  We report the theoretical prediction of a new class of spintronic materials,
namely bipolar magnetic semiconductor (BMS), which is also supported by our
experimental data. BMS acquires a unique band structure with unequal band gaps
for spin up and down channels, and thus are useful for tunable spin transport
based applications such as spin filters. The valence band (VB) and conduction
band (CB) in BMS approach the Fermi level through opposite spin channels, and
hence facilitate to achieve reversible spin polarization which are controllable
via applied gate voltage. We report the quaternary Heusler alloy VNbRuAl to
exactly possess the band structure of BMS. The alloy is found to crystallize in
LiMgPdSn prototype structure (space group $F\bar{4}3m$) with B$2$ disorder and
lattice parameter 6.15 \AA . The resistivity and Hall measurements show a two
channel semiconducting behavior and a quasi linear dependence of negative
magneto resistance (MR) indicating the possible semiconducting nature.
Interestingly, VNbRuAl also shows a fully compensated ferrimagnetic (FCF)
behavior with vanishing net magnetization (m$_s$$\sim$ $10^{-3}$ $\mu_B/f.u.$)
and significantly high ordering temperature ($> 900$ K). Unlike conventional
FCF, vanishing moment in this case appears to be the result of a combination of
long range antiferromagnetic (AFM) ordering and the inherent B2 disorder of the
crystal. This study opens up the possibility of finding a class of materials
for AFM spintronics, with great significance both from fundamental and applied
fronts.
