The effect of thermal treatment on superconductivity in HoNi2B2C

We present detailed studies of the dependence of the low-temperature proper ties of HoNi2B2C on the annealing temperature. We could shift the supercond ucting transition temperature T-c reversibly and continuously by changing t he thermodynamic state of a sample via thermal treatment. The magnitude of this shift is as large as about 10% of T-c. The superconducting transition temperature is correlated with the occurrence and strength of re-entrant be haviour. Upper critical field measurements of two samples annealed at 800 d egrees C and 1100 degrees C indicate that the magnetic ordering temperature s are not affected by the thermal treatment. A reversible shift of T-c with annealing temperature has also been observed for YNi2B2C and LuNi2B2C with non-magnetic rare earth ions. (C) 1999 Elsevier Science B.V. All rights re served.