  Surface magnetic domains of a UCoGa single crystal during
magnetization/demagnetization processes in increasing/decreasing magnetic
fields were investigated by means of magnetic-force-microscopy (MFM) images at
low temperatures. The observed domain structure is typical for a ferromagnet
with strong uniaxial anisotropy. The evolution of magnetic domains during
cooling of the crystal below TC has also been manifested by MFM images.
Analysis of the available data reveals that the high uniaxial
magnetocrystalline energy in combination with the relatively small
ferromagnetic exchange interaction in UCoGa gives rise to the formation of very
narrow domain walls formed by the pairs of the nearest U neighbor ions with
antiparallel magnetic moments within the basal plane. Since the very high
anisotropy energy is a common feature of the majority of the uniaxial U
ferromagnets, analogous domain-wall properties are expected for all these
materials.
