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Study on Magnetoelectric Coupling Effect and Resistive Switching Characteristics of Multiferroic Heterostructual Films

Author: LiZiWei
Tutor: WanJianGuo
School: Nanjing University
Course: Condensed Matter Physics
Keywords: multiferroic materials composite films magnetoelectric effect resistiveswitching
CLC: O484.4
Type: PhD thesis
Year: 2012
Downloads: 25
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Abstract


Multiferroic magnetoelectric composite films can produce a unique physical characteristic from the coupling interaction of ferroelectric and magnetic phases, i.e. magnetoelectric effect. Based on such effect, magnetoelectric composite films have recently attracted increasing interests and show potential applications in novel multifunctional devices, such as sensors, memories, and spintronic devices. Moreover, multiferroic materials combined with some functional materials, i.e. semiconductor, show the desired switching of resistivity, which has been given considerable attention due to their promising applications in resistance-based random access memories. In this paper, magnetoelectric coupling modulated by external electric field and modulation mechanism in magnetoelectric composite films as well as the resistive switching behavior of ferroelectric-semiconductor heterostructures were investigated. According to these studies, we further designed a heterostructual film consisting of ferromagnetic, ferroelectric and semiconductor components and investigated its multistate resistive switching behavior and mechanism upon the combined stimuli of electric field and magnetic field. The main results of this thesis are described as follows:The CoFe2O4(CFO)-Pb(Zr0.52Ti0.48)03(PZT) bilayer composite film was prepared by a sol-gel process. An integrating current method was adopted to characterize the ferroelectric polarization behavior of the film at various magnetic fields. We find that the ferroelectric polarization of the film is suppressed by external magnetic field, which is originated from the mechanical stress in CFO layer induced by the magnetic field preventing the movement of the ferroelectric domains in PZT layer. After undergoing cycling electric polarization treatment, the ferroelectric polarization of the film enhances and the suppression of ferroelectric polarization by external magnetic field remarkably weakens. Furthermore, we measured the temperature dependence of dielectric loss of the film. After the bilayer film is subjected to the cycling electric polarization treatment N-2.2×108, we find the activation energy of the oxygen vacancies increases from0.58eV to0.68eV. It is concluded that the concentration of oxygen vacancies in PZT layer is reduced and the oxygen vacancies migrate to CFO layer, which is also comfirmed by the leakage current meaurement. Such cycling electric polarization modulation on the magnetoelectric properties of the film is closely correlated with the oxygen vacancies migration between CFO and PZT layers upon the cycling electric polarization, which leads to more ferroelectric domains in PZT layer to be unpinned and reversed and then enhances the ferroelectric characteristic of the film upon external magnetic field.The bilayer Pb(Zr0.52Ti0.48)03/ZnO (PZT/ZnO) heterostructural films with different ZnO-layer thickness were prepared by a sol-gel process. The heterostructures exhibit asymmetry diodelike current-voltage and polarization-voltage behaviors, which strongly depend on the thickness of the ZnO layer. The heterostructures are at the high resistance state (HRS) for the positive bias voltage and switch to the low resistance state (LRS) for the negative bias voltage. Reversible two-state resistive switching between HRS and LRS upon external electric field is originated from the coupling between the reversible ferroelectric polarization in PZT layer and the irreversible spontaneous polarization in ZnO layer. Charge carriers in the heterostructures are accumulated or depleted at the interface giving rise to different local conductivities. The rectification ratio of the heterostructures increases with increasing the ZnO-layer thickness, which is attributed to the enhanced interface polarization coupling and energy band structure differences caused by the ZnO-layer thickness. On one hand, the ZnO-layer thickness increasing decreases the hole barrier of the heterostructures, which leads to the current increase at LRS. On the other hand, as the thickness of the ZnO layer increaseing, the degree of its preferential crystallographic orientation along c-axis is enhanced, which leads to the enhancement of spontaneous polarization in the ZnO layer and result in the enhancement of the interface polarization coupling between ZnO and PZT layers. And then the current decreases at the HRS. Finally, the maximum value of rectification ratio of-1750is obtained where the ZnO-layer thickness is125nm.Moreover, the electric conduction mechanisms of the heterostructures at low and high resistance states were revealed. The electric conduction mechanism in low electric fields is ohmic conduction for all heterostructures, however, in high electric fields, SCLC process is dominative at high resistance states, while FN tunneling process is responsible for the electric conduction process at low resistance states when the ZnO layer is thicker.The three-layer heterostructural film combining multiferroic CoFe2O4-Pb(Zr0.52Ti0.48)O3composite film with semiconductor ZnO layer was designed and prepared. The CFO/PZT/ZnO heterostructural film exhibites three-state resistive switching behavior under the different stimuli combination of voltage pulse and magnetic bias. Three resistance states are HRS, LRS and lower resistance state (LerRS), the resistance value at LRS and LerRS are approximately6and20times smaller than that at HRS, respectively. And also three resistance states can retain at least for103ms without the external field stimuli. The asymmetry diodelike current-voltage, capacitance-voltage and polarization-voltage loops are observed, which seriously depend on magnetic bias. We revealed that three-state resistive switching was dominated by interface polarization coupling between PZT and ZnO layers upon external electric field and the magnetoelectric coupling between CFO and PZT layers upon external magnetic bias.

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CLC: > Mathematical sciences and chemical > Physics > Solid State Physics > Thin Film Physics > The nature of the films
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