Superconductivity, magnetism, and their coexistence in R(Ni1-xCox)(2)B2C (R=Lu, Tm, Er, Ho, Dy)

The effect of Co substitution on superconductivity and magnetism in the system R(Ni1-xCox)(2)B2C (R = Lu, Tm, Er, Ho, Dy) is investigated by ac-susceptibility measurements in zero and applied magnetic fields and zero-field resistivity measurements. For all R series a rapid decrease of the superconducting transition temperature T-c is observed with increasing Co substitution, while the magnetic transition temperatures show only small changes. It is observed that the initial depression rate of T-c grows in magnitude in the R series from Lu to Ho. For the nonmagnetic system Lu(Ni1-xCox)(2)B2C the mechanisms leading to the decrease in T-c are analyzed and discussed. It was found that a transition from clean to dirty limit behavior occurs with Co doping. For the magnetic systems R(N1-xCox)(2)B2C R = Tm, Er, where antiferromagnetism and superconductivity coexist, the influence of Co doping results in the occurrence of anomalous behavior of ac-susceptibility, resistivity and the upper critical fields when T-c comes close to the magnetic transition temperatures T-N. For Er(Ni1-xCox)(2)B2C reentrant-like behavior is observed, similar to that present in HoNi2B2C. The reentrant behavior of HoNi2B2C is enhanced with Co doping. Possible reasons for that behavior are discussed.