Magnetoelectric Effect And Resistive Switching Behaviors In Multiferroic Composites

Recently, with the development of the information storage industry, it is necessary to achieve multi-functionality in one device. Therefore, the mutual manipulation of the magnetic and electric properties in multiferroics which show both magnetic and ferroelctric properties is of great fundamental and technological interest. In single-phase multiferroic materials, the room-temperature magnetoelectric coupling effect is very weak due to the contradiction of symmetry between ferroelelctric and ferromagnetic. The multiferroic composite materials expected to show a much stronger magnetoelectric effect than that of the single-phase magnetoelectric materials. Although many efforts have been made in this system, there are lots of problems that need further investigation, such as the influence of energy loss on magnetoelectric properties. Moreover the relation between the magnetoelectric and magnetodielectric effects is not clear so far.In this dissertation, the magnetoelectric, magnetodielectric and multi-state resistive switching behaviors were studied in multiferroic composite materials. The possible factors to influence the magnetoelectric and magnetodielectric effect were discussed, and the design of resistive switching prototype memory device were explored.In chapter1, the magnetoelectric, magnetodielectric and resistive switching behaviors in multiferroic composites were briefly reviewed, and mechanisms of all these effects were introduced.In chapter2, the magnetoelectric properties and energy loss behaviors in Terfenol-D/PbZr0.52Ti0.48O3/Terfenol-D laminate composite were systematically investigated. At quasi-static frequency, the maximum magnetoelectric voltage coefficient was about1V cm-1Oe-1, while increased up to23V cm-1Oe-1for resonance conditions. By analyzing the frequency dependence of resonance magnetoelectric coefficients, the total energy dissipation increased with increasing magnetic field at first, reached its maximum near500Oe and then shifted down. After comparing the possible contribution of each energy dissipation mechanism, it was found that energy dissipation of the system was mainly dominated by the mechanical damping due to the domain wall motion in Terfenol-D under the actions of dc and ac magnetic fields as well as the stress at the interface.In chapter3, the magnetic field dependence of the dielectric permittivity of magnetoelectric composites composed of Terfenol-D and PZT was studied systematically. A large magnetodielectric (MDE) effect following a conventional MDE definition was achieved. In addition, an improved definition (MDEM) and method were developed to describe this MDE more appropriately for resonance conditions, and maximum of MDEM at room temperature was up to15%in the magnetic field of5kOe. The results indicated that the variations of dielectric permittivity with magnetic fields were associated with mechanical energy loss due to magnetic domain wall motion in the magnetostrictive layer Terfenol-D. In addition, by optimizing the structure of the magnetoelectric composite, the resonance frequency was decreased to effectively avoid the high-frequency eddy-current loss of Terfenol-D. Besides, the magnetoelectric and MDE effect was successfully connected after investigating the MDE effect in both ac and dc magnetic fields, and the characterization of magnetoelectric effect via MDE effect was realized.In chapter4, the bipolar (BRS) and unipolar resistive switching (URS) behaviors were investigated systematically in Au/BiFe0.95Mn0.05O3/La5/8Ca3/8MnO3heterostructure. For the conductive filament related URS, the ON/OFF ratio was up to3orders of magnitude and the good retention characteristic was obtained. The BRS was related to the interface barrier change at the asymmetric electrodes, which can be modulated by ferroelectric polarization. The results showed that after conductive filaments were formed, the previously polarized ferroelectric state would keep almost unchanged. By combining the two resistive switching mechanisms together under appropriate programming conditions, a tri-state-like resistive switching behavior was realized, providing effective routes in designing high-density storage. According to these distinctive characteristics, a prototype memory device with secure information storage was properly designed as an example of promising applications.In chapter5, magnetodielectric and magnetoelectric effect in PbZro.52Tio.4803/La0.62Ca0.3gMnO3(PZT/LCMO) heterostructures were investigated. A large relative change of capacitance up to60%was observed near the ferromagnetic transition temperature (Tc～220K) at H=0.8T and f=3MHz. This magnetodielectric effect may be led by the combination of Maxwell-Wagner effect as well as magnetoresistance effect in LCMO. In addition, it was found that the coercive electric field Ec decrease dramatically with increasing magnetic field near Tc, which may be related to by the depolarization effect caused by the semiconducting electrode LCMO.