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The ground-state magnetic properties of a hexagonal equiatomic alloy of nominal composition Mn0.8Fe0.2NiGe were investigated through dc magnetization and heat capacity measurements. The alloy undergoes a 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, but 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 the 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 a similar phase separation can lead to a glassy ground state.

Superconducting transition temperature TC of some of the cubic β-phase Mo1− xRex alloys with x > 0.10 is an order of magnitude higher than that in the elements Mo and Re. We investigate this rather enigmatic issue of the enhanced superconductivity with the help of experimental studies of the temperature dependent electrical resistivity (ρ(T)) and heat capacity (CP(T)), as well as the theoretical estimation of electronic density of states (DOS) using band structure calculations. The ρ(T) in the normal state of the Mo1− xRex alloys with x >=0.15 is distinctly different from that of Mo and the alloys with x < 0.10. We have also observed that the Sommerfeld coefficient of electronic heat capacity γ, superconducting transition temperature TC and the DOS at the Fermi level show an abrupt change above x > 0.10 . The analysis of these results indicates that the value of electron-phonon coupling constant λep required to explain the TC of the alloys with x > 0.10 is much higher than that estimated from γ. On the other hand the analysis of the results of the ρ(T) reveals the presence of phonon assisted inter-band s-d scattering in this composition range. We argue that a strong electron-phonon coupling arising due to the multiband effects is responsible for the enhanced TC in the β-phase Mo1− xRex alloys with x > 0.10 .

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