Studies On The Synthetic Process And Properties Of Co-doped Polycrystalline BiFeO₃ Nanoparticles

Multiefrroics are materials in which two or more properties of ferromagneti(antiferromagnetism), ferroelectric(antiferroelectricity) and/or ferroelasticity(antiferroelasticity) occur in the same phase. Multiferromagnetoelectrics are the ones pocess ferroelectricity and ferromagnetism, which enables a coupling interaction between them, called the magnetoelectric(ME) effect. The mutual control between ferroelectric and magnetic ordering makes multiferroic materials have potiential applications in magnetoelectric storage, spintronics devices, sensors and so on. BiFeO3 exhibits ferroelectric and antiferromagnetic at room temperature(TC ~1100K, TN~640K). Furthermore, the difficulty in preparation pure sample, high leakage current and rather weak magnetism may prohibit the application of BiFeO3 in practice. In our studies, we aim to improve the properties of BiFeO3 through A and B site doping. The structural phase transition, morphology, ferroelectric and magnetic characteristic of all the samples have been explored. The main results of this paper as follows:BiFeO3(BFO), Bi0.8Er0.2FeO3(BEFO), Bi0.8Er0.2Fe0.9Mn0.1O3(BEFMO) and Bi0.8Er0.2Fe0.9Co0.1O3(BEFCO) nanoparticles were prepared by sol–gel method having an average size of 200 nm for BFO, under100 nm for BEFO and under 60 nm for BEFMO and BEFCO. Phase transition from a rhombohedral symmetry(R3c) for BFO to an orthorhombic symmetry(Ibmm) for BEFO, BEFMO and BEFCO ha s been observed. The phase transformation combined with size reduction has significantly improved both ferroelectric and ferromagnetic behaviors of the doped samples in a similar way. The formation of dipolar defect complexes(DDCs) in the doped samples also contributes to the improved ferroelectric property with saturated polarization(Ps) of 0.375μC/cm2 and remanent polarization(Pr) of 0.244μC /cm2 for BEFMO. Size effect may also impact the simultaneously developed Pr for BEFMO and BEFCO. Owning to the interactions between the ferromagnetic and antiferromagnetic microdomains, improved saturated magnetization(Ms) and remanent magnetization(Mr) are also observed in BEFMO.Mn, Zn doped and pure BiFeO3 were prepared by sol–gel method. The 5% and 10% Zn-doped phase crystallites have a rhombohedral symmetry(R3c) structure. Doped with 5% Mn, BiFeO 3 contains both rhombohedral(R3c) and tetragonal phase(P4mm) while 10% Mn gives a rise to tetragonal phase(P4/mbm). Raman and Fourier Transformed Infrared spectra a lso confirmed the structure transformation. Compared with pure BiFeO3, Mn-Zn doped samples present intense blue shift as shown in UV-Visible diffuse absorption together with the decreased direct optical band gap. The substitution of Mn ions for Fe enhances ferromagnetic properties while the Zn-doped ones show linear paramagnetism.Owning to the interactions between the ferromagnetic and antiferromagnetic microdomains, Mn and Zn co-doping shows the largest remanent magnetization(Mr) of 0.045emu/g among all the samples.