  The ground-state magnetic properties of hexagonal equiatomic alloy of nominal
composition Mn_{0.8}Fe_{0.2}NiGe were investigated through dc magnetization and
heat capacity measurements. The alloy undergoes first order martensitic
transition below 140 K with simultaneous development of long range
ferromagnetic ordering from the high temperature paramagnetic phase. The
undoped compound MnNiGe has an antiferromagnetic ground state and it shows
martensitic like structural instability well above room temperature. Fe doping
at the Mn site not only brings down the martensitic transition temperature, it
also induces ferromagnetism in the sample. Our study brings out two important
aspects regarding the sample, namley (i) the observation of exchange bias at
low temperature, and (ii) spin glass like ground state which prevails below the
martensitic and magnetic transition points. In addition to the observed usual
relaxation behavior the spin glass state is confirmed by zero field cooled
memory experiment, thereby indicating cooperative freezing of spin and/or spin
clusters rather than uncorrelated dynamics of superparamagnetic like spin
clusters. We believe that doping disorder can give rise to some islands of
antiferromagnetic clusters in the otherwise ferromagnetic background which can
produce interfacial frustration and exchange pinning responsible for spin glass
and exchange bias effect. A comparison is made with doped rare-earth manganites
where similar phase separation can lead to glassy ground state.
