Investatigation Of Synthetic Process And Properties Based On Ca-Doped BiFeO₃ Nanoparticles

Multiferroics are unique materials for their unique coupling of electric, magnetic, and structural order, which are useful for several applications including novel storage media, spintronic devices, sensors and multi-state electric devices. Among many single-phase magnetoelectric multiferroics, BiFeO₃（BFO） is the minority material presently known to express coupling between magnetic and ferroelectric order at room temperature. It exhibits ferroelectricity（TC=1103k） and G-type antiferromagnetism（TN=640K） with superimposed cycloidal modulation at room temperature simultaneously. However, the rather weak magnetization and magnetoelectric effect may prohibit the application of BFO in practice. Ion substitution is a general method to solve this problem. In our studies, we aim to improve the properties of BFO through alkaline earth Ca doping and Ca co-doped with rare earth elements or transition metal element. The main contents of this paper listed as followed:1. Experimental methods were used to study the structure, electronic, optical and magnetic properties of BFO and Bi0.9Ca0.1FeO3 accompanied with first-principles methods verification. BFO and Bi0.9Ca0.1FeO3 were prepared by sol-gel method. X-ray diffraction（XRD） and Raman results show that Bi-site doped with Ca could result in a transition of crystal structure（from single phase rhombohedral（R3c） to two phase coexistence）. Experiment results show that band gap of BFO is decreased to 1.42 e V after doping, which is consistent with density of states cac ulations results. After Ca doping in BFO the leakage current decreased and the magnetism increased apparently.2. Ca doped and Ca, Nd co-doped BFO were synthesized by sol- gel method. The structural, electronic, magnetic and optical properties are studied s ystematically. The crystal size decreases to ⁵0 nm after doping. The crystal structure of dopped BFO are well described with two phase model of rhombohedral and cubic. Although a large remnant polarization（Pr） value of about 54.7 uC/cm2 is obtained for the Bi0.9Ca0.05Nd0.05FeO3 sample, an obvious contribution from the leakage current is observed. An apparent blue shift can be observed in the co-doped samples along with a decrease of the direct optical band gap. Moreover, the leakage current was decreased due to the introduction of nonvolatile Ca and Nd at Bi3+ site. Analysis of MPMS-VSM magnetic hysteresis data reveals a further enhancement in magnetism in the Nd doped Bi0.9Ca0.1FeO3, which is further explained by XPS characterization.3. BFO and transition metal（Co,Ni,Cu） doped BFO nanoparticles have been synthesized by the sol–gel method. At first, comparative study of pure, Ca-doped, Co-doped and co-doped BFO nanoparticles was done by us, then we analyze the influence of transition metal element（Co, N i, Cu）doping on structural, electrical and magnetic properties of Bi0.9Ca0.1FeO3 nanoparticles. Rietveld refinement of X-ray diffraction data and Raman spectra reflect a structural phase transition after doping. An apparent blue-shift can be observed in Bi0.9Ca0.1FeO3, Bi0.9Ca0.1Fe0.9Co0.1O3, Bi0.9Ca0.1Fe0.9Ni0.1O3 and Bi0.9Ca0.1Fe0.9Cu0.1O3 nanoparticles along with a decrease of the direct optical band gap compared with pure BFO. XPS results reveal that the concentration of Fe2+ and oxygen vacancy decreased after transition metal elements（Co, N i, C u） doped into Bi0.9Ca0.1FeO3. Moreover Co, Ni doping can enhance the saturation magnetization, while Cu doping can enhance the coercive field in Bi0.9Ca0.1FeO3.