掺杂对BiFeO3薄膜电、磁特性影响综述

随着科技的高速发展,多铁性材料已经成为传感器、微波器件、数据存储、自旋电子学及太阳能电池等领域的研究热点,在智能材料与器件方向显示出可观的应用潜力。BiFeO_3及其衍生的一系列材料Bi_(1-x)AxFeO_3(A=La,Nd,Sm)、BiFe_x B_(1-x)O_3(B=Ni,Mn,Co)的发现使得多铁性材料获得了更迅猛的发展。这类材料属于单相钙钛矿氧化物型多铁材料,在室温以上同时具有铁电、压电、介电、电光、铁磁、光伏、磁电耦合、光催化等效应。BiFeO_3作为一种单相多铁性材料,与同类的多铁性材料相比,其具有较高的居里温度、尼尔温度以及较小的光学禁带宽度和较好的化学稳定性等特点。然而,在制备BiFeO_3的过程中,部分Fe~(3+)向Fe~(2+)转变,并且铋元素熔点较低容易挥发,产生大量的氧空位,造成漏电流较大,很难得到具有较高剩余极化强度的样品;并且BFO薄膜室温下弱的磁性等性质使其实际应用受到极大的限制。多年来国内外学者致力于改善制备条件和参数,使用更先进的制备方法,改用更合适的衬底材料及进行离子掺杂等,以制备多层复合薄膜。其中,离子掺杂对减小漏电流,提高铁电性及室温磁性方面的效果最为理想。各国研究者已经制备出比纯BiFeO_3材料性能更好的掺杂和复合BiFeO_3材料。在不同的位置掺杂多种元素较掺杂单一元素能更好地改善材料的性能。最新报道的采用溶胶-凝胶法制备的多个混合掺杂离子Bi_(0.88)Sr_(0.03)Gd_(0.09)Fe_(0.94)Mn_(0.04)Co_(0.02)O_3薄膜的剩余极化强度增加到108μC/cm~2,显著高于La、Mn、Zn等元素单掺杂得到的极化强度(69.47μC/cm2)。同时,掺杂BiFeO_3薄膜的磁化强度比纯BiFeO_3薄膜提高了3～4倍。这可能是源于:掺杂离子抑制Bi3+的挥发和Fe3+的还原,减小氧空位和缺陷浓度,从而减小漏电流,进一步改善BiFeO_3薄膜的铁电性能;掺杂离子也会导致结构的畸变而打破其螺旋磁结构,从而产生较强的室温磁性。本文首先简单介绍了BiFeO_3材料的结构及其掺杂元素的种类,然后讨论了A位、B位和AB位共掺杂离子对提高BiFeO_3薄膜弱的室温磁性以及减小漏电流、提高铁电性产生的影响,并进一步分析了产生影响的原因,最后提出了未来研究工作的方向。     

钛酸铋铁电薄膜的改性及其复合薄膜的磁电耦合效应研究

对铋层状钙钛矿结构钛酸铋无铅铁电薄膜的制备、改性、失效行为,以及钛酸铋-铁酸钴多铁复合材料进行了研究。通过分析A位和B位离子的极化率和半径关系,设计了一系列复合掺杂钛酸铋基无铅铋层状钙钛矿铁电体系,并制备出了一系列性能优良的单掺杂和复合掺杂的钛酸铋基无铅铁电薄膜,结果表明A位La~(3 )、Nd~(3 )、Eu~(3 )掺杂,B位V~(5 )、Zr~(4 )、Mn~(4 )都可以明显提高薄膜的性能,特别是A位Nd~(3 )掺杂可以很好地提高剩余极化强度和抗疲劳性,B位Mn~(4 )掺杂可以降低矫顽场和漏电电流,有望突破无铅铁电薄膜的应用瓶颈;同时,研究了铁电薄膜及其在辐照条件下的疲劳、保持性能损失和印记失效等失效行为,提出了一个能合理解释印记失效的双界面层的理论模型;还利用化学溶液沉积法制备了不同复合结构(颗粒复合和层状复合)的Nd掺杂钛酸铋/铁酸钴多铁薄膜,并研究了不同复合结构中的磁电耦合效应,研究发现在铁电/铁磁多铁复合材料中,铁电/铁磁界面处两种不同材料之间的原子相互耦合对磁电耦合效应具有很大的贡献。     

铁酸铋薄膜的电学特性及掺杂影响分析

利用溶胶-凝胶法在氧化铟锡(ITO)衬底上制备了多铁性铁酸铋薄膜,发现这种方法制备的薄膜具有典型的钙钛矿晶体结构,并表现出较好的介电性能。薄膜的漏电和铁电性能测试表明由于漏电流比较大,薄膜在室温下表现出不饱和的电滞回线,难以精确获得剩余极化强度值。而在外加扫描电压或电流时,薄膜均呈现出明显的阻变效应,可应用于阻变信息存储。最后,分析讨论了元素掺杂对薄膜铁电性能和阻变效应的影响。     

多铁性复合薄膜的结构及2-2 型双层复合磁电薄膜的制备方法

多铁性材料是一种新型材料,即一种材料同时具备铁电性、铁磁性和磁电耦和性。多铁性材料已成为当前国际上一个新的研究热点。介绍了多铁性复合薄膜的结构、2-2型多铁性复合薄膜的制备方法以及制备B iFeO3-Fe双层多铁性薄膜的最佳生长条件。     

六角铁氧体的多铁性

单相多铁性材料一般指铁电性和铁磁性共存的一类特殊功能材料.铁电性和铁磁性间的相互耦合导致外加磁场能影响其铁电序或者外加电场能影响其磁有序.这种磁电耦合效应使得诸多新型功能器件的实用成为可能,例如超敏传感器、电写磁读技术、高密度四态存储,低能耗逻辑器件等.按铁电性起源可以将单相多铁性材料划分为第一类和第二类多铁性材料:第一类多铁性材料中,铁电序和磁有序的物理起源相互独立;而第二类多铁性材料的铁电序来源于其特殊的磁有序结构.六角铁氧体作为第二类多铁性材料中的典型代表,近十年来得到广泛而深入的研究.归纳了近期六角铁氧体多铁性研究的主要进展,介绍了其多铁性起源、磁电耦合机制以及功能器件应用方面的科研成果,并对六角铁氧体多铁性的研究进行了展望.     

弛豫多铁性材料研究进展

多铁性材料同时具有多种铁性(铁电性、铁磁性或铁弹性)的有序, 可实现电磁信号的相互控制, 成为近年来研究热点。在具有成分无序的复杂体系中, 长程铁性有序有可能被打破, 材料将表现出弛豫特性。我们将至少存在一种铁性弛豫特性的多铁性材料称之为弛豫多铁性材料。这类多铁性材料的极化强度(或磁化强度)在外加电场(或外加磁场)作用下响应更加灵敏, 其磁电耦合机制与长程有序的多铁性材料不同。本文结合国内外最新研究成果, 首先介绍了和弛豫铁性有序相关的物理概念, 重点阐述了多铁性材料在铁电和铁磁双弛豫态下的磁电耦合机制; 然后, 详细介绍了钙钛矿结构(包括PbB₁B₂O₃基和BiFeO₃基材料)和非钙钛矿结构(包括层状Bi结构和非正常铁电体)弛豫多铁性材料的研究进展; 最后, 对该领域亟待解决的问题进行了展望。     

水热法制备稀土掺杂铁酸铋粉末及其性能研究

采用水热法制备了不同稀土元素(La、Ce、Nd)A位掺杂铁酸铋粉末。X射线衍射表明该方法制备的铁酸铋粉末为R3c三方晶相结构;扫描电子显微镜图像表明,稀土元素掺杂对铁酸铋粉末形貌影响较大,不同稀土元素掺杂的粉末呈现不同的颗粒形貌和尺寸;光电子能谱扫描分析证实了掺杂稀土的存在,得到了Fe2+的含量;紫外-可见光吸收光谱显示,掺杂铁酸铋粉末在蓝绿光区(400~550nm)有着更强吸收,稀土掺杂使其吸收边蓝移;磁滞回线结果表明A位稀土元素(La、Ce、Nd)掺杂使得铁酸铋粉末铁磁性得到明显增强。     

六角铁氧体磁电特性的研究进展

多铁性是指材料同时具有铁电性和铁磁性,研究表明铁电性能由复杂的磁序列来诱导产生,但常常在较低的温度下和较高的磁场下发生(0.1T),最近发现的六角铁氧体在室温弱场下(0.01T)表现出了磁电耦合现象,在新型器件的应用方面具有潜在价值。     

铁电薄膜光伏效应及形成机制研究进展

铁电薄膜具有反常光生伏打效应,且光伏特性可以通过电场进行调控,在铁电光伏电池、光驱动器、光传感器等方面具有广阔的应用前景.本文对锆钛酸铅、锆钛酸镧铅等铁电薄膜及铁酸铋多铁薄膜的铁电性与光伏特性的关系、界面效应、尺度效应、空间电荷效应等方面及铁电光伏形成机制进行了归纳和分析.今后的研究重点将集中在铁电薄膜电畴驱动光伏形成机制和光伏特性的电、磁场调控机制上.     

掺杂氧化铪基薄膜铁电性能的研究进展

铁电薄膜的研究多集中于钙钛矿结构材料,然而,这些传统的铁电材料存在与硅Si兼容性差、含铅而污染环境、物理厚度大、电阻低、带隙小等问题。不同的掺杂剂,如Si、Zr、Al、Y、Gd、Sr和La可以在HfO₂薄膜中诱导铁电或反铁电性,使其剩余极化率达到45μC·cm^-2,矫顽力(1～2 MV·cm^-1)比传统铁电薄膜大约1个数量级。同时,HfO₂薄膜厚度可以非常薄(低于10 nm),并具有很大的带隙(约5 eV)。这些优于传统铁电材料的特质可以克服包括铁电场效应晶体管和三维电容传统铁电材料等在薄膜存储器应用中的障碍。除此之外,反铁电薄膜的热电耦合性将有望用于能量收集、存储、固态冷却和红外传感器等多种应用中。HfO₂掺杂薄膜可以通过不同的沉积技术如ALD、溅射和CSD来制备,其中ALD技术沉积的薄膜优势更加明显。本文综述了近年来掺杂HfO₂薄膜材料铁电性和反铁电性的研究进展,详细介绍了不同掺杂元素、薄膜厚度、晶粒尺寸、电极、退火及应力等对薄膜铁电性的影响。     

β-NaFeO2, a new room-temperature multiferroic material

‘Multiferroic’ materials possessing simultaneously magnetic and ferroelectric orders are scarce and most of them order at low temperatures. So far, bismuth ferrite, BiFeO₃, is the only reliable room-temperature multiferroic: it is ferroelectric and antiferromagnetic. The absence of a net magnetisation in this compound is a problem when one wants to use the magneto-electric effect to address magnetic information with an electric field for potential applications in spintronic devices. We show here that β-NaFeO₂ is also a multiferroic material at room-temperature but with the most interesting extra property of showing weak ferromagnetism. This makes it a potentially very promising material for applications and a model compound for fundamental studies of the interaction between ferroelectricity and magnetism.     

Development of Bismuth Ferrite as a Piezoelectric and Multiferroic Material by Cobalt Substitution

Bismuth ferrite (BiFeO₃) is the most widely studied multiferroic material with robust ferroelectricity and antiferromagnetic ordering at room temperature. One of the possible device applications of this material is one that utilizes the ferroelectric/piezoelectric property itself such as ferroelectric memory components, actuators, and so on. Other applications are more challenging and make full use of its multiferroic property to realize novel spintronics and magnetic memory devices, which can be addressed electrically as well as magnetically. This progress report summarizes the recent attempt to control the piezoelectric and magnetic properties of BiFeO₃ by cobalt substitution.     

MnFe2O4纳米薄膜的反常磁性研究

采用磁控溅射法制备MnFe2O4薄膜,利XPS、XRD和MPMS分别对薄膜的成分、结构和磁性进行研究,结果表明,温度导MnFe2O4薄膜中金属离子在A位和B位的分布发生变化,从而导致A亚晶格与B亚晶格磁矩随温度变化规律不同,造成MnFe2O4薄膜出现反常的热磁曲线 负剩磁现象与锰铁氧体的亚晶格磁各向异性与磁场导致的磁矩翻转之间的竞争效应有关。     

Experimental evidence of enhanced ferroelectricity in Ca doped BiFeO3

Calcium (Ca)-doped bismuth ferrite (BiFeO₃) thin films prepared by using the polymeric precursor method (PPM) were characterized by X-ray diffraction (XRD), field emission gun scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM), polarization and piezoelectric measurements. Structural studies by XRD and TEM reveal the co-existence of distorted rhombohedral and tetragonal phases in the highest doped BiFeO₃ where enhanced ferroelectric and piezoelectric properties are produced by internal strain. Resistive switching is observed in BFO and Ca-doped BFO which are affected by the barrier contact and work function of multiferroic materials and Pt electrodes. A high coercive field in the hysteresis loop is observed for the BiFeO₃ film. Piezoelectric properties are improved in the highest Ca-doped sample due to changes in the crystal structure of BFO for a primitive cubic perovskite lattice with four-fold symmetry and a large tetragonal distortion within the crystal domain. This observation introduces magnetoelectronics at room temperature by combining electronic conduction with electric and magnetic degrees of freedom which are already present in the multiferroic BiFeO₃.     

One-dimensional multiferroic bismuth ferrite fibers obtained by electrospinning techniques

We report the fabrication of novel multiferroic nanostructured bismuth ferrite (BiFeO(3)) fibers using the sol-gel based electrospinning technique. Phase pure BiFeO(3) fibers were prepared by thermally annealing the electrospun BiFeO(3)/polyvinylpyrrolidone composite fibers in air for 1 h at 600?°C. The x-ray diffraction pattern of the fibers (BiFeO(3)) obtained showed that their crystalline structures were rhombohedral perovskite structures. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images revealed that the BiFeO(3) fibers were composed of fine grained microstructures. The grains were self-assembled and self-organized to yield dense and continuous fibrous structures. The magnetic hysteresis loops of these nanostructured fibers displayed the expected ferromagnetic behavior, whereby a coercivity of ～ 250 Oe and a saturation magnetization of ～ 1.34 emu g(-1) were obtained. The ferroelectricity and ferroelectric domain structures of the fibers were confirmed using piezoresponse force microscopy (PFM). The piezoelectric hysteresis loops and polar domain switching behavior of the fibers were examined. Such multiferroic fibers are significant for electroactive applications and nano-scale devices.     

层状镍基锂离子电池正极材料的改性研究

详细阐述了国内外关于层状LiNiO2正极材料的改性研究进展,并对体相掺杂和表面包覆改性层状LiNiO2正极材料电化学性能提高的机理进行了讨论,最后对层状LiNiO2正极材料今后的发展方向进行了展望。     

Magnetoelectrically Driven Catalytic Degradation of Organics

Here, the catalytic degradation of organic compounds is reported by exploiting the magnetoelectric nature of cobalt ferrite–bismuth ferrite (CFO–BFO) core–shell nanoparticles. The combination of magnetostrictive CFO with multiferroic BFO gives rise to a magnetoelectric engine that purifies water under wireless magnetic fields via advanced oxidation processes, without involvement of any sacrificial molecules or cocatalysts. Magnetostrictive CoFe₂O₄ nanoparticles are fabricated using hydrothermal synthesis, followed by sol–gel synthesis to create the multiferroic BiFeO₃ shell. Theoretical modeling is performed to study the magnetic‐field‐induced polarization on the surface of magnetoelectric nanoparticles. The results obtained from these simulations are consistent with experimental findings of the piezoforce microscopy analysis, where changes in piezoresponse of the nanoparticles under magnetic fields are observed. Next, the magnetoelectric‐effect‐induced catalytic degradation of organic pollutants is investigated under AC magnetic fields, and 97% removal efficiency for synthetic dyes and over 85% removal efficiency for routinely used pharmaceuticals are obtained. Additionally, trapping experiments are performed to elucidate the mechanism behind the magnetic‐field‐induced catalytic degradation of organic pollutants by using scavengers for each of the reactive species. The results indicate that hydroxyl and superoxide radicals are the main reactive species in the magnetoelectrically induced catalytic degradation of organic compounds.     

From Magnetite to Cobalt Ferrite

We synthesized Fe_3–x Co _x O₄ (x = 0–1) using the hydrothermal method in order to demonstrate the compositional modulation of magnetite to cobalt ferrite. Our Mössbauer spectroscopy results provided direct evidence for the presence of the Co substitution in the B sublattice, which was found to be accompanied by a systematic increase of the hyperfine magnetic field at these sites. The mechanism we propose relies on the substitution of Fe²⁺ by Co²⁺ in the B sublattice and is supported by the observed dependence of the populations of the (A) and (B) sites on content x of cobalt substitution. The X-ray diffraction (XRD) determinations demonstrated a linear increase in the lattice parameter when going from magnetite to cobalt ferrite. For the particular value x = 0.1, we report that the two sublattices of magnetite become equally populated with Fe. For this particular value of the cobalt content, we obtained a thin film sample by laser ablation deposition and characterized its properties by XRD and conversion electron Mössbauer spectroscopy (CEMS).     

Electric-field control of local ferromagnetism using a magnetoelectric multiferroic

Multiferroics are of interest for memory and logic device applications, as the coupling between ferroelectric and magnetic properties enables the dynamic interaction between these order parameters. Here, we report an approach to control and switch local ferromagnetism with an electric field using multiferroics. We use two types of electromagnetic coupling phenomenon that are manifested in heterostructures consisting of a ferromagnet in intimate contact with the multiferroic BiFeO(3). The first is an internal, magnetoelectric coupling between antiferromagnetism and ferroelectricity in the BiFeO(3) film that leads to electric-field control of the antiferromagnetic order. The second is based on exchange interactions at the interface between a ferromagnet (Co(0.9)Fe(0.1)) and the antiferromagnet. We have discovered a one-to-one mapping of the ferroelectric and ferromagnetic domains, mediated by the colinear coupling between the magnetization in the ferromagnet and the projection of the antiferromagnetic order in the multiferroic. Our preliminary experiments reveal the possibility to locally control ferromagnetism with an electric field.     

Improved magnetic and piezoresponse behavior of cobalt substituted BiFeO3 thin film

Piezoresponse force microscopy (PFM) technique has been utilized to characterize the nanoscale electromechanical properties of chemical solution deposited pristine (BiFeO₃, BFO) and 5% Cobalt-substituted bismuth ferrite (BiFe_0.95Co_0.05O_3, BFCO) thin films. Distinct PFM contrast was observed on the films' surface both in out-of-plane and in-plane modes. Piezoresponse signal was larger in BFCO film than BFO and the former film also exhibited negative self polarization. The calculated self polarization factor was − 0.94. Lateral signal (in-plane) changed its sign upon 180° sample rotation which ruled out spurious electrostatic contribution and confirmed piezoelectric nature of the effect. Square patterns were written by local poling and reversible nature of the piezoresponse behavior was established. Well saturated piezoelectric hysteresis loops were acquired with improved piezoelectric coefficient value in case of Co-substituted BFO film. The effect has been described on the basis of polarization rotation with doping. Magnetization of BFO film also increases with Co-substitution. The enhancement in magnetic and ferro/piezoelectric properties should be useful for multiferroic applications.