  Nearly 30% increase of the ferromagnetic phase transition temperature has
been achieved in strained MnAs nanocrystals embedded in a wurtzite GaAs matrix.
Wurtzite GaAs exerts tensile stress on hexagonal MnAs nanocrystals, preventing
a hexagonal to orthorhombic structural phase transition, which in the bulk MnAs
is combined with the magnetic one. This effect results in a remarkable shift of
the magneto-structural phase transition temperature from 313 K in the bulk MnAs
to above 400 K in the tensely strained MnAs nanocrystals. This finding is
corroborated by the state of the art transmission electron microscopy,
sensitive magnetometry and the first-principles calculations. The effect relies
in defining a nanotube geometry of molecular beam epitaxy grown core-multishell
wurtzite (Ga,In)As/(Ga,Al)As/(Ga,Mn)As/GaAs nanowires where the MnAs
nanocrystals are formed during the thermal-treatment-induced phase separation
of wurtzite (Ga,Mn)As into the GaAs:MnAs granular system. Such a unique
combination of two types of hexagonal lattices provides possibility of
attaining quasi-hydrostatic tensile strain in MnAs (impossible otherwise),
leading to the substantial ferromagnetic phase transition temperature increase
in this compound.
