MAGNETIC AND DIELECTRIC PROPETIES OF DOPED BIFEO3 AND MULTIFERROIC COMPOSITES

Abstract

Magnetoelectric (ME) coupling is exhibited by materials which posses ferroelectric/antiferroelectric and ferromagnetic/antiferromagnetic orders. These materials are termed as multiferroic materials. To be potentially important for applications, magnetoelectric materials should have magnetic and electric ordering above room temperature. This requirement sharply decreases the range of possible materials. BiFeO3 (BFO) possesses antiferromagnetic Néel temperature (TN) of 370 oC and ferroelectric Curie temperature (TC) of 810 oC. The ferroelectric polarization measurement in BiFeO3 is always hampered by its low resistivity and high coercive field. Also, the inhomogeneous spin structure of BiFeO3 leads to the cancellation of macroscopic magnetization which prohibits the linear magnetoelectric effect from being observed. This incommensurate spiral spin structure can be suppressed by strain, high magnetic field and by various doping. In the present thesis we have synthesized pure/modified BiFeO3 with good crystallinity and homogeneous microstructure. We have studied the magnetodielectric properties of the synthesized materials. In order to study the magnetoelectric coupling we studied the effect of various applied external magnetic field on dielectric and ferroelectric properties at room temperature, so as to suggest materials which may be most suited for device application. The effects of transition metal ion doping on the structural, magnetodielectric properties of BiFeO3 were studied and it is presented in Chapter 3. With the substitution of nickel ions BFO showed superparamagnetism over all the range of BiFe1-xNixO3 (x=0.25) at room temperature. Quenching of samples (quenched in liquid nitrogen) had enhanced the magnetic and dielectric properties of BFO as well as it showed spin glass transition at lower temperature (~20 K). Cr doped bismuth ferrite (leaching done in diluted HNO3) was found to have enhanced magnetic and magnetodielectric properties. Value of magnetoelectric interaction coefficient ( ) was calculated for Cr doped bismuth ferrite nanoceramics. Abstract ii Chapter 4 deals with the study of double (Zr, La) doped BiFeO3. We obtained ferroelectric (P-E) loops for Zr doped BiFeO3. Remnant polarization (Pr) was found to decrease as we increased the Zr4+ content in BiFeO3, at 80 kOe magnetic field. Low temperature step magnetization was observed in Zr doped bismuth ferrite. In addition to this, enhanced magnetic moment was observed in double (Zr, La) doped BiFeO3 which may be due to modification in Fe-O-Fe bond length. Chapter 5 presents the effect of Lu substitution at A site along with La on the magnetoelectric properties of BiFeO3. The antiferromagnetic Neél temperature was found to decrease to 375 ºC for (Lu and La) doped BiFeO3. At low temperature (~20 K) magnetic anisotropy change ( Sm , also called Magnetocaloric effect), of Lu doped Bismuth Ferrite was found to be higher than pure bismuth ferrite as well as some intermetallic compound. In another multiferroic system, Eu and Gd were substituted at A site with a fixed 10 mol% concentration. These double rare earth (Eu and Gd) doped BiFeO3 showed enhancement in magnetodielectric properties. Chapter 6 reports the multiferroic properties of composite La doped BiFeO3 with Zn doped CoFe2O4, and composites of BiFeO3-MnFe2O4. In addition to this, we have synthesised composite films of PVDF with nanosize single- phase bismuth ferrite. Enhancement in the magnetodielectric properties of these composite are discussed in this chapter. The magnetoelectric coupling coefficient ( ) was found be to higher than some reported value in literature. The optical properties study of composite films prepared by hot press method showed that the optical energy band gap of bismuth ferrite increases with increase in the concentration of PVDF. Chapter 7 presents the conclusion of the present thesis, which is mainly concerned with synthesis and study of structural, electrical, magnetic and magnetoelectric properties of the aforesaid compounds. By substituting various small concentrations of aliovalent ions in the A or and B site of bismuth ferrite, we found that the final structures were invariant. Systematic and exhaustive studies of electrical (dielectric constant, dielectric loss, P-E loop) properties and magnetic properties of the above compounds have provided important information about the magnetoelectric coupling