  Structural and magnetic properties of (In,Ga)As-(Ga,Mn)As core-shell
nanowires grown by molecular beam epitaxy on GaAs(111)B substrate with gold
catalyst have been investigated.(In,Ga)As core nanowires were grown at high
temperature (500 {\deg}C) whereas (Ga,Mn)As shells were deposited on the
{1-100} side facets of the cores at much lower temperature (220 {\deg}C). High
resolution transmission electron microscopy images and high spectral resolution
Raman scattering data show that both the cores and the shells of the nanowires
have wurtzite crystalline structure. Scanning and transmission electron
microscopy observations show smooth (Ga,Mn)As shells containing 5% of Mn
epitaxially deposited on (In,Ga)As cores containing about 10% of In, without
any misfit dislocations at the core-shell interface. With the In content in the
(In,Ga)As cores larger than 5% the (In,Ga)As lattice parameter is higher than
that of (Ga,Mn)As and the shell is in the tensile strain state. Elaborated
magnetic studies indicate the presence of ferromagnetic coupling in (Ga,Mn)As
shells at the temperatures in excess of 33 K. This coupling is maintained only
in separated mesoscopic volumes resulting in an overall superparamagnetic
behavior which gets blocked below ~17 K.
