Effect Of Eu And Co Codoping On The Dielectric And Magnetoelectric Properties Of BiFeO₃ Ceramics

The effects of A-site Eu doping and B-site Co co-doping on the dielectric and magnetoelectric properties of BiFeO₃ were investigated. Series of specimens of Bi_1-xEu_xFe_1-yCoyO₃ (x = 0, 0.05, 0.1, 0.01; y = 0, 0.01, 0.05, 0.1) were prepared by rapid liquid phase sintering method. The crystal structure of Bi_1-xEu_xFe_1-yCoyO₃ samples was examined by using a DX-2000 X-ray diffractometer (XRD). The dielectric and magnetoelectric properties of all the samples were studied using Agilent 4294A impedance analyzer in a frequency range from 40 Hz to 1 MHz. The resistivity of samples at room temperature was measured using a HP3458A multimeter. The results indicate that:(1) All the ceramic samples Bi_1-xEu_xFe_1-yCoyO₃ have the rhombohedral structure with R3c space group. No impurity phase peaks appear in the image of XRD for undoped BiFeO₃ and slightly doped Bi_0.99Eu_0.01Fe_0.99Co_0.01O₃. An impurity phase Bi₁₂Co_0.6O_18.8 was detected between 25 and 30 degrees of 2 theta when the quantity of Eu and Co reached 0.05 and above. Therefore, the best doping level of Eu and Co should not be more than 0.05 respectively.(2) The dielectric constants of all samples decrease with increasing frequency. The dielectric constants of Bi_1-xEu_xFe_1-yCoyO₃ is larger compared with those for undoped BiFeO₃. However, the dielectric constants of undoped BiFeO₃ depend less on the frequency, indicating higher frequency stability than those doped samples. At a particular frequency, the dielectric constant increases with the Eu and Co content first and then decreases with increasing Eu and Co doping. The dielectric constant of Bi_0.99Eu_0.01Fe_0.99Co_0.01O₃ ceramic sample is 1790.5 at 1000Hz, which is nearly an order of magnitude larger than that for undoped BFO. The dielectric loss of all the samples decreases with frequency. A small amount of Eu, and Co doping reduces dielectric loss of BFO significantly. The dielectric loss of Bi_0.99Eu_0.01Fe_0.99Co_0.01O₃ reaches a minimum, but it varies more complicatedly with further increased doping. The dielectric loss is biggest when x= 0.05 and y= 0.05. It falls again when x and y = 0.1.(3) At a particular frequency, the dielectric constant of undoped BiFeO₃ changes little with magnetic field, indicating a weak magnetoelectric coupling. Dielectric constants of Eu and Co co-doped samples Bi_1-xEu_xFe_1-yCoyO₃ increase with magnetic field, suggesting stronger magnetoelectric coupling effect. In particular, the magnetic-dielectric coupling coefficient (MD) for samples Bi_0.99Eu_0.01Fe_0.99Co_0.01O₃ and Bi0.95Eu0.05Fe0.95Co0.05O₃ in a magnetic field of 4500Gs was measured at 10 kHz to be 3.2% and 4.37% respectively, increased 20 and 25 times from that for undoped samples. To improve sample magnetoelectric coupling coefficient the best doping quantity should be x= 0.05, y = 0.05.(4) The resistivity of Eu and Co co-doped spemens increases with the Eu and Co content until x, y = 0.01where the resistivity reaches a maximum, and then decreases with increasing Eu and Co doping. Minimum resistivity is reached at x, y = 0.05. Compared with Eu, the effect of Co doping on the resistivity is larger; when the Co content equal or greater than 0.05, the resistivity of Eu and Co co-doped samples is lower than that for undoping sample.