Use this URL to cite or link to this record in EThOS: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.781014
Title: Multiferroics in perovskite and aurivillius structured materials
Author: Cao, Jun
ISNI:       0000 0004 7966 6509
Awarding Body: Queen Mary University of London
Current Institution: Queen Mary, University of London
Date of Award: 2019
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Abstract:
Multiferroics (MF) are types of novel materials that possess two- or three- of the so called 'ferroic' properties, including ferroelectricity, ferromagnetism and ferroelasticity, which have recently simulated vast number of research activities. Specifically, the couplings between ferroelectricity and ferromagnetism provide the possibilities to create new generation multifunctional devices such as "electric-write, magnetic-read" high density memory media, electric field tunable targeting therapy, micro-magnetic field detection gyro-sensors, etc. There are two main types of materials to realize the multiferroicity, including multiferroics composites and multiferroics single phase compounds. The multiferroics composites, composed by ferromagnetic and ferroelectric phase, are famous for yielding large magnetoelectric (ME) coupling effects above room temperature. However, the multiferroics composites are restricted for applications because their ME coupling effects are commonly achieved by the interaction between piezoelectricity and magnetostriction. The macroscopical couplings are only effective at low frequency range of electric and magnetic fields (below GHz level) and insensitive for low external magnetic fields, which restricts their applications. For the single phase room temperature multiferroics materials, they possibly possess intrinsic ME coupling effects that the magnetic and electric interact mutually through the changes of electron spin. That means the single phase multiferroics are possible to build intrinsic multiferroics for the high speed read/write memory media and low magnetic field detector devices. This study l mainly focus on the preparation, characterization and study on the Aurivillius and perovskite structure based single phase ferroelectric and multiferroics ceramics. The materials in this work were built based on the recent interesting ferroelectric systems including Ba2Bi4Ti5O18, Na0.5Bi0.5TiO3 and x(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-(1-x)BiFeO3. Characterizations on crystal structures, ferroelectric domain structures, dielectric and ferroelectric properties and magnetic properties are intensively investigated and discussed for each composition. For textured Ba2Bi4Ti5O18 ceramics, their dielectric relaxation behaviour has been proved by the diffusion behaviour of dielectric permittivity peaks around Tm (dielectric maxima). Additionally, the other fascinating finding is the electric field induced phase transition behaviour between strong polar-nano-regions (PNRs) to weak PNRs , which is manifested by the current-electric field loops with four peaks on the in-plane and out-of-plane measurement directions. Multiferroics (MF) are types of novel materials that possess two-or three-of the so called 'ferroic' properties, including ferroelectricity, ferromagnetism and ferroelasticity, which have recently simulated vast number of research activities. Specifically, the couplings between ferroelectricity and ferromagnetism provide the possibilities to create new generation multifunctional devices such as "electric-write, magnetic-read" high density memory media, electric field tunable targeting therapy, micro-magnetic field detection gyro-sensors, etc. There are two main types of materials to realize the multiferroicity, including multiferroics composites and multiferroics single phase compounds. The multiferroics composites, composed by ferromagnetic and ferroelectric phase, are famous for yielding large magnetoelectric (ME) coupling effects above room temperature. However, the multiferroics composites are restricted for applications because their ME coupling effects are commonly achieved by the interaction between piezoelectricity and magnetostriction. The macroscopical couplings are only effective at low frequency range of electric and magnetic fields (below GHz level) and insensitive for low external magnetic fields, which restricts their applications. For the single phase room temperature multiferroics materials, they possibly possess intrinsic ME coupling effects that the magnetic and electric interact mutually through the changes of electron spin. That means the single phase multiferroics are possible to build intrinsic multiferroics for the high speed read/write memory media and low magnetic field detector devices. This study I mainly focus on the preparation, characterization and study on the Aurivillius and perovskite structure based single phase ferroelectric and multiferroics ceramics. The materials in this work were built based on the recent interesting ferroelectric systems including Ba2Bi4Ti5O18, Na0.5Bi0.5TiO3 and x(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-(1-x)BiFeO3. Characterizations on crystal structures, ferroelectric domain structures, dielectric and ferroelectric properties and magnetic properties are intensively investigated and discussed for each composition. For textured Ba2Bi4Ti5O18 ceramics, their dielectric relaxation behaviour has been proved by the diffusion behaviour of dielectric permittivity peaks around Tm (dielectric maxima). Additionally, the other fascinating finding is the electric field induced phase transition behaviour between strong polar-nano-regions (PNRs) to weak PNRs , which is manifested by the current-electric field loops with four peaks on the in-plane and out-of-plane measurement directions. For Na0.5Bi0.5Ti0.8Mn0.2O3 with Nb additive ceramics, its single phase ceramic has been successfully prepared. In terms of it magnetic nature, the low temperature (< 30 K) remnant magnetization of the M-H loops, the zero-field cooling and field cooling (ZFC-FC) results indicate that the ceramic of Na0.5Bi0.5Ti0.8Mn0.2O3 with Nb additive possesses the low temperature ferromagnetic nature. The suggested Tc for ferromagnetic-paramagnetic phase transition is around 50 K. The research of room-temperature multiferrocity on this single phase ceramic reveals that the BNT based multiferroic is a valuable research direction for the new generation of magnetic-electric coupling devices. For 0.5(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-0.5BiFe0.8Mn0.2O3 ceramics, the single phase ceramic was successfully prepared. The visible magnetic field tuneable ferroelectric domain switching behaviour under PFM observation present its room temperature magneto-electric coupling effect, which is rarely presented among multiferroics researches. Moreover, the room-temperature ferromagnetism was also strongly confirmed by the apparent remnant magnetization in M-H loops, along with the ZFC-FC results, which indicates that 0.5(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-0.5BiFe0.8Mn0.2O3 possesses the structure with the transition from low temperature ferrimagnetic to room temperature ferromagnetic nature. These encouraged magnetic properties make this composition worthy to be further investigated on its magnetoelectric (ME) coupling effect mechanism or even its high frequencies (THz) ME coupling effect.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.781014  DOI: Not available
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