MAGNETIC TRANSITIONS AND NEARLY REENTRANT SUPERCONDUCTING PROPERTIES OF HONI2B2C

A low-temperature phase diagram H(T) of the 7.8-K superconductor HoNi2B2C (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 de magnetic susceptibility, magnetic hysteresis, specific heat, and electrical resistivity measurements. The results yield a superconducting upper critical field H-c2(0) of 3.5 kG, a lower critical field H-c1(0) of 250 G, and a Ginzburg-Landau parameter K of 3.5. A nearly reentrant deep minimum at 5.2 K with very small H-c2 of 400 G and H-c1 of 5 G are observed. Two distinct magnetic transitions are observed with an incommensurate magnetic ordering temperature T-m of 5.7-6 K and an antiferromagnetic Neel temperature T-N of 5.2 K. The magnetic entropy Delta(S-m + S-N) estimated between 2 and 10 K is 10.4 J/mol K. The effective internal field which causes the nearly reentrant behavior is 2 kG at 5.2 K.