Dielectric Relaxations In Rare Earth Iron Garnet Ceramics

Rare earth iron garnet,a ferromagnetic material known as microwave ferrite,has been widely applied in passive microwave devices and magneto-optical devices, because it posses excellent magetic and magneto-optical properties.Recently,the investigations of magneto-electric and magneto-dielectric effects in rare earth iron garnet have been attracting much interest in view of the direct interaction between dielectric and magnetic properties.Rare earth iron garnets are excellent candidates of magnetic dielectrics with high magnetic ordering temperatures.In order to well understand the physical nature of dielectric relaxations in rare earth iron garnets,the dielectric properties of Y₃Fe₅O₁₂,and Tb_0.91Yb_1.38Bi_0.71Fe₅O₁₂ceramics in a broad temperature and frequency range were investigated.Three dielectric relaxations of Y₃Fe₅O₁₂(YIG)ceramics are identified in the temperature range from 125 K to 620 K.The low temperature relaxation is typical of Debye-type relaxation with activation energy close to 0.29 eV.This is agreement with the activation energy of a two-site polaron hopping process of charge transfer between Fe²⁺and Fe³⁺(E_a=0.29 eV),suggesting that the low temperature dielectric relaxation originates from the dipolar effects associated with the charge carrier hopping between Fe²⁺and Fe³⁺inside the grains.The low frequency dielectric relaxation in the higher temperature range has a similar activation energy with the grain boundaries.Taking into account its giant low frequency response,it is most likely arises from the inhomogeneous structure,such as grain boundaries.Another dielectric relaxation in the high temperature range with the similar activation energy of conduction can be attributed to the conduction.In the Al substituted yttrium iron garnet ceramic,the dielectric constant plateu at low temperatures significantly rises and the corresponding activation energy slightly increases.Because the valence of Al³⁺is stable,partial substitution of Fe³⁺by Al³⁺ increases the hopping distance between the Fe²⁺and Fe³⁺in the lattice which increase the activation energy of charge carriers hopping.On the other hand,the thermal relaxation polarization increases with the increase of the distance between the dipoles. This leads to the rise of the dielectric constant plateu at low temperatures.Two dielectric relaxations in Tb_0.91Yb_1.38Bi_0.71Fe₅O₁₂(TYBIG)ceramics are identified in the temperature range from 125 K to 620 K.The similar activation energy of the relaxation in the frequency range of 10 Hz～4 MHz and the carriers hopping between Fe²⁺and Fe³⁺suggested that it is a dipolar-type relaxation associated with the hoping process.Another relaxation in the high temperature range from 400 K to 620 K has an activation energy of 1.08 eV,which is comparable to that of conduction.The giant low frequency response indicates that the inhomogeneous structures,such as grain boundaries,interfaces,and in_homogeneous distribution of oxygen vacancies may contribute to the low frequency dielectric relaxation.