  In order to investigate the mechanism of ferromagnetic ordering in the new
n-type magnetic semiconductor (In,Fe)As co-doped with Be, we have performed
X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD)
studies of ferromagnetic and paramagnetic samples. The spectral line shapes
suggest that the ferromagnetism is intrinsic originating from Fe atoms
incorporated into the Zinc-blende-type InAs lattice. The magnetization curves
of Fe measured by XMCD were well reproduced by the superposition of a Langevin
function representing superparamagnetic (SPM) behavior of nano-scale
ferromagnetic domains and a T-linear function representing Curie-Weiss
paramagnetism even much above the Curie temperatures. The data at 20 K showed a
deviation from the Langevin behavior, suggesting a gradual establishment of
macroscopic ferromagnetism on lowering temperature. The existence of nano-scale
ferromagnetic domains indicated by the SPM behavior suggests spatial
fluctuations of Fe concentration on the nano-scale.
