Superconductivity and magnetism in quaternary F(F &#8801; Y or F elements) transition metal borocarbides and magnetic properties of RNi<SUB>4</SUB>B (R&#8801; Y or rare earth)

Our recent pioneering discovery of bulk superconductivity (SC) in Y-Ni-B-C at an elevated temperature (Tc &#8776; 12 K) has triggered intense research activity in borocarbide materials containing 4f and 5f elements. This was an offshoot of our studies of RNi4B (R&#8801; Y or rare earth) series of materials. Replacing Ni by Pd produces a superconducting phase in Y-Pd-B-C with record Tc ( &#8776; 23 K) in bulk intermetallics. Here we present the results of some of our investigations in borocarbide materials. Our specific heat studies suggest that YNi2B2C is a strong-coupling type II superconductor. The temperature dependence of Hc2(T) is rather unusual. &#956; -SR measurements indicate the possibility that Ni atoms carry a magnetic moment. SC and magnetism coexist in RNi2B2C (R &#8801; Ho, Er or Tm). CeNi2B2C is found to be a mixed-valence material. NdNi2B2C and GdNi2B2C, which we have been able to synthesize as a single phase, do not superconduct but order magnetically at Tm &#8776; 4.5 K and Tm &#8776; 19 K respectively. We have observed SC in Th-Ni-B-C and Th-Pd-B-C systems. There are at least two superconducting transitions in the Pd-containing systems Th-Pd-B-C and Y-Pd-B-C. YT2B2C (T &#8801; Co, Ir, Rh or Os) crystallize in a YNi2B2C-type structure but do not superconduct down to 4.2 K. Substitution studies show that Th (non-magnetic) and Gd (magnetic) suppress Tc by nearly the same extent. We also summarize our results on RNi4B series of materials which show remarkable physical properties