Magnetodielectric And Magnetoelectric Effects In Multiferroic Heterostructure

As for the trends toward device miniaturization and multi-functionalization, the conventional electronic element size is approaching quantum limits. One way to continue the current trends in computer power and storage increase, is to use multifunctional materials that would bring out novel physical phenomena and enable new device capabilities. Multiferroic materials, as a result of a coupling between the magnetic and electric orders, enable controlling the ferroelectric polarization by a magnetic field, and conversely manipulating the magnetization by an electric field, which exhibit great potential applications in multi-level memories and magnetic-field sensors. Due to very weak magnetoelectric coupling in single-phase multiferroics, much more attention has been paid on the composite multiferroics, especially the artificial ferromagnetic/ferroelectric (FM/FE) multiferroic heterostructures. Even though much remarkable progress has been made in FM/FE multiferroic heterostructures, several key issues remain to be further investigated, such as giant magnetocapacitance (MC) effect with high temperature and low magnetic field, the effect of magnetic field on the ferroelectric properties, and the possibility for multi state storage devices in multiferroic tunnel junction (MFTJ).In this dissertation, the magnetocapacitance and effect of magnetic field on the ferroelectric properties in multiferroic heterostructures are studied. The possible factors influencing the magnetocapacitance and magnetoelectric coupling are also systematically investigated, which provides a route to achieve a giant room-temperature magnetoelectric coupling with low magnetic field. Furthermore, the design of prototype device with the coexistence of four resistance states and exchange bias in MFTJ was explored.In chapter1, we make a brief introduction to the recent advances in magnetocapacitance and manetoelectric coupling effects of FM/FE multiferroic heterostructure, and the magnetoelectric coupling in MFTJs, putting emphasis on the interfacial effects at the heterostructure and tunnel jucntion and routes to crease and control the properties of these materials.In chapter2, the colossal magnetocapacitance about1100%enhancement around 220K was obtained in BiFeO3/La5/8Ca3/8MnO3(BFO/LCMO) multiferroic heterostructure, which is attributed to the contribution of magnetoimpedance effect and Maxwell-Wagner effect. By systematically studying the temperature, frequency and magnetic field dependencies of dielectric properties of BFO/LCMO, it was found that the magnetocapacitance increases with increasing magnetic fields and reaches a maximum up to1100%enhancement near the ferromagnetic transition temperature (Tc) of LCMO at H=10T and f=500kHz. From the analysis of the dielectric relaxation, one can see that above Tc, the relaxation time τ decreases with increasing temperature, and follows the Arrhenius law. The obtained activation energy Ea is about93meV at H=0T, and reduces rapidly with increasing magnetic fields. While for temperatures below Tc, τ decreases approximately two orders of magnitude with decreasing temperature.In chapter3, we found that the reduction of apparent coercive electric field Ec for Au/BFO/LCMO heterostructure is closely related to the voltage drops across the LCMO layer and the magnetoelectric coupled interface between BFO and LCMO, which can be influenced by magnetic fields due to the phase separation in LCMO. Through systematically investigating the temperature and magnetic field dependencies of ferroelectric properties of Au/BFO/LCMO heterostdructure, it was found that Ec decreases obviously with increasing magnetic fields around Tc～220K. The remanent polarization Pr increases with increasing temperatures, but changes little with magnetic fields at f=2kHz. From the magetnic field dependence of impedance of BFO, LCMO, and BFO/LCMO, we found the variations of ferroelectricity are mainly related to the voltage drops across the LCMO layer and BFO/LCMO interface. The decrease of Pr with decreasing temperature is probably due to the temperature dependencies of the lattice mismatch between the substrate and BFO films.In chapter4, with10nm thick ferroelectric-antiferromagnetic BFO as a barrier in La0.6Sr0.4MnO3/BiFeO3/La0.6Sr0.4MnO3(LSMO/BFO/LSMO) MFTJ, both exchange bias and four resistance states have been realized in one memory cell, due to the tunnel magnetoresistance manipulated by ferroelectric polarizations and the magnetic interaction between antiferromagnetic BFO and ferromagnetic LSMO layers. As the negative and positive pulse voltages applied, the tunnel magnetoresistance manipulated by ferroelectric polarizations, i.e., four non-volatile resistance states, is observed in this tunnel junction. The most important finding is that the exchange bias is achieved in this MFTJ. This exchange bias field obtained from electric measurement follows the similar variation trend as observed from the magnetization measurements in the unpatterned sample. In addition, we found giant tunnel electroresistance effect (8×104%) in LSMO/BFO/LSMO MFTJs with3nm thick BFO as tunnel barrier fabricated by pulse laser deposition.In chapter5, we systematically investigate the temperature, frequency and magnetic field dependence of magnetoelectric coupling effect in x[0.92Pb(Mg1/3Nb2/3)03-0.04Pb(Zn1/3Nb2/3)03-0.04PbTi03]-(1-x)Ni0.2Cuo.2Zno.6Fe204(xP MZNT-(1-x)NiCuZn)(x=0.4,0.6,0.8) by Super ME measurement system. At quasi-static frequency (f=1kHz), the magnetoelectric voltage coefficient a first increases, reaches a maximum value αmax and then decreases with increasing magnetic field for all samples. With decreasing temperature, αmax increases, and the corresponding magetnic field increases as well. It is found that the variation trend of the αmax as the function of PMZNT volume fraction x is not monotonous, which almost reaches minimum at x=0.6.