Synthesis and investigation of Cu doped Ni-Zn ferrites and La, Dy doped BiFeO3 multiferroic composites

Abstract

Multiferroic materials with the coexistence of ferroelectric and magnetic orders have been stimulating significant interest both from the basic science and application point of view. The magnetoelectric (ME) effect in multiferroic composites with piezoelectric and magnetostrictive phases results from the cross interaction between the two phases in the composite. In the present research four series of multiferroic composites i) (1-y) BiFeO3(BFO)-yNi0.5Zn0.5Fe2O4(NZFO), ii) (1-y)BiFeO3(BFO)-yNi0.50Cu0.05Zn0.45Fe2O4 (NCZFO), iii) (1-y)Bi0.7La0.3FeO3(BLFO)-yNi0.5Zn0.5Fe2O4(NZFO) and iv) (1-y) Bi0.8Dy0.2FeO3(BDFO)-yNi0.5Zn0.5Fe2O4(NZFO) (where y = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 1.0) have been prepared by standard solid state reaction method. The structural, magnetic, electrical and ME properties of these composites have been studied in details. Pellet- and toroid-shaped samples are prepared from each composite and sintered at different temperatures for 4 hours in air. Structural and morphological analyses are carried out by Xray diffraction (XRD) and field emission scanning electron microscope, respectively. The complex initial permeability, dielectric constant and ac conductivity have been measured using a Wayne Kerr Impedance Analyzer. The XRD analysis reveals the coexistence of both the structures of ferrite and ferroelectric phases in the composites with no other phase thereby indicating that there is no chemical reaction between the two phases in the composites. The NZFO and NCZFO show spinel structure whereas BFO, BLFO and BDFO show distorted rhombohedral, tetragonal and orthorhombic perovskite structures, respectively. Impurity phase has been suppressed significantly in (1-y)BLFO-yNZFO and (1-y)BDFO-yNZFO composites due to the La and Dy substitution in BFO, respectively. There is a slight change in the lattice parameter of both the ferrite and ferroelectric phases in the composites which may be due to the stress exerted on each other by the two phases. The X-ray density decreases almost linearly obeying the rule of mixture for all the series of composites. The microstructural study shows that both the sintering temperature and the ferrite content in the composites have significant influence on the average grain size. The average grain size of all the composites increases with increasing sintering temperature. The average grain size initially decreases with the ferrite content up to certain concentration and then increases with further increase of ferrite content in the (1-y)BFO-yNZFO and (1- y)BDFO-yNZFO composites. On the other hand, the average grain size decreases for (1- y)BFO-yNCZFO and increases for (1-y)BLFO-yNZFO with the ferrite content in the composites. A significant enhancement in the initial permeability and relative quality factor has been observed with the increase of ferrite content in the composites. The optimum initial permeability is obtained for (1-y)BFO-yNZFO composites. The saturation magnetization, remanent magnetization and coercivity have been calculated from the M-H loop at room temperature. The saturation magnetization increases with the increase in ferrite content in the composites and it fairly follows the sum rule. The dielectric dispersion at lower frequency is due to the Maxwell-Wagner type interfacial polarization. The highest dielectric constant is obtained for (1-y)BDFO-yNZFO composites. The complex impedance spectroscopy is used to distinguish between the grain and grain boundary contribution to the total resistance. The grain resistance is extremely low as compared to the grain boundary resistance indicating the conducting nature of the grain. The ME voltage coefficient ����������� is measured as a function of applied dc magnetic field. The highest ��� (~66 mV/cm Oe) is obtained for 0.6BDFO-0.4NZFO composite which is attributed to the enhanced mechanical coupling between the two phases. This value of ��� is found to be larger than the values of some reported bulk composites and thus the (1-y)BDFO-yNZFO composites can be better alternatives to single phase ME materials.