The physics of nanoscopic metal rings and honeycombs
Both magnetic and superconducting systems exhibit long range order physics. In the magnetic case, long range order is described by the exchange length of the magnetic material while superconducting long range order is defined by its coherence length. If the dimensional length scales of the physical systems become comparable to the long range length scale of the parent material then interesting physical phenomena are observed. This thesis is mainly about the study of physical systems whose dimensional length scales are comparable to the long range length scales of the parent materials. It is mainly divided into three groups: ferromagnetic rings, superconducting honeycomb films and ultrasmall rings on thin Nb film.
Chapter 1 and 2 provide an introduction and overview. Chapter 3 discusses the fabrication techniques used for this research. It is relevant to note that a new fabrication technique is revealed, which results in the fabrication of metal rings of unprecedented small size and also small symmetric and asymmetric rings. Chapters 4 discuss the experiments on ferromagnetic ultrasmall rings (diameter 13 nm) and asymmetric small rings (diameter 150 nm). Magnetic measurements and analytical investigations indicate that magnetic transition process is simple and occurs via the formation of vortex state in the case of ultra-small rings. It is also shown that by creating asymmetry in the ring's width, the direction of vortex magnetic circulation can be controlled by simple application of magnetic field.
A related structure is a honeycomb film where hole sizes are of the order of 13 nm and center-to-center distance between neighboring holes are 28 nm. Chapters 5 discuss the physics of honeycomb films made of superconducting niobium. It is found that as the dimension scale becomes comparable to the coherence length of the superconductor, interesting physical behavior is observed, which includes: Unusual suppression of transition temperature for thin honeycomb films as compared to plain films, change in dimensionality of the films and a jump in resistivity for a sample etched for a particular amount of time.
Chapter 6 discusses the electrical transport measurements of ultrasmall magnetic rings on a metal film.