and this can be rationalized in terms of a micromagnetic model. These results show promise for the miniaturization of skyrmion size and their stability at above room temperature, with potential applications in information processing