Colossal dielectric response and relaxation properties in Co2Z-type hexaferrites

Polycrystalline samples of Sr3Co2Fe24O41 and Ba3Co2Fe24O41 have been prepared by a sol-gel method. The XRD data indicate that the samples are Co(2)Z-type hexaferrites with the centrosymmetric space group of P6(3)/mmc. The SEM micrographs suggest that mixture plate-like grains are formed and Sr3Co2Fe24O41 shows more dense grain structure. High-temperature dielectric relaxation behaviors are studied in a wide range of frequency and temperature through dielectric and impedance spectroscopies analysis. Colossal dielectric permittivity is observed at low frequency and dielectric relaxation behavior is clearly seen at high temperatures. Two types of relaxations are observed in Sr3Co2Fe24O41 and it is attributed to the thermally activated model which follows the Arrhenius law. The X-ray photoelectron spectroscopy indicates that Fe2+ and Fe3+ coexist in the system and the dielectric relaxations originate from the carriers hopping between Fe2+ and Fe3+. Moreover, the high concentration of Fe2+ in Ba3Co2Fe24O41 causes the high dielectric permittivity at low frequency range and the relatively low conductivity at high temperature.