Off-campus UNL users: To download campus access dissertations, please use the following link to log into our proxy server with your NU ID and password. When you are done browsing please remember to return to this page and log out.

Non-UNL users: Please talk to your librarian about requesting this dissertation through interlibrary loan.

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

Rabindra Pahari, University of Nebraska - Lincoln


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' 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.

Subject Area

Physics|Condensed matter physics|Molecular physics

Recommended Citation

Pahari, Rabindra, "Quantum Phase Transition and Chiral Magnetism in Nanostructured Cobalt-Based Compounds" (2021). ETD collection for University of Nebraska - Lincoln. AAI28419178.