  We present a combined theoretical and experimental study of the energetic
stability and accessibility of different domain wall spin configurations in
mesoscopic magnetic iron rings. The evolution is investigated as a function of
the width and thickness in a regime of relevance to devices, while Fe is chosen
as a material due to its simple growth in combination with attractive magnetic
properties including high saturation magnetization and low intrinsic
anisotropy. Micromagnetic simulations are performed to predict the lowest
energy states of the domain walls, which can be either the transverse or vortex
wall spin structure, in good agreement with analytical models, with simulations
revealing the expected low temperature configurations observable on relaxation
of the magnetic structure from saturation in an external field. In the latter
case, following the domain wall nucleation process, transverse domain walls are
found at larger widths and thicknesses than would be expected by comparing the
competing energy terms demonstrating the importance of metastability of the
states. The simulations are compared to high resolution experimental images of
the magnetization using scanning electron microscopy with polarization analysis
to provide a phase diagram of the various spin configurations. In addition to
the vortex and simple symmetric transverse domain wall, a significant range of
geometries are found to exhibit highly asymmetric transverse domain walls with
properties distinct from the symmetric transverse wall. Simulations of the
asymmetric walls reveal an evolution of the domain wall tilting angle with ring
thickness. Analysis of all the data reveals that in addition to the geometry,
the influence of materials properties, defects and thermal activation all need
to be taken into account in order to understand and reliably control the
experimentally accessible states, as needed for devices.
