EPR of ${\mathrm{Co}}^{2+}$-doped ${\mathrm{NiSO}}_4$⋅${7\mathrm{H}}_2$O and ${\mathrm{MgSO}}_4$⋅${7\mathrm{H}}_2$O: ${\mathrm{Co}}^{2+}$-${\mathrm{Ni}}^{2+}$ exchange interaction

X-band EPR measurements on ${\mathrm{Co}}^{2+}$-doped isostructural single crystals of paramagnetic host ${\mathrm{NiSO}}_4$⋅${7\mathrm{H}}_2$O and diamagnetic host ${\mathrm{MgSO}}_4$⋅${7\mathrm{H}}_2$O have been made at liquid-helium temperature. The principal values of the g→ ${}_{}{}^2{}_{}{}^{}$ and A→ ${}_{}{}^2{}_{}{}^{}$ tensors, as well as the orientations of the principal axes of the g→ ${}_{}{}^2{}_{}{}^{}$ and A→ ${}_{}{}^2{}_{}{}^{}$ tensors, are evaluated from the data using a rigorous least-squares-fitting program suitable for electron-nuclear spin-coupled systems. Using the g shift of ${\mathrm{Co}}^{2+}$ in the paramagnetic lattice from that in the diamagnetic lattice the value of -0.81 GHz for the average ${\mathrm{Co}}^{2+}$-${\mathrm{Ni}}^{2+}$ exchange constant in ${\mathrm{NiSO}}_4$⋅${7\mathrm{H}}_2$O has been estimated using the molecular-field approximation.