Competition between magnetism and superconductivity in rare-earth nickel boride carbides

The competition between magnetism and superconductivity was investigated for the recently discovered RNi2B2C superconductors with the magnetic rare-earth elements R=Tm,Er,Ho,Dy. The systematic decrease of Tc, approximately scaled by the de Gennes factor, implies a very weak coupling between the rare-earth magnetic moments and the conduction electrons due to a small conduction-electron density at the rare-earth site. Associated with the antiferromagnetic order of the rare-earth moments, a pronounced dip structure in the upper critical field Hc2(T) was observed; a similar structure has been seen in the previously known magnetic superconductors RRh4B4 and RMo6S8. For HoNi2B2C, the pair-breaking associated with the magnetic transition is strong enough to bring about a resistive reentrant behavior even under zero field, an effect which, to our knowledge, has not previously been observed in antiferromagnetic superconductors.