Modification Of Rare Earth Iron Garnets Magnetodielectric Ceramics

Rare earth iron garnets possessing specific dielectric, magneto-optical properties have been widely utilized in microwave devices and magneto-optical record medium. The permittivity of garnets can be modified by external magnetic field, which makes garnet a promising material for adjustable capacitors and magnetic field sensors. In this thesis, Y3-xYbxFe5O12, Y3-xYbxFe4AlO12 and Yb3-xBixFe5O12 ceramics were prepared by standard solid state reaction method and the effects of ion substitution on the structure, dielectric properties, magnetic properties and magneto-dielectric properties were investigated.Y3-xYbxFe5O12(x=0,0.5,1,2,3) ceramics exhibit three dielectric relaxations in the temperature range of 133K-573K and frequency range of 1Hz-1MHz. No significant effect of ytterbium partial substitution for yttrium on low temperature dielectric relaxation was observed. The activation energy of middle temperature dielectric relaxation decreases and the activation energy of high temperature dielectric relaxation increases as the amount of Yb increases. It was found that the saturation magnetization of Y3-xYbxFe5O12 ceramics is independent of the amount of ytterbium. The lattice constant and superexchange interaction Fe3+(a)-O-Fe3+(d) decrease with increasing x, which results in decrease of magnetic ordering temperature. The intrinsic magnetodielectric effect plays a dominant role in Y3Fe5O12, Y2.5Yb0.5Fe5012 and Y2YbFe5O12 ceramics, and the magnetodielectric coefficient increases with increasing amount of Yb3-. YYb2Fe5O12 and Yb3Fe5O12 ceramics exhibit small magnetodielectric coefficients because of significant weakening of extrinsic magnetodielectric effect.It was found that magnetic ordering temperature and saturation magnetization of Y3-xYbxFe4AlO12(x=0,0.5,1,2) ceramics depend on Fe3T(a)-O-Fe3+(d) superexchange interaction. Substitution of Fe3- greatly affects the magnetic properties and dielectric properties of Y3-xYbxFe4AlO12 ceramics. Partial substitution of Y3+ by Yb3+ slightly decreases the magnetic ordering temperature and saturation magnetization, which can be attributed to the change in lattice constant and bond angle of Fe3 (a)-O-Fe3+(d).Bismuth substitution of ytterbium significantly decreases the optimal sintering temperature of Yb3Fe5O12 ceramics. As the amount of Bi3+ increases, the dielectric constant and dielectric loss decrease. Partial substitution of Yb3+ by Bi3+ weakens the Fe2 -Fe3+ double exchange interaction, which results in the decrease of saturation magnetization. The magnetic ordering temperature increases and magnetodielectric coefficient decreases with increasing x because of the enhancement of Fe3+(a)-O-Fe3+(d) superexchange interaction.