Structural And Magnetic Properties In Rare Earth Doped BiFeO₃ Polycrystalline Ceramics

The new-type multifunctional material has become the focus of research in the field of material science owing to the requirements on function materials such as miniaturization and integration. At present, magnetic materials and ferroelectric materials are both extensive applied as functional materials, which make multiferroics the hotspot. Multiferroics are materials in which the ferroelectric and ferromagnetic phases coexist, the theoretical study of which can solve many fundamental physical problems; moreover, this kind of material can bring new technological breakthrough. In recent years, much of exploration has been done, involving trying various ingredients and improving methods of preparation for better characters. Many remarkable materials have been reported, such as BiFeO3. But it is not in widespread use mainly because BiFeO3 itself is antiferromagnetic materials. The magnetism in BiFeO3 is too weak to be applied. In this paper, we synthesized samples by solid-state reaction method. A series of rare-earth doped BiFeO3 samples, Bi1-xRxFeO3(R=La,Pr,Sm,Nd,Eu,Gd,Tb,Dy;x=0-1) were prepared and studied in depth in order to improve the magnetism in samples. We paid much attention to the structure and the magnetic properties of all the samples and gained some useful results.We observed the changes in crystal structures as R and x taking different values. The results appeared that different R brought the structures vary in similar. The lattice constants decreased by increasing x. Furthermore, series samples with the same x displayed lanthanide contraction. Later, R is represented by Nd. We detected the structure of Bi1-xNdxFeO3. We found all samples can form steady phases with right temperature and time of retaining heat. With increasing x, the little impurity phases disappeared, which proved doping can resist the impurity phases. There was a transformation from rhombohedral lattice to orthorhombic one when x≥0.4. We used refined software to determine the sample's structure, the lattice constant and atomic occupancy. The analysis of the performance of the sample structures had a very important role since oxides were very much influenced by the structure. We used a variety of means to test the magnetic properties of the samples, including hysteresis loop in different temperature and the magnetization as a function of temperature. The results showed that samples with different rare earth elements had the same variation. Doping improved the magnetic properties, which contained the saturation magnetization, coercivity and the Néel temperature. Experimental data showed that the sample magnetization and Néel temperature rising linearly, while the coercivity increased to a maximum value and down with x increasing. We studied in depth the magnetic properties of samples and had explanations as follows: the increase of magnetization was due to the replacement of rare earth, which damaged the structure of BiFeO3 and showed up the inherent magnetic moment. In addition, rare earth magnetic moment has its own magnetic moment that can increase the overall magnetic properties. On the coercive force, considering he different components and changes in the coercivity value, it can be determined that the coercivity of samples mainly from the magnetocrystalline anisotropy, simultaneously the role of magnetoelastic anisotropy. Talking about the increase of Néel temperature, it shows in high field hysteresis loops that the total magnetism came from the tilt of antiferromagnetic magnetism moment. We could infer the conclusion that the addition of R enhanced the antiferromagnetic exchange interaction among magnetic moments.Interestingly, Bi1-xNdxFeO3 series of samples showed complicated magnetic properties in low temperature. We detected magnetism by field cooling and zero field cooling with different field and established a simple model for the results. We discussed the interaction and independence of Nd and Fe sublattice with temperature changing. This model could explain the magnetic behavior of the various temperature ranges. Discussion above can also explain the phenomenon that coercivity down to zero at low temperature.