Abstract
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 dc magnetic susceptibility, magnetic hysteresis, specific heat, and electrical resistivity measurements. The results yield a superconducting upper critical field Hc2(0) of 3.5 kG, a lower critical field Hc1(0) of 250 G, and a Ginzburg-Landau parameter of 3.5. A nearly reentrant deep minimum at 5.2 K with very small Hc2 of 400 G and Hc1 of 5 G are observed. Two distinct magnetic transitions are observed with an incommensurate magnetic ordering temperature Tm of 5.76 K and an antiferromagnetic Néel temperature TN of 5.2 K. The magnetic entropy (Sm+SN) 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. © 1995 The American Physical Society.