Magnetism and Superconductivity of Itinerant D - Systems.

From magnetization and electrical resistivity measurements we have determined the ferromagnetic transition temperature T_{c} = 81K for a single crystal of Ni_3Al, which is the highest value reported so far. The results which include a T^{4/3} dependence of magnetization squared, a T^ {5/3} dependence of resistivity and an upturn behavior of the C/T vs T^2 plot at low temperatures are consistent with the predictions of the Self-Consistent Renormalization (SCR) theory of spin fluctuations for weakly and nearly itinerant ferromagnets. The alloying studies for (rm Ni_{1 -x}M_{x})_3Al with M = Cu and Pd and rm Ni_3(Al_ {x}Si_{1-x}) as well as Hf(rm Fe_{x}Co_ {1-x})_2 have shown that the electronic specific heat coefficient gamma is enhanced when the system approaches a magnetic instability. This also agrees with gamma~ ln(1 - alpha) predicted by the SCR theory. We have also investigated the strong electron-phonon coupling superconducting HfV_2 system. Our specific heat measurements show that it undergoes a structural phase transition at 116K, and the transition is of the first order. In addition, it becomes superconducting at 8.24 K. The gamma value of 50 mJ/mol -K^2 for HfV_2 is much higher than that for typical pure metal superconductors. Recent band structure calculations have shown that HfV _2 has a high density of states at the Fermi level and exhibits a Fermi surface nesting which results in the structural phase transition. The structural phase transition is suppressed to a low temperature by both dilute Fe and Co doping in HfV_2. Surprisingly, a relatively large amount of both Fe and Co concentrations is required to quench the superconductivity of HfV _2.