Investigations of multiferroic materials

Abstract

This thesis presents the synthesis and investigations of physical properties of multiferroic materials. Multiferroics are materials in which two or all the three properties: ferroelectricity, ferromagnetism and ferroelasticity occur in the same phase. Such materials have alluring potential applications because of the existence of many order parameters. The thesis is organized into four sections. Section 1 gives an overview of multiferroics, explaining the origin of multiferroicity, occurrence of magnetoelectric coupling, their possible technological applications and the challenges involved. Section 2 gives the scope of the investigations. The specific objectives of the present research on hexagonal rare-earth manganites like ErMnO3 and LuMnO3, Raman evidence of multiphonon scattering in TbMnO3, multiferroic heavy rare earth chromites, solid solutions of Y(Lu)CrO3 with Y(Lu)MnO3, cationic-substituted BiFeO3, chargeordered rare earth ferrites, LnFe2O4 and rare earth manganites doped with alkaline earth metals are outlined. Experimental aspects are described in section 3. In section 4, results of the investigations are discussed. In 4.1, Single crystal ErMnO3 and polycrystalline LuMnO3 have been established to be multiferroic through magnetic and ferroelectric properties studies, where support by Raman Spectra has also been reported. In 4.2, orbiton–phonon mixed nature of the high frequency Raman mode has been suggested. One of the first-order phonon modes shows anomalous softening below TN (~ 46 K), suggesting a strong spin–phonon coupling. Magnetic and dielectric properties of heavy rare earth chromites and Y(Lu)Cr1-xMnxO3 are reported in 4.3, where Néel temperature and ferroelectric transition temperatures are found to be influenced by ionic radii of rare-earth ions. In 4.4, we have investigated separately the impact of the substitution of manganese in the B-site and that of lanthanum in the A-site of BiFeO3 on its multiferroic properties. In 4.5, we describe the results of charge-ordered rare earth ferrites which show appreciable magnetoelectric effect. Magnetic, dielectric and magneto-dielectric properties of Ln1-xAxMnO3 and its many binary systeM.S. (Ln = rare earth, A = alkaline earth) are discussed in 4.6.