The effects of Fe, Co, and Ni doping on the superconductivity and structure of HfV2

The temperature dependence of the specific heat has been measured for Fe-, Co-, and Ni-doped HfV2 systems. HfV2 undergoes a structural transformation at T-S=116 K and becomes superconducting at T-C=9 K. The low temperature sp ecific heat of HfV2 shows an enhanced electronic specific heat coefficient indicating a large density of states at the Fermi level. T-C shows nonmonot onic behavior with magnetic ion concentration. It increases initially as on e dopes HfV2 with small amounts of Fe, Co, or Ni and reaches a maximum of 9 .4 K for about 3.0 to 4.0 at. % doping and then decreases for higher concen trations. On the other hand, the specific heat data at higher temperatures show that both T-S and the size of the specific heat anomaly at T-S decreas e monotonically and dramatically when the concentration of Fe, Co, or Ni is increased. No evidence of a structural transformation is detected for samp les with more than 5 at. % Fe, Co, or Ni. The variations of T-C and T-S can be explained qualitatively by recent theoretical calculations. (C) 2000 Am erican Institute of Physics. [S0021-8979(00)69308-8].