Annihilation of superconductivity by Co substitution for Cu in ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_4$

The effect of Co substitution for Cu in the ${\mathrm{Nd}}_{1.85}$ ${\mathrm{Ce}}_{0.15}$ ${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Co}}_{\mathit{x}}$ ${\mathrm{O}}_4$ system has been investigated by powder x-ray-diffraction analysis and resistivity and magnetic susceptibility measurements. It is observed that only 0.6 at. % Co substitution for Cu destroys the superconductivity of the sample. For heavily doped samples (with x≥0.01), the effective magnetic moment per Co ion is found to decrease with increasing x. The antiferromagnetic correlation between doped Co ions is also discovered to increase with increasing x. For lightly doped samples (with x≤0.008), the dependence of ${\mathit{T}}_{\mathit{c}}$ on x may be caused by a pair-breaking effect. The absolute value of the exchange interaction energy between a ${\mathrm{Co}}^{2+}$ ion and an electron is estimated at ∼0.09 eV for S=3/2 or ∼0.19 eV for S=1/2, where S is the total spin of a ${\mathrm{Co}}^{2+}$ ion.