Dielectric and magnetic properties of Fe- and Nb-dopedCaCu3Ti4O12

Detailed studies of the properties of ceramic ${\mathrm{CaCu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ (CCTO) have clarified the physics of this interesting material and revealed several features not reported before. The dielectric relaxational properties of CCTO are explained in terms of a capacitive-layer model, as for an inhomogeneous semiconductor, consisting of semiconducting grains and insulating grain boundaries as also concluded by others. The kinetics of the main [low-temperature $(T)$] relaxation reveal that two different thermally activated processes in CCTO grains control the dynamics. A likely candidate defect responsible for the two processes is the oxygen vacancy which is a double donor. A higher-$T$ relaxation is determined by grain boundary conduction. Both Nb and Fe doping lowered both the apparent dielectric constant ${\ensuremath{\epsilon}}^{\ensuremath{'}}$ and the dielectric loss, but increased Fe doping led to more dramatic effects. At 3 at. % Fe doping, the anomalous ${\ensuremath{\epsilon}}^{\ensuremath{'}}(T)$ response was removed, making the CCTO an intrinsic, very-low-loss dielectric. The intrinsic ${\ensuremath{\epsilon}}^{\ensuremath{'}}(\ensuremath{\simeq}75)$ and its $T$ dependence are measured and shown to be largely determined by a low-lying soft TO phonon. At low $T$, cubic CCTO transforms into an antiferromagnetic phase at ${T}_{N}=25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. ${T}_{N}$ is essentially independent of Nb doping (up to 4 at. %) and of hydrostatic pressure (up to $\ensuremath{\sim}7\phantom{\rule{0.3em}{0ex}}\mathrm{kbar}$), but decreases significantly with Fe doping. Analysis of the high-$T$ dependence of the magnetic susceptibility provided insight into the role of Fe as a dopant. Finally, an ${\ensuremath{\epsilon}}^{\ensuremath{'}}(\mathrm{T})$ anomaly associated with the onset of antiferromagnetic order has been discovered, providing evidence for coupling between the polarization and sublattice magnetization. The possible origin of this coupling is discussed.