  MnAs has been grown by means of MBE on the GaN(0001)-(1x1) surface. Two
options of initiating the crystal growth were applied: (a) a regular MBE
procedure (manganese and arsenic were delivered simultaneously) and (b)
subsequent deposition of manganese and arsenic layers. It was shown that
spontaneous formation of MnAs dots with the surface density of 1$\cdot 10^{11}$
cm$^{-2}$ and $2.5\cdot 10^{11}$ cm$^{-2}$, respectively (as observed by AFM),
occurred for the layer thickness higher than 5 ML. Electronic structure of the
MnAs/GaN systems was studied by resonant photoemission spectroscopy. That led
to determination of the Mn 3d - related contribution to the total density of
states (DOS) distribution of MnAs. It has been proven that the electronic
structures of the MnAs dots grown by the two procedures differ markedly. One
corresponds to metallic, ferromagnetic NiAs-type MnAs, the other is similar to
that reported for half-metallic zinc-blende MnAs. Both system behave
superparamagnetically (as revealed by magnetization measurements), but with
both the blocking temperatures and the intra-dot Curie temperatures
substantially different. The intra-dot Curie temperature is about 260 K for the
former system while markedly higher than room temperature for the latter one.
Relations between growth process, electronic structure and other properties of
the studied systems are discussed. Possible mechanisms of half-metallic MnAs
formation on GaN are considered.
