Probing the magnetization reversal of patterned cobalt and nickel-iron nanostructures

As the size of magnetic structures is decreased to the nanoscale, many physical properties such as magnetization reversal begin to change dramatically from those observed in bulk materials. An in-depth understanding of the magnetization reversal, which is of great scientific and technological importance is the focus of this dissertation. NiFe and Co nanostructures were fabricated and systematically studied as a function of size, shape, and temperature. The magnetic nanoelement arrays were fabricated using a bi-layer electron beam lithography process followed by electron beam evaporation and lift-off or by ion milling. The magnetization reversal of the samples was characterized using a variable temperature (2 K to 325 K), focused Magneto-optic Kerr effect measurement system that I constructed here at the University of Utah. Additional information about the magnetization states of the elements including inter-element coupling was obtained by micromagnetic simulations and through imaging by photoemission electron microscopy and magnetic force microscopy. It was found that the element shape and therefore the magnetostatic energy plays an important role in magnetization reversal. The temperature dependence of the switching field for many rectangular elements follows a linear behavior, which is predicted by the two-dimensional thermally activated nucleation theory. To better understand this nucleation process, long (60 μm) nanolines (where the magnetization must break into domains during reversal) ranging in width from 55 nm to 1 μm were studied as a function of width and thickness. The coercivity was found to have a universal dependence, inversely proportional to the width and proportional to both the saturation magnetization and the thickness of the element, which is consistent with the coherent rotation model prediction for smaller aspect ratio elements, but not expected for such long elements. An effective understanding of this behavior was found and is here outlined. Temperature dependence investigation in combination with micromagnetic simulation and imaging are very powerful tools in the illumination of the fundamental physics involved in nanoscale magnetism.