  Spin-based electronics or spintronics is an emerging field, in which we try
to utilize spin degrees of freedom as well as charge transport in materials and
devices. While metal-based spin-devices, such as magnetic-field sensors and
magnetoresistive random access memory using giant magnetoresistance and
tunneling magnetoresistance, are already put to practical use,
semiconductor-based spintronics has greater potential for expansion because of
good compatibility with existing semiconductor technology. Many
semiconductor-based spintronics devices with useful functionalities have been
proposed and explored so far. To realize those devices and functionalities, we
definitely need appropriate materials which have both the properties of
semiconductors and ferromagnets. Ferromagnetic semiconductors (FMS), which are
alloy semiconductors containing magnetic atoms such as Mn and Fe, are one of
the most promising classes of materials for this purpose, and thus have been
intensively studied for the past two decades. Here, we review the recent
progress in the studies of the most prototypical III-V based FMS, p-type
(GaMn)As, and its heterostructures with focus on tunneling transport, Fermi
level, and bandstructure. Furthermore, we cover the properties of a new n-type
FMS, (InFe)As, which shows electron-induced ferromagnetism. These FMS materials
having zinc-blende crystal structure show excellent compatibility with
well-developed III-V heterostructures and devices.
