The effect of Co substitution on superconductivity and magnetism in the system R(${\mathrm{Ni}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Co}}_{\mathrm{x}}$${)}_2$${\mathrm{B}}_2$C (R = Lu, Tm, Er, Ho, Dy) is investigated by ac-susceptibility measurements in zero and applied magnetic fields and zero-field resistivity measurements. For all R series a rapid decrease of the superconducting transition temperature ${\mathrm{T}}_{\mathrm{c}}$ is observed with increasing Co substitution, while the magnetic transition temperatures show only small changes. It is observed that the initial depression rate of ${\mathrm{T}}_{\mathrm{c}}$ grows in magnitude in the R series from Lu to Ho. For the nonmagnetic system Lu(${\mathrm{Ni}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Co}}_{\mathrm{x}}$${)}_2$${\mathrm{B}}_2$C the mechanisms leading to the decrease in ${\mathrm{T}}_{\mathrm{c}}$ 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(${\mathrm{Ni}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Co}}_{\mathrm{x}}$${)}_2$${\mathrm{B}}_2$C R = Tm, Er, where antiferromagnetism and superconductivity coexist, the influence of Co doping results in the occurrence of anomalous behavior of ac-susceptibility, resistivity and the upper critical fields when ${\mathrm{T}}_{\mathrm{c}}$ comes close to the magnetic transition temperatures ${\mathrm{T}}_{\mathrm{N}}$. For Er(${\mathrm{Ni}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Co}}_{\mathrm{x}}$${)}_2$${\mathrm{B}}_2$C reentrant-like behavior is observed, similar to that present in ${\mathrm{HoNi}}_2$${\mathrm{B}}_2$C. The reentrant behavior of ${\mathrm{HoNi}}_2$${\mathrm{B}}_2$C is enhanced with Co doping. Possible reasons for that behavior are discussed.

• Received 22 October 1996

DOI:https://doi.org/10.1103/PhysRevB.55.8497