COMPETITION BETWEEN MAGNETISM AND SUPERCONDUCTIVITY IN RARE-EARTH NICKEL BORIDE CARBIDES

The competition between magnetism and superconductivity was investigat ed for the recently discovered RNi2B2C superconductors with the magnet ic rare-earth elements R = Tm,Er,Ho,Dy. The systematic decrease of T(c ), approximately scaled by the de Gennes factor, implies a very weak c oupling between the rare-earth magnetic moments and the conduction ele ctrons due to a small conduction-electron density at the rare-earth si te. Associated with the antiferromagnetic order of the rare-earth mome nts, a pronounced dip structure in the upper critical field H(c2)(T) w as observed; a similar structure has been seen in the previously known magnetic superconductors RRh4B4 and RMo6S8. For HoNi2B2C, the pair-br eaking associated with the magnetic transition is strong enough to bri ng about a resistive reentrant behavior even under zero field, an effe ct which, to our knowledge, has not previously been observed in antife rromagnetic superconductors.