Bismuth ferrite BiFeO3 (BFO) is one of the most studied single-phase multiferroic materials with both ferroelectricity and G-type antiferromagnetism above room temperature. It undergoes a ferroelectric-paraelectric phase transition at TC = 830°C and an antiferromagnetic-paramagnetic phase transformation at Néel temperature TN = 370 °C. Despite these wonderful properties of BFO, there are some drawbacks associated with this material including the formations of impurity phases, weak magnetic properties, weak magnetoelectric coupling, and large leakage current density. Therefore, the appropriate chemical modifications are required to improve the electrical and magnetic properties of BFO. In this work, the substitutions of rare earth (RE) ions, such as Dy3+, Er3+, Yb3+, etc., for the A-site Bi3+ ion have been performed and the structures and physical properties of the resulting solid solutions have been investigated.First, new multiferroic materials (1−x)BiFeO3-xDyFeO3 (denoted BDF-x) and (1-x)BiFe(1-y)Ti(y)O(3+y/2)-xDyFeO3 (denoted BDFT-x-y) were synthesized by solid-state reactions. Compared with pure BFO, the ferromagnetism in the BDF-x solid solution is substantially enhanced by the structural distortion and unpaired electrons due to A-site substitution of Dy3+ for Bi3+. The electrical properties, including the ferroelectric and dielectric properties, are further improved by the substitution of Ti4+ for Fe3+ on the B-site, which substantially diminishes the conductivity and consequently the dielectric loss. Well-developed ferroelectric hysteresis loops are displayed in BDFT-x-y with a large remnant polarization Pr = 23 μC/cm2 at room temperature, which is significantly higher than the previously reported Pr = 3.5 μC/cm2 in pure BiFeO3 ceramic. Moreover, weak ferromagnetism is found in it at room temperature (Ms = 0.1 μB/f.u.). The structure-composition phase diagram of the BiFeO3-DyFeO3 system is established.The chemically modified (1-x)BiFe(1-y)Ti(y)O(3+y/2)-xLuFeO3 ceramics exhibit ferromagnetism with a saturated magnetization (Ms = 0.03 μB/f.u) and a remnant polarization of 0.30 μC/cm2 at room temperature. In the (1−x)BiFeO3-xYbFeO3 solid solution, a calculated spontaneous polarization of 7.7 μC/cm2 is obtained for the x = 0.11 ceramics which exhibit a weak ferromagnetism with Ms = 0.025 μB/f.u at roomivtemperature. Interestingly, an unusual magnetization reversal behavior is discovered in the (1−x)BiFeO3-xErFeO3 solid solution. At x = 0.12, the magnetic pole inversion occurs at 30 K. Lastly, the (1−x)BiFeO3-xEuFeO3 solid solution is found to exhibit an interesting magnetization behavior, with the magnetic properties undergoing a crossover from an antiferromagnetic to ferromagnetic state at x = 0.12.
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Thesis advisor: Ye, Zuo Guang
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