  The layers of a high-temperature novel GaAs:Fe diluted magnetic semiconductor
(DMS) with an average Fe content up to 20 at. % were grown on (001) i-GaAs
substrates using a pulsed laser deposition in a vacuum. The transmission
electron microscopy (TEM) and energy-dispersive X-ray spectroscopy
investigations revealed that the conductive layers obtained at 180 and 200 C
are epitaxial, do not contain any second-phase inclusions, but contain the
Fe-enriched columnar regions of overlapped microtwins. The TEM investigations
of the non-conductive layer obtained at 250 C revealed the embedded coherent
Fe-rich clusters of GaAs:Fe DMS. The X-ray photoelectron spectroscopy
investigations showed that Fe atoms form chemical bonds with Ga and As atoms
with almost equal probability and thus the comparable number of Fe atoms
substitute on Ga and As sites. The n-type conductivity of the obtained
conductive GaAs:Fe layers is apparently associated with electron transport in a
Fe acceptor impurity band within the GaAs band gap. A hysteretic negative
magnetoresistance was observed in the conductive layers up to room temperature.
Magnetoresistance measurements point to the out-of-plane magnetic anisotropy of
the conductive GaAs:Fe layers related to the presence of the columnar regions.
The studies of the magnetic circular dichroism confirm that the layers obtained
at 180, 200 and 250 C are intrinsic ferromagnetic semiconductors and the Curie
point can reach up to at least room temperature in case of the conductive layer
obtained at 200 C. It was suggested that in heavily Fe-doped GaAs layers the
ferromagnetism is related to the Zener double exchange between Fe atoms with
different valence states via an intermediate As and Ga atom.
