Spin glass like ground state and observation of exchange bias in Mn_{0.8}Fe_{0.2}NiGe alloy

Abstract: 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.