Electron-paramagnetic-resonance spectra of ions substituted into transition-metal ion lattices

Below 2 K it is possible to observe well-resolved electron paramagnetic resonance spectra for ${\mathrm{Cu}}^{2+}$ substituted into $\alpha -\mathrm{NiS}{\mathrm{O}}_4·6{\mathrm{H}}_2\mathrm{O}$. Comparison with the $g$ values observed in the diamagnetic isostructural ZnSe${\mathrm{O}}_4$ · 6${\mathrm{H}}_2$O shows a positive $g$ shift of about 0.2 units in the ${\mathrm{Ni}}^{2+}$ lattice due to ${\mathrm{Cu}}^{2+}$-${\mathrm{Ni}}^{2+}$ exchange interaction. Two equivalent theories for the incorporation of guest-host exchange interaction into the spin Hamiltonian of the guest ion are presented. One of these theories is an extension of the standard perturbation techniques and the other utilizes a molecular-field approach. Both give similar results and indicate a nearest-neighbor ferromagnetic ${\mathrm{Cu}}^{2+}$-${\mathrm{Ni}}^{2+}$ exchange with $-2J=-0.145\mathrm{}\pm 0.01$ ${\mathrm{cm}}^{-1\mathrm{}}$. Experiments are also reported for ${\mathrm{Co}}^{2+}$, ${\mathrm{Mn}}^{2+}$, and ${\mathrm{V}}^{2+}$ substituted into either $\alpha -\mathrm{NiS}{\mathrm{O}}_4·6{\mathrm{H}}_2\mathrm{O}$, NiSe${\mathrm{O}}_4$·6${\mathrm{H}}_2$O, or ZnSe${\mathrm{O}}_4$·6${\mathrm{H}}_2$O. The ${\mathrm{Co}}^{2+}$-${\mathrm{Ni}}^{2+}$ exchange is only slightly ferromagnetic, but our theories cannot give quantitative agreement with experiment. The ${\mathrm{Mn}}^{2+}$-${\mathrm{Ni}}^{2+}$ exchange is too small to be measured, but the ${\mathrm{V}}^{2+}$-${\mathrm{Ni}}^{2+}$ exchange indicates $-2J=+0.06\mathrm{}\pm 0.01$ ${\mathrm{cm}}^{-1\mathrm{}}$. All of these exchange interactions are for hydrated ions and take place through hydrogen bonds. The trends that we observe for exchange in the ${\mathrm{Ni}}^{2+}$ lattice are discussed.