Temperature dependence of electron spin resonance in CaCu3Ti4O12 substituted with transition metal elements

The temperature dependence of the electron spin resonance (ESR) spectrum of copper in CaCu₃Ti₄O₁₂ (CCTO) polycrystalline samples doped with transition metal elements Mn, Fe, Ni is reported. The frequency dependence of the dielectric constant of the sample is also reported at room temperature. While the dielectric constant of undoped CCTO samples reaches ～10,000, it is found one hundred times lower in samples doped with only 0.5 or 1% of Mn or Fe. Copper is confirmed to give a g = 2.14 signal at room temperature for substituted and unsubstituted samples. Below the antiferromagnetic transition that occurs near 25 K, the signal is found shifted down to low fields for all samples. However the downshift is 10–20 times more important in Mn and Fe-doped samples compared to undoped or Ni-doped CCTO. ESR results in an undoped CCTO thin film grown by pulse laser deposition are also reported. While in the low-temperature antiferromagnetic phase the spectrum is multi-line and magnetic-field-orientation-dependent, it reduces to a narrow single line, independent of the orientation of the magnetic field, in the upper paramagnetic phase, similar to the undoped polycrystalline sample. All doped samples display much broader response in the upper phase. The results are discussed within the framework of the relationship between the high effective dielectric constant and the electrical conductivity of CCTO bulk.