Effect of hydrostatic and chemical pressure on the exchange interaction in magnetic borocarbide superconductors

The investigation of pair-breaking effects in magnetic rare-earth nickel borocarbide superconductors reveals a considerable increase of the magnetic exchange integral ${\mathcal{J}}_{\mathrm{sf}}$ by hydrostatic as well as chemical pressure. In both, ${\mathcal{J}}_{\mathrm{sf}}$ is governed by the $R-\mathrm{C}$ distance (or lattice constant $a)$ and is described quantitatively by a simple phenomenological model. Thereby, just two parameters ${\mathcal{J}}_{\mathrm{sf}0\mathrm{}}=31\mathrm{}\hspace{0.5em}\mathrm{meV}$ and $\Delta {\mathcal{J}}_{\mathrm{sf}}/\Delta a=165\mathrm{}\hspace{0.5em}\mathrm{meV}/\AA$ explain well the influence of chemical pressure upon the initial depression rates of $T_c$ in solid solutions $R_{1-x}^{\prime }{R}_x{\mathrm{Ni}}_2{\mathrm{B}}_2\mathrm{C}$ with $R=\mathrm{Gd},$ Tb, Dy, Ho and $R^{\prime }=\mathrm{Y}$ and Lu.