Strong magnetic pair breaking and weak magnetic exchange interaction among R ions in ( Y , R ) Pd 2 B 2 C

In multiphase material ${\mathrm{YPd}}_{5}{\mathrm{B}}_{3}{\mathrm{C}}_{0.35}$ of the Y-Pd-B-C system, ${\mathrm{YPd}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ (space group I4/mmm) is believed to be the superconducting phase ${(T}_{c}\ensuremath{\approx}23 \mathrm{K}).$ Despite the nonavailability of the superconducting phase ${\mathrm{YPd}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ in pure form, we have been able to obtain crucial information on the magnetic pair breaking and magnetic exchange interaction among R ions in the $R{\mathrm{Pd}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ $(R=\mathrm{Y},$ rare-earth elements) system by studying multiphase $\mathrm{Y}(R){\mathrm{Pd}}_{5}{\mathrm{B}}_{3}{\mathrm{C}}_{0.35}$ materials. Strong pair-breaking effects are observed in ${\mathrm{Y}}_{0.9}{R}_{0.1}{\mathrm{Pd}}_{5}{\mathrm{B}}_{3}{\mathrm{C}}_{0.35};$ for instance, $\ensuremath{\Delta}{T}_{c}$\char22{}the depression of the superconducting transition temperature ${T}_{c}$\char22{}is $\ensuremath{\approx}8 \mathrm{K}$ for $R=\mathrm{Gd}.$ $\ensuremath{\Delta}{T}_{c}(R),$ except for $R=\mathrm{Ce},$ Eu, and Yb, nearly follows the de Gennes scaling $\ensuremath{\Delta}{T}_{c}\ensuremath{\propto}{(g}_{J}\ensuremath{-}{1)}^{2}J(J+1),$ both for light as well as heavy rare-earth atoms. $\ensuremath{\Delta}{T}_{c}$ $(\ensuremath{\approx}8.5 \mathrm{K})$ is anomalously large for $R=\mathrm{Ce}.$ Magnetic exchange interaction among R ions is rather weak in $R{\mathrm{Pd}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ as is inferred from our studies in ${\mathrm{DyPd}}_{5}{\mathrm{B}}_{3}{\mathrm{C}}_{0.35},$ which does not exhibit a magnetic transition down to 1.7 K. Our present investigations bring out striking dissimilarities in the superconducting and magnetic properties of the materials of $R{\mathrm{Pd}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ vis-\`a-vis those of $R{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}.$