Superconductivity and magnetism in quaternary F(F ≡ Y or F elements) transition metal borocarbides and magnetic properties of RNi₄B (R≡ Y or rare earth)

Abstract

Our recent pioneering discovery of bulk superconductivity (SC) in Y-Ni-B-C at an elevated temperature (T_c ≈ 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≡ Y or rare earth) series of materials. Replacing Ni by Pd produces a superconducting phase in Y-Pd-B-C with record T_c ( ≈ 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 H_c2(T) is rather unusual. μ -SR measurements indicate the possibility that Ni atoms carry a magnetic moment. SC and magnetism coexist in RNi₂B₂C (R ≡ 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 T_m ≈ 4.5 K and T_m ≈ 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 ≡ 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 T_c by nearly the same extent. We also summarize our results on RNi₄B series of materials which show remarkable physical properties.