We report experiments of electron spin resonance (ESR) of ${\mathrm{Cu}}^{2+}$ in polycrystalline samples of ${\mathrm{CaCu}}_3{\mathrm{Ti}}_4{\mathrm{O}}_{12}$ post-annealed in different atmospheres. After being synthesized by solid state reaction, pellets of ${\mathrm{CaCu}}_3{\mathrm{Ti}}_4{\mathrm{O}}_{12}$ were annealed for $24\mathrm{h}$ at $1000°\mathrm{C}$ under air, Ar or ${\mathrm{O}}_2$. Our temperature dependent ESR data revealed for all samples nearly temperature independent $g$ value (2.15(1)) and linewidth for $T⪢T_N\approx 25\mathrm{K}$. However, the values of ESR linewidth are strongly affected by the oxygen content in the sample. For instance, argon post-annealed samples show a much larger linewidth than the ${\mathrm{O}}_2$ or air post-annealed samples. We attribute this broadening to an increase of the dipolar homogeneous broadening of the ${\mathrm{Cu}}^{2+}$ ESR lines due to the presence of oxygen vacancies which induce an $S=1∕2$ spin inside the ${\mathrm{TiO}}_6$ octahedra. Correlation between a systematic dependence of the ESR linewidth on the oxygen content and the high dielectric constant of these materials is addressed. Also, ESR, magnetic susceptibility, and specific heat data for a single crystal of ${\mathrm{CaCu}}_3{\mathrm{Ti}}_4{\mathrm{O}}_{12}$ and for polycrystals of ${\mathrm{CdCu}}_3{\mathrm{Ti}}_4{\mathrm{O}}_{12}$ are reported.

• Received 4 October 2005

DOI:https://doi.org/10.1103/PhysRevB.73.224404