Direct Measurement of Weak Exchange Interactions in Pairs of ${\mathrm{Co}}^{2+}$ Ions in ${\mathrm{Cs}}_3$Zn${\mathrm{Cl}}_5$

In ${\mathrm{Cs}}_3$Zn${\mathrm{Cl}}_5$ doped with 3 mole% ${\mathrm{Co}}^{2+}$, satellites to the -½ → ½ transition were observed in the ESR spectrum of ${\mathrm{Co}}^{2+}$. They are attributed to weak exchange and dipole-dipole interactions. A detailed analysis showed that the exchange interaction between two nearest-neighboring Co ions in a plane perpendicular to the tetragonal crystal axis is antiferromagnetic ($\frac{j}{k}=-0.0204\mathrm{}°$K), while the interaction between two nearest-neighboring Co ions on a line parallel to the tetragonal axis is ferromagnetic ($\frac{j}{k}=+0.0154\mathrm{}°$K).