  Spin gapless semiconductors (SGS) form a new class of magnetic
semiconductors, which has a band gap for one spin sub band and zero band gap
for the other, and thus are useful for tunable spin transport based
applications. In this paper, we report the first experimental evidence for spin
gapless semiconducting behavior in CoFeMnSi Heusler alloy. Such a behavior is
also confirmed by first principles band structure calculations. The most stable
configuration obtained by the theoretical calculation is verified by
experiment. The alloy is found to crystallize in the cubic Heusler structure
(LiMgPdSn type) with some amount of disorder and has a saturation magnetization
of 3.7 Bohr's magneton/f.u.. and Curie temperature of 620 K. The saturation
magnetization is found to follow the Slater-Pauling behavior, one of the
prerequisites for SGS. Nearly temperature-independent carrier concentration and
electrical conductivity is observed from 5 to 300 K. An anomalous Hall
coefficient of 162 S/cm is obtained at 5 K. Point contact Andreev reflection
data has yielded the current spin polarization value of 0.64, which is found to
be robust against the structural disorder. All these properties are quite
promising for the spintronic applications such as spin injection and can bridge
a gap between the contrasting behavior of half-metallic ferromagnets and
semiconductors.
