Breakdown of de Gennes Scaling in ( ${\mathit{R}}_{1-\mathit{x}}{R^{\prime }}_{\mathit{x}}$)N${\mathrm{i}}_2$${\mathrm{B}}_2$C Compounds

The superconducting transition temperatures $\left(T_c\right)$ and Néel temperatures $\left(T_N\right)$ have been determined for thirty $\left(R_{1-x}{R}_x^{\prime }\right){\mathrm{Ni}}_2{\mathrm{B}}_2\mathrm{C}$ single crystals. For $T_c>T_N$, deviations from de Gennes scaling occur that are probably due to crystalline electric field effects. For $T_c<T_N$ there is a complete breakdown of de Gennes scaling. In $\left({\mathrm{Dy}}_{1-x}{R}_x^{\prime }\right){\mathrm{Ni}}_2{\mathrm{B}}_2\mathrm{C}$, $R^{\prime }=\mathrm{Lu}$ suppresses $T_c$ an order of magnitude more rapidly than $R^{\prime }=\mathrm{Ho}$. This breakdown is inferred to arise from pair breaking by collective magnetic excitations.