  Hybridization of semiconducting and magnetic materials into a single
heterostructure is believed to be potentially applicable to the design of novel
functional spintronic devices. In the present work we report epitaxial
stabilization of four magnetically ordered iron oxide phases (Fe3O4,
{\gamma}-Fe2O3, {\alpha}-Fe2O3 and most exotic metastable {\epsilon}-Fe2O3) in
the form of nanometer sized single crystalline films on GaN(0001) surface. The
epitaxial growth of as many as four distinctly different iron oxide phases is
demonstrated within the same single-target Laser MBE technological process on a
GaN semiconductor substrate widely used for electronic device fabrication. The
discussed iron oxides belong to a family of simple formula magnetic materials
exhibiting a rich variety of outstanding physical properties including peculiar
Verwey and Morin phase transitions in Fe3O4 and {\alpha}-Fe2O3 and multiferroic
behavior in metastable magnetically hard {\epsilon}-Fe2O3 ferrite. The physical
reasons standing behind the nucleation of a particular phase in an epitaxial
growth process deserve interest from the fundamental point of view. The
practical side of the presented study is to exploit the tunable polymorphism of
iron oxides for creation of ferroic-on-semiconductor heterostructures usable in
novel spintronic devices. By application of a wide range of experimental
techniques the surface morphology, crystalline structure, electronic and
magnetic properties of the single phase iron oxide epitaxial films on GaN have
been studied. A comprehensive comparison has been made to the properties of the
same ferrite materials in the bulk and nanostructured form reported by other
research groups.
