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Access Type

WSU Access

Date of Award

January 2016

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Physics and Astronomy

First Advisor

Boris Nadgorny

Second Advisor

Ratna Naik

Abstract

With the increasing demand on cheaper and better performance multifunctional materials for different applications, it is becoming more crucial to have a better understanding of the physics needed to tailor more devices and materials to fit better in every day’s technological needs. Materials which show more than one ferroic order simultaneously –namely, multiferroics– are of particular importance for their potential applications as multiple state memory elements, transducers and electrically tunable microwave devices.

In this work, we studied FeVO4 single crystals as an example on low symmetry multiferroics. We focused on the anisotropy in those crystals in an attempt to nail the origin of the ferroelectric and magnetoelectric behaviors. Our data suggest the crystallographic a-axis to be a favorable direction for magnetic and ferroelectric alignment. FeVO4 single crystals doped with Zn and Mn were also prepared to investigate the role of magnetically induced lattice distortions (in case of Zn) in creating the spontaneous ferroelectric moment, and to test for the role played by Dzyaloshinskii-Moriya interaction (in case of Mn) in FeVO4 ferroelectricity. Doped crystals were shown to retain the ferroelectric nature as well.

Gd-doped Fe3O4 nanoparticle system –a charge ordered ferroelectric– was also studied. Partial substitution of Fe3+ with Gd3+ aims to induce chemical pressure, based on the considerable difference in ionic radii. The dielectric constant of Fe3O4 was found to increase upon doping with Gd. Also, the ferroelectric polarization moment was found to be higher for the 5% Gd-doped sample. Our magneto-dielectric data were not conclusive about the nature of magnetoelectric coupling in this system.

Strain induced magnetoelectric coupling in polyvinylidene difluoride (PVDF)-based film composites was investigated as well. The use of reduced graphene oxide (RGO) and ZnO as nanofillers in PVDF-Fe3O4 hybrid films was found to reduce the piezoelectric β-phase (i.e.: the ferroelectric phase) content, even though PVDF-RGO composite films showed the highest ferroelectric polarization moment in the measured set. Using graphene oxide (GO), on the other hand, was shown to increase the β-phase content and to enhance the ferroelectric response as well. We also show that PVDF films carrying surface-functionalized Ni nanoparticles show enhanced ferroelectric polarization and higher magneto-dielectric coupling as compared to films with non-functionalized Ni. Finally, an enhancement in the dielectric permittivity and magneto-dielectric behavior was seen in barium hexaferrite (BHF)-loaded films.

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