【作者】 刘愉快；

【导师】 李晓光；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2014， 博士

【摘要】 随着人们对器件小型化、多功能化的需求越来越高,传统电子器件尺寸已经接近了量子极限,因而尺寸的进一步降低变得越来越困难。在保证器件尺寸不变的情况下,一种有效的解决办法就是采用多功能材料提供新颖的物理现象和功能,从而使得计算机能力和存储密度增加等。而同时具有铁电极化和磁极化、并且它们之间具有耦合作用的多铁材料的出现,使得电控磁或者磁控电成为可能,并且其在多功能存储器件、磁传感器件等领域具有很大的潜在应用前景,有望解决传统电子器件的瓶颈。天然的室温单相的多铁材料的匮乏、而且其磁电耦合效应比较小,而人工复合的多铁材料在材料的选取上和磁电耦合效应上都具有单相多铁材料所不可比拟的优势,因而引起了人们的广泛关注,特别是具有铁磁／铁电(FM/FE)结构的多铁异质结。尽管人们在多铁异质结中开展了广泛的研究工作并取得一些进展,然而仍然存在着一些问题亟待解决,比如如何在高温、低磁场下实现大的磁介电效应,磁场对铁电性的影响如何,以及如何在多铁隧道结中实现交换偏置与四阻态的共存等等。针对上述问题,本论文主要研究了多铁异质结的磁介电效应和磁场对铁电性的影响,并探讨了多铁异质结中影响磁介电效应和磁电耦合内在的因素,为实现室温大的磁电耦合效应提供了途径；初步探索了在多铁隧道结中同时实现四阻态和交换偏置效应的可能性。论文内容共分为五章,每一章的主要内容分别概括如下：第一章主要综述了多铁异质结中的磁介电效应、磁电耦合效应以及多铁隧道结等方面的最新研究进展；并着重的介绍了多铁异质结和隧道结中的界面效应产生的机理及性能的调控。第二章中,我们研究了BiFeO3/La5/8Ca3/8MnO3(BFO/LCMO)多铁异质结的磁介电效应。在BFO/LCMO异质结中实现了由LCMO的磁阻以及Maxwell-Wagner效应共同导致的高达1100%的巨磁介电效应。通过对BFO/LCMO多铁异质结在不同温度、频率以及磁场下的介电频谱的研究发现：磁介电效应随着磁场的增加而增加,并在LCMO的铁磁转变温度Tc=220K、H=10T、f=500kHz处达到最大值1100%；而且在f=500kHz,介电的损耗随着磁场的增加而降低。通过对其介电弛豫分析,发现在Tc以下,弛豫时间τ随着温度的降低而降低；而在Tc以上,τ随着温度的增加而降低,符合热激活的Arrhenius公式,在零磁场下热激活能大约为93meV,且激活能随着磁场的增加而降低。第三章我们研究了磁场对BFO/LCMO异质结铁电行为的影响。实验发现磁场对BFO/LCMO异质结的表观矫顽力Ec有很大的调控作用,这是由于LCMO的相分离的变化改变了LCMO和BFO/LCMO界面的导电能力,从而影响铁电矫顽力。通过系统的研究Au/BFO/LCMO复合薄膜在不同温度和磁场下铁电性质的变化,我们发现在零磁场下BFO/LCMO的Ec随着温度的增加先减小再增加,并在在220K达到峰值。而随着外磁场的增加,这个峰明显减弱并逐渐向高温方向移动。而剩余极化强度Pr随着温度的升高而增加,且在外磁场下变化不明显。通过对不同磁场下BFO/LCMO系统以及单层LCMO和BFO的阻抗的模分析发现：这种明显的变化主要来源于LCMO单层和BFO/LCMO界面处分压的变化。在磁场下LCMO的金属相和绝缘相的体积分数会发生变化,因而LCMO和界面的导电能力也会发生变化,进而改变界面处的电压降,从而影响BFO/LCMO的表观矫顽力Ec。由于BFO与衬底的晶格失配随着温度的降低而减小,面内压应变减少,c轴缩短,因而Pr随着温度的降低而减少。第四章我们用10nm厚的反铁磁—铁电材料BiFeO3作为势垒层构建了La0.6Sr0.4MnO3/BiFeO3/La0.6Sr0.4MnO3(LSMO/BFO/LSMO)多铁隧道结,由于BFO铁电极化对隧穿磁阻效应的调控和BFO/LSMO界面的磁交换作用,我们实现了四阻态和交换偏置效应的共存。通过对隧道结的铁电性、隧穿电致电阻结电阻的电输运测量,我们在LSMO/BFO/LSMO多铁隧道结中发现了四阻态行为以及界面磁电耦合作用即BFO铁电极化对磁致隧穿电阻的调控作用。而且在隧道结中通过这种电测量获得交换偏置场与未刻蚀的样品通过磁测量的交换偏置场的大小以及其随温度的变化趋势都一致。此外,采用脉冲激光沉积技术制备了高质量的LSMO/BFO/LSMO//SrTiO3铁电隧道结,发现其在5K下具有大约8×104%的TER效应,且高低组态之间的转换具有很好的稳定性。第五章中,我们研究了x[0.92Pb(Mg1/3Nb2/3)O3-0.04Pb(Zn1/3Nb2/3)O3-0.04PbTiO3]-(1-x)Ni0.2Cu0.2Zn0.6Fe2O4(PMZNT-NiCuZn)复合材料的磁电耦合系数随温度、频率以及组分的变化关系。在准静态频率下(1kHz),样品的磁电耦合系数αE随着磁场的增加先增加到一最大值αmax后,然后随着磁场的继续增加而减小；随着温度的降低,峰值向高磁场方向移动,而且αmax也随着温度的降低而增加。而αmax随着组分x并不是单调的变化,而是对于所有温度几乎都是在x=0.6有一最小值。随着频率的增加,磁电耦合系数都是随着频率的增加而增加。更多还原

【Abstract】 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.更多还原