The Effect Of Rare Earth Element Substitution On The Electric And High Temperature Magnetic Behavior Of BiFeO₃ Ceramic

Multiferroic materials with simultaneous anti/ferromagnetic and ferroelectric orderings have been extensively studied for their potential applications in functional sensors and spintronic devices. One of the most widely investigated multiferroics is BiFeO₃（BFO） due to its ferroelectric and magnetictransition temperatureslocated well above room temperature, giving rise to possibilities of RT multiferroic devices. BiFeO₃ has arhombohedral distorted perovskite structurein the spacegroup R3 c with a Neel temperature of ⁶40 K and Curie temperature of¹100K. However, the high leakage current and weak macroscopic magnetism of BiFeO₃ are the main barriers to its practical applications. It is essential in device applications to reduce leakage current and improve the magnetic behavior without disturbing the ferroelectric properties. One possible strategy is partial ionic substitution to obtain spontaneous magnetization in BiFeO₃. Recently, great efforts have been focused on ferroelectric property, ferromagnetic property and magneto-electric effect of the multiferroic material BiFeO₃.We have studied the effect of oxygen, rare earth doping in the BiFeO₃ compound on its crystalline structure, electronic transport, ferromagnetism properties and the high temperature magnetic phase transition.（1） Effects of oxygen content on the magnetic properties of BiFeO₃ compoundMultiferroic Bi Fe Oδ（δ=2.67, 2.95, 3.02） compound of various oxygen contents（δ） have been prepared by gel sol method. All the samples of Bi Fe Oδ have the same crystal structure regardless of oxygen content. The SEM micrograph reveals microstructures comprising of grains with various sizes from 200 nm to 1μm. XPS study confirms the coexistence of Fe3+ and Fe2+ ions in Bi Fe Oδ compound. The M（H） curves exhibit weak ferromagnetic behavior with unsaturated magnetization at room temperature, independent of the oxygen content. The M（T） curves of Bi Fe O2.95 and BiFeO₃.02 samples suggest anti-ferromagnetic behavior with Neel temperature of³70°C and paramagnetic behavior of Bi Fe O2.67 sample from RT up to 500℃. The experimental results show that the antiferromagnetic ordering is strongly correlated with the oxygen content and is almost entirely suppressed in Bi Fe Oδ withδ=2.67. The effect of oxygen vacancies is to weaken the magnetic ordering rather than to enhance it as previously suggested in the literature.（2） High temperature magnetic behavior studies on Bi1–xCaxFe O3 ceramicsThe characteristic diffraction peaks of Bi_1-xCa_xFeO₃ samples became gradually wider and the（104） and（110） peaks of BiFeO₃ merged partially to formed a broadened peak（110） with Ca²⁺ doping. XRD analysis revealed a phase transition in Ca-doped BiFeO₃ from rhombohedral to orthorhombic when x≥0.15. The average grain size of Bi_1-xCa_xFeO₃ samples varies from 0.5 to 2μm by SEM images.This dielectric behavior of Bi1–xCaxFe O3 ceramics varies with content x and frequency, the dielectric constant measured at 1 k Hz reaches a maximum value of er=4603.9 when x=0.1, seven times as big as that of pure BiFeO₃.. With further increasing the Ca content（x=0.15, 0.2）, the value of the dielectric constant back to the level of pure BiFeO₃. The dielectric constant of Bi0.8Ca0.2Fe O3（er=57） is less than one-tenth of BiFeO₃（er=629.9）. The dielectric losses of Bi1-XCaxFe O3 samples become smaller than that of BiFeO₃ ceramic. This dramatic change in the dielectric properties of Bi_1-xCa_xFeO₃ samples can be understood in terms of orientational relaxation of dipoles and the space charge limited conduction associated with crystal defects at low frequency.The magnetic measurements show that all samples possess strong ferromagnetism at room temperature expect BiFeO₃ which are weakly ferromagnetic. It indicates that coexistence of Fe2 + and Fe3 + in Bi1- xCaxFe O3 samples according to the XPS spectrum. The ratio of Fe2+/Fe3+ is increased with doping Ca content and the magnetic preparation of BiFeO₃ is enhanced. It evidences that the ferromagnetic phase transition of Bi_1-xCa_xFeO₃ samples occurs at 878 K from M-T curve and the phase transition of BiFeO₃ happen at 878 K by DSC measurement.（3） Effects of Ho³⁺ substitution on the ferromagnetism properties of BiFeO₃All the peaks for Bi_1-xHo_xFeO₃ samples can be indexed according to the crystal structure of pure BiFeO₃ by XRD. When x=0.15 and 0.2, the samples consist of two phases including rhombohedral and orthorhombic. Ho doping BiFeO₃ enhanced the electrical properties with lower leakage current density. This dielectric behavior of Bi_1-xHo_xFeO₃ ceramics varies with doping content and frequency, which might be understood in terms of oxygen vacancy, the displacement of Fe3+ ions and lattice phase transition. The magnetic moment of Bi_1-xHo_xFeO₃ ceramics varies with temperature from 300 to 1000 K at 5k Oe. It shows that the TN of BiFeO₃ changes slightly from 644 K to 632 K with Ho³⁺ doping. The magnetic phase transition temperature of Bi_1-xHo_xFeO₃ will reduce from 878 K to 840 K with increasing Ho³⁺ content. All the samples exhibit weak ferromagnetic behavior under 840 K and paramagnetism above 890 K. It evidences that the ferromagnetic phase transition of BiFeO₃ samples occurs at 878 K.（4） Effects of Gd3+ and Co3+ co-substitution on the ferromagnetism properties of BiFeO₃ ceramicMultiferroic Bi0.95Gd0.05Fe1-xCoxO3（x=0, 0.05, 0.1, 0.15, 0.2） ceramics were prepared by rapid liquid phase sintering method. All the peaks of XRD for Bi0.95Gd0.05Fe1-xCoxO3 samples can be indexed according to the crystal structure of pure BiFeO₃. XRD analysis revealed a phase transition in Gd3+ and Co3+ co-doped BiFeO₃ when x 3 0.1. The SEM images indicated that Gd3+ and Co3+ doping significantly decreased the grain sizes of BiFeO₃ ceramic. Gd3+ and Co3+ co-doping BiFeO₃ enhanced the electrical properties with lower leakage current. The magnetic hysteresis loops and the magnetization were greatly improved in co-substituted specimens at room temperature. The Mr of Bi0.95Gd0.05Fe1-xCoxO3（x=0, 0.05, 0.1, 0.15, 0.2） was 34, 60, 105, 103 and 180 times of that of BiFeO₃ at 30 k Oe, respectively. All the samples exhibited ferromagnetic behavior at 750 K and paramagnetism at 900 K, indicating a high temperature magnetic phase transition of BiFeO₃ at 878 K, which shifted to 780 K by Gd3+ and Co3+ doping. This can be attributed to the Fe3+-O2--Fe3+ super-exchange strength and the relative stability of the magnetic structure.