Magnetic transitions and nearly reentrant superconducting properties ofHoNi2B2

Abstract

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...

Torque magnetometry study of metamagnetic transitions in single-crystalHoNi2B2CatT≈1.9Rathnayaka1, Белевцев2, Naugle3 2007Phys. Rev. B4260View full textAdd to dashboardCite

“…The angle is then changed to the next value at approximately zero field, still at 1.9 K.) In Ref. [22], a noticeable hysteresis in the magnetization at T = 2 K in polycrystalline HoNi 2 B 2 C was found at H < 0.1 T, which is consistent with the torque behavior found in this study. Recent Bitter decoration experiments [23] indicate the possibility that pinned vortex structures could be important in the AFM phase.…”

“…The decrease of torque to nearly zero with increasing field suggests a transforma- tion to a one-domain state. It should be mentioned that the upper critical field H c2 for HoNi 2 B 2 C is about 0.3 T at T = 2 K [9,22]. Thus, the torque hysteresis in the AFM phase is in the superconducting state.…”

Neutron scattering studies of RENi{sub 2}B{sub 2}C: Magnetic structures and lattice dynamics

“…Single crystals of LuNi2B2C were grown at the Ames Laboratory by the hightemperature flux technique as described elsewhere [24]. The phonon frequencies of the acoustic and first optical A4 branches in the vicinity of the zone boundary point G1 on the other hand, decrease with decreasing temperature.…”

Thermal expansion of single-crystalline HoNi2B2C

“…Much interest has been focused on HoNi 2 B 2 C because of its comparable values of the transition temperatures of the two ordering phenomena [1]. In the specific heat, a large magnetic contribution was observed [2][3][4][5][6][7]. In comparison, the superconducting phase transition causes only relatively weak changes in the specific heat.…”