Magnetic charge correlation and quantum disorder in honeycomb spin ice systems
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The artificial spin ice system consists of islands comprised of nanomagnets arranged in desired lattice structures, including squares, triangles, and honeycombs. These nanomagnets can also be arranged in an aperiodic lattice structure in some cases. These systems are artificial imitation of naturally ubiquitous systems such as geometric frustration which often lead to novel magnetic phases and its phase transitions. The research in artificial spin ice systems have made it possible to explore properties that have been mostly studied in the bulk systems such as pyrochlore spin-ice crystals. Theoretically it has been predicted that magnetic honeycomb lattice manifest depth-tunable magnetic phase transition based on the temperature of the system. The vertex of the honeycomb can have four unique configurations whose arrangements over the lattice would determine the magnetic phase of the system. At high temperature, honeycomb spin ice has been predicted to exhibit paramagnetic gas state, which upon cooling exhibits a phase transition to spin-ice, charge order, and magnetic vortex spin-solid state, respectively. The state-of-the-art method for developing an artificial spin ice in the laboratory is via lithography techniques which usually renders micrometer size elements. This inhibits the study of temperature dependent phase transitions in these micrometer-size elements as it leads to a large inter-elemental energy in the order of approx 10^5 K. However, it is the aim of this thesis to explore a new technique of preparing magnetic honeycomb lattice with nanome ter size elements by utilizing diblock-copolymer of polystyrene and poly-4-vinyl-bpyridine (PS-P4VP) template and physical vapor deposition of magnetic Permalloy in oblique geometry. The resulting magnetic honeycomb elements have a length of approx 10-12 nm and 5 nm width and a variable thickness, which give rise to a very small inter-elemental energy of approx 10 K, hence making this system tunable based on the system temperature. In this thesis, we have used these nanoscale artificial magnetic honeycomb lattice to explore various novel magnetic phases and their temperature dependent properties using a combination of electrical and magnetic measurements along with neutron scattering techniques such as polarized neutron reflectivity and neutron spin echo spectroscopy. We have discovered the existence of a massively degenerate quantum disordered state in these magnetic honeycomb lattice as a result of frustrations arising from competing nearest neighbor and next neighbor exchange interactions by using polarized neutron reflectivity. By using neutron spin echo technique, we have also estimated the magnetic charge relaxation and its transport properties. Furthermore, we also discovered key underlying mechanisms of a magnetic diode in these magnetic honeycomb lattices by using neutron reflectivity, micromagnetic simulations and distorted wave born approximations (DWBA).We showed that the magnetic honeycomb lattice undergoes a fractional transformation of magnetic charge ordering which gives rise to the magnetic diode behavior via neutron reflectometry and DWBA simulations. We also highlight two complementary studies based on neutron scattering of bulk materials namely: CeAuSb2 and CoAsSe. In both the cases, we have used elastic and inelastic neutron scattering to study the magnetic structure and its order parameters. These studies are supported by first principles calculations based on the density functional theory.
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Ph. D.