A low-temperature phase diagram H( T) of the 7.8-K superconductor HoNi~B~C (with an onset of 8.3 K) is generated through characterization of well-prepared samples by various experimental techniques including ac magnetic susceptibility, superconducting quantum interference device dc magnetic susceptibility, magnetic hysteresis, specific heat, and electrical resistivity measurements. The results yield a superconducting upper critical field H,.&(0) of 3.5 kG, a lower critical field H"(0) of 250 G, and a Ginzburg-Landau parameter a of 3.5. A nearly reentrant deep minimum at 5.2 K with very small H, z of 400 G and H, & of 5 G are observed. Two distinct magnetic transitions are observed with an incommensurate magnetic ordering temperature T of 5.7 -6 K and an antiferromagnetic Neel temperature T& of 5.2 K. The magnetic entropy A(S +Sz) estimated between 2 and 10 K is 10.4 J/molK. The efFective internal field which causes the nearly reentrant behavior is 2 kG at 5.2 K.
I. INTR&DUCTIONRelatively high superconducting transition temperatures T, up to 23 K have been reported in the quaternary borocarbide RTzBzC compounds (R = Sc, Y, Th, U or a rare earth; T=Ni, Pd, or Pt). ' " The superconducting phase has been identified to be of the body-centeredtetragonal LuNizBzC type with space group I4/mmm.The structure is a three-dimensionally connected framework with LuC layers alternated with NizBz layers, where nickel is tetrahedrally coordinated by four boron atoms. 4 Among many nonmagnetic compounds in the Ni system, LuNizBzC exhibits the highest T, of 16.6 K, followed by 15 -16 K for YNizBzC and metastable ScNi~B~C, ' 7 K for ThNi~B~C, and no superconducting transition was found down to 2 K for LaNizBzC. Band-structure calculations on LuNizBzC (Refs. 12,13) indicate a high density of states X(E~) at the Fermi level near the top of the almost-filled Ni(3d) band, with only modest admixture from B and C. All characteristics are indicative of a good, three-dimensional metal. A strongcoupled phonon mechanism for the occurrence of superconductivity is deduced with a very large electronphonon coupling parameter k, which is related to an unusual combination of electronic states at the Fermi level and a substantial contribution from the vibration of the light atoms. ' For compounds containing magnetic rare earth elements such as R=Dy, Ho, Er, and Tm, lower T, values were observed due to the magnetic pair-breaking effect. In fact, HoNizBzC is the most intensively studied compound of the Ni-based system due to its nearly reentrant behavior around 5 -6 K below the superconducting transition temperature T, of 7.5 -8 K. ' ' However, the reported magnetic transition temperatures are ill defined. For example, while yielding consistently an antiferromagnetic transition temperature T& around 5 K, ' ' two neutron diffraction measurements give two different incommensurately modulated/spiral magnetic transition ternperatures T of 8 K (Ref. 15) and 6 K (Ref. 16), respectively. Meanwhile,prior specific-heat measurements show vaguely two shoulders a...
