SUPERCONDUCTIVITY AND MAGNETISM IN THE HO-NI-B-C SYSTEM

We investigated the annealing temperature, the carbon non-stoichiometr y and the Fe doping effects on the physical properties of the supercon ductor HoNi2B2Cx (1 less than or equal to x less than or equal to 1.35 ), using X-ray diffraction, magnetisation or electrical resistance mea surements and Fe-57 Mossbauer spectroscopy. We show that the spontaneo us re-entrant behaviour of HoNi2B2C which has been attributed to the o ccurrence of incommensurate magnetic states destructive to superconduc tivity, is strongly enhanced by quenching or by decreasing the anneali ng temperature. The re-entrant behaviour also increases with the carbo n content and the Fe-57 (0.5 at.%) doping. We conclude that the chemic al defects introduced by quenching, deviation from stoichiometry or Fe doping, depress superconductivity and favour the appearance of the re -entrant behaviour. Our Fe-57 Mossbauer investigation shows that the p resence of the incommensurate magnetic orders in the re-entrant region does not lead to the introduction of a hyperfine field at the Fe prob e. (C) 1997 Elsevier Science S.A.