金属材料外文翻译--FeBaTio3异质外延生长【中文3900字】【PDF+中文WORD】
金属材料外文翻译-FeBaTio3异质外延生长【中文3900字】【PDF+中文WORD】,中文3900字,PDF+中文WORD,金属材料,外文,翻译,FeBaTio3,外延,生长,中文,3900,PDF,WORD
【中文3900字】 Fe/BaTio3异质外延生长 摘要涉及外延异质结构铁电(FE)和铁磁(FM)材料是为实现利用庞大的磁电效应的设备一个可能路线。在这论文我们将演示钛酸钡铁的外延生长(001),因为这代表一个典型的例子。众所周知铁电和铁磁材料与BCC和钙钛矿结构材料居里温度高于300 K。Fe生长在钛酸钡45旋转的立方晶格中,为了减少晶格不匹配。Ba和Fe原子的相互扩散区别,通过X-射线光电子能谱表明,在发生相当大的铁氧化界面层厚度小于3 nm。 1 .介绍 磁电( ME)耦合最近成为非常热门的话题在自旋电子学和氧化电子领域 ,因为它可以促进这样新的技术和设备概念的发展1。在控制磁特性的特定的材料施加电场将使克服实际自旋电子器件的限制,以取代当前行磁信息的电子信息。有一个相关的例子例如磁性的隧道结(MTJ),都是非挥发性的磁性随机存取存储( NVMRAMs )应用,包括磁阻装置。在磁存储的角度来看,这是由于磁阻的电读行为,也可以开发一个合适的磁电效应。对许多不同的路线进行了探讨,in the search for a magnetoelectric effect: single phase multiferroic在一个磁电效应搜索:单相多铁性materials, displaying magnetic and electric polarization at the sametime资料显示 , 在同一时间显示磁场和电场极化 ,取得了铁电(FE)的三维复合材料和铁磁材料2,3,二维异质结构和接口4。后者是特别有吸引力,它是根据著名的铁and FM properties of materials while magnetoelectric coupling is电材料的调频特性,为了最大化的影响磁电耦合,confined at the interface which can be controlled and engineered in限制了在可控制与接口设计order tomaximize the effect. In this context epitaxial interfaces between。在这种背景下外延接口FM和FE材料,如铁和钛酸钡(BTO),非常吸引人。事实上这些材料的整体性质符合基本的应用程序,即坚固性,稳定性和实时操作性。此外,Fe/ BTO界面进行了预测,显示相当大的磁电效应。第一个段等人工作5。报告关于一个ME机构组成的界面的Fe的磁化变化,由于改造的Fe和Ti之间的配比,从在中心位置连接到Ti离子的位移外部电场的反向的铁电极化得到应用。提出了另一种机制蔡等人6基于界面的FM层的BTO铁电极化自旋相关的筛选层。在这两种情况下应该改变磁化的模量,电场具有相反的应用极性。该界面电场的变化各向异性的铁层的影响也已在参考文献中介绍7,他们发现,该电场凭借修改磁晶各向异性能修正Ti和Fe原子之间的界面配比。磁电效应实验中,已经有很多人在FM薄膜与Fe单晶基体之间的界面的报道,但基本上是归因于Fe的形成范畴。不过,他们需要高电压和FE范围的有限尺寸对小型化的器件是一个明显的障碍。另一方面,只有少数的结果是报道了Fe膜界面 。最近Zhang等人8报道了一种用于控制生长在镍铁层的磁化在BTO薄膜的几十伏的应用。 本文的目的是展示外延生长在Fe/SrTiO3(001)(STO)基底上,这是一个基本的BTO接口鉴于ME耦合在两种材料之间。 2 .实验细节 脉冲激光沉积法(PLD)和分子束外延已被用于Fe/ BTO/ STO异质结构的生长, 在众多工具中,其中两个沉积技术是可在文献9介绍。 BTO薄膜在STO和Nb:SrTiO3基底生长。在一个特定的PLD基底上,生长的parameters for BTO have been optimized in order to obtain high参数的BTO已被优化,以获得高quality epitaxial films with low surface roughness.表面粗糙度低质量的外延薄膜。在整个沉积过程中的氧的压力一直保持在2.67Pa(0.02毫米汞柱)。生长之前,基底的退火达1003 K,温度由一个高温计控制。一个四倍Q开关Nd:YAG激光(波长266 nm ),提供了脉冲的7nm长玉5.6 J/cm2的能量密度,一直在重复频率运行2Hz,等离子体放置在前面基板在30毫米的距离。在这些淀积速率条件是0.24 A /脉冲,从RHEED振荡推导通过X光对不同的BTO薄膜厚度测量证实反射( XRR )采用日本理学ROTAFLEX RU- 200B衍射仪。沉积之后,退火在1个大气压下在873 K的才能被执行,以提高表面结构质量。通过X-射线衍射(XRD )使用铜的K -辐射对晶体结构进行了研究。-2扫描使用西门子D500衍射反射进行了测量,得到面外和面内的晶格参数,从位置计算出衍射峰。面内晶格参数,确定从倒易空间映射(RSMs)围绕STO(103)和BTO(103)不对称的反射,用Rigaku D8推进区域检测器测定。铁膜已生长分子束外延( MBE )在超高真空条件下与基材保持在373 K在沉积过程压力中总是在10-10毫米汞柱范围内。该沉积速率对Fe校准用石英微量天平测量,然后通过X-射线光电子能谱(XPS)进行检查,这也是用于调查的界面化学。铝K- a和镁K-a被用于的化学物种的浓度进行定量评价和线形分析。磁性表has been performed ex-situ with a standard Magneto Optical Kerr Effect (MOKE) setup operating in longitudinal configuration, as described in Ref 10.已与一个标准的磁光克尔效应(MOKE)进行易地安装在纵向配置操作,如在文献10中描述。3 .结果与讨论 这项工作的第一步是PLD的优化生长一直在STO (001)基体上的BTO薄膜。两种材料之间晶格错配是相当高(2.3),因此BTOfilms grow on STO under in-plane compressive strain in case of cube on-cube epitaxial growth, with an elongation in the out of plane生长在STO薄膜的立方体上外延生长立方的情况下,面内压缩应变在平面外的一种延伸direction.方向。这种结构关系修复了BTO四边形中的c轴的方向垂直于膜的平面,从而迫使铁电极保持垂直于BTO表面,即along the natural direction for the application of an electric field in the typical structure of a vertical capacitor.沿自然方向的电场在一个垂直的电容器的典型结构中的应用。这应导致BTO的电极化的极化增强等。11和Neaton等 12 。在图1的STO上生长17 nm厚的BTO薄膜(001)XRD分析a 17 nm thick BTO film grown on STO(001) are shown. In panel (a) a theta-2theta scan is shown where only the (001) and (002) reflection所示。在面板上(一)个-2扫描显示只有(001)和(002)反射from the BTO film and the cubic STO substrate are visible.从BTO薄膜和立方STO基体上是可见的。这表明BTO增长完全取向STO的C轴,而不检测虚假阶段,也不是一个面向域。基片和图中所示的膜是不对称(103)反射。 1B表明,膜和基板的面内的参数是等效的。指示一个完美的外延,参数of BTO from the bulk value of 0.3995 nm to the value of the STO lattice parameter (0.3905 nm).从0.3995 nm的体积值的BTO压缩STO晶格参数的值(0.3905 nm)。相反,平面外参数是0.420.005 nm13 ,即大于C参数BTO散装值(0.403 nm)如预期的平面应变 14 。初步的C-V和庞德(正负下)的测量结果表明滞后的平面组件,polarization in our BTO films (data not shown) thus indicating the FE nature of our films.在我们的BTO薄膜极化(数据未显示)表明我们的薄膜的铁电性质。另外,在面板(c)中显示RHEED表明,薄膜的结晶质量会保留到表面,这是为了获得一个外延FE / FM接口基本点。细长的条痕的存在清楚的表示降低粗糙度,由AFM分析,给出了一个55平方微米面积的0.2纳米的RMS值。注意的是,表面质量是独立于膜厚达300nm时,作为检查通过RHEED和AFM (数据未显示),而晶格参数变更为弛豫应变的结果。当薄膜厚度增加到保持完全的c轴取向,而无错误阶段,但他们往往逐步降慢应变。事实上,在图2我们总结的演变晶格参数为BTO薄膜厚度的函数。开始从顶部,该C /A比,晶胞参数和晶胞体积V1 / 3显示出。图表中的红线,而是代表了相应数量的批量BTO晶体的值。薄膜是外延和完全匹配的基板:该参数与STO基板一致,而c参数是相应增加。这形成了一个增强四方性,由数量的C / A表示,这是大部分有价值薄膜厚度在微米范围(数据未显示)。晶胞体积,而是保持不变,可能是由于一些氧气的损失,对于所有的研究厚度和价值略大。注意,这些值很好地与以往赵等人15 和Suzuki 16 等人报告的数据相对一致。高品质的钛酸钡的表面是一个很好的增长外延的Fe / BTO接口起点。这个Fe膜在1-3nm范围内已用MBE在BTO / STO (001)生长 ,根据上述配方。 Fe膜出现了良好外延生长相对于所述BTO晶格立方结构呈现出45度的旋转。如图3所示了2nm的Fe膜一个LEED模式下50v拍摄,其中从我们的LEED设备的校准产生在单晶的Fe (001)薄膜,所述偏离中心的电子能量点对应于第一阶衍射峰来自Fe晶体的晶格。由转动的(10)和(-10)斑点45 ,对于所述STO基底,正好与100 BTO的方向一致,从而使铁和BTO之间的外延关系为Fe 100 / / BTO 110 。图4表示在Fe薄膜标称厚度为2nm(如校准与石英微量天平)的XPS分析。铁2P , 2P钛和Ba 4D峰在不同的收集角强度(0,30 ,45和60)相对于表面法线已记录。Fe膜的二维生长的情况与厚度t铁峰强度和钛,钡峰强度应该遵循的趋势中的一个相同,集合的角度分别由下式给出IFE ( 1 - EXP ( -t/Fecos ) )和ITI ,Ba EXP ( -t/Ti ,巴COS ) 17 ,其中Fe ,钛,钡是电子逸出深度。如图4示出了2P铁,钛2P或Ba4d峰的实验强度为的1/cos (黑色方块,红色圆圈和蓝色三角形函数分别),最适合功能上述报告(连续线)。通过采用逸出深度的值根据公式计算 = 0.54 KE 18 我们发现,钛,钡,铁强度的得出一个厚度值t = 2.2nm0.4nm,与所述估算好的设定用于校准积液细胞石英微量天平。这清楚地表明,2 nm厚的铁片在BTO是二维的和连续的,通过电气测量,也证实了钛,钡和铁之间的相互扩散有限。界面形成过程中铁的氧化态是很敏感的,通过接口形成的氧化可以调查铁线性2P峰。图4显示一个Fe 2p的光谱对BTO生长2 nm厚的Fe膜的一个典型的反褶积。金属铁提供的2P3/2和2P1/2峰的特征双峰分别在719.9 eV和706.7 eV 19 ,而该铁氧化物的形成的双峰的需要特定较高的结合能存在:由3.2eV和2.9eV的为2P 3/2和2P 1/2的FeO ( Fe2 +的),4.3 eV和以Fe2O3 ( Fe3 +的)的情况下, 4.4伏特的情况下。图的铁2P反褶积。图4示出一个额外的双重峰与在709.5 eV和723.2 eV的,即相干带的存在额外的双重峰与峰氧化铁原子2 +氧化态。角分辨分析表明:增加时的收集角和相应的表面灵敏度,氧化和金属原子的信号之间的比率没有相当大的变化(0.3),从而这表明氧化并不限于非常接口。在另一方面,增加Fe的厚度为3nm时,在零度角收集的比值减0.2:这表明存在界面氧化层,其厚度大于3nm的下部。该铁覆盖层的磁性能已核对MOKE 。在图3对应于1nm和Kerr磁滞回线2nm厚的铁片(分别黑色和红色曲线)的报告。较薄的膜具有一个非常小的矫顽场(1奥斯特)显示一个方形环路,而对于2纳米厚度的矫顽场变为6.5奥斯特, 其余基本上是正方形的线圈的形状。注意,这些可与薄外延铁层相比的矫顽场, 在MgO,代表了高质量的外延Fe薄膜的生长参考基板。类似于的Fe / MgO膜,在我们低活化能磁化翻转的情况下基底特征为通过畴壁传播与缺陷数量减少(钉扎点)。 4.结论 在本文中,我们用PLD和MBE的结合使用成长(001)异质结构,展示了铁/ BTO/ STO的外延生长。铁生长用45旋转的立方晶格在BTO,而立方体上的BTO对STO立方外延生长在平面上观察是压缩应变。 X射线衍射,反射高能电子衍射和LEED数据表示Fe和BTO层的结晶度高。无阳离子的(Ti,Ba)和铁相互扩散已被检测到,而一个相当大的界面Fe原子发生氧化或多或少内均匀的厚度小于3nm。初步的铁电特性显示出残余平面介质极化BTO的,而Fe膜的磁行为是典型的高品质单晶薄膜。对在Fe / BTO接口的磁电耦合的研究这些结果,代表一个基本步骤的成功。 致谢作者深表感激马可里昂熟练技术援助。这项工作是由基金会Cariplo viathe项目集市(项目号2007.5095)。财政支持的西班牙政府(项目:mat2008 - 06761 c03 Nanoselectcsd2007 - 00041)。Epitaxial growth of Fe/BaTiO3 heterostructuresS.Brivioa,C.Rinaldia,D.Pettia,R.Bertaccoa,F.SanchezbaLNESS,Dipartimento di Fisica,Politecnico di Milano,via Anzani 42,22100,Como,ItalybInstitut de Cincia de Materials de Barcelona,Consejo Superior de Investigaciones Cientficas,Campus Universitat Autnoma de Barcelona,Bellaterra 08193,Catalunya,Spaina b s t r a c ta r t i c l ei n f oAvailable online xxxxThe realization of epitaxial heterostructures involving ferroelectric(FE)and ferromagnetic(FM)materials isone of the possible routes towards the realization of devices exploiting sizable magnetoelectric effects.In thispaper we demonstrate the epitaxial growth of Fe on BaTiO3(001)as this system represents a prototypicalexample of interface between well known FE and FM materials with bcc and perovskite structure respectively,both with Curie temperature well above 300 K.Fe grows on BaTiO3with 45 rotation of its cubic lattice withrespect to that of the substrate in order to reduce the lattice mismatch.Negligible interdiffusion of Ba and Tications or Fe atoms is found by X-ray photoemission spectroscopy,while a sizable Fe oxidation occurs withinan interfacial layer with thicknesses thinner than 3 nm.2010 Published by Elsevier B.V.1.IntroductionMagnetoelectric(ME)couplinghasrecentlybecomeaveryhottopicin the field of spintronics and oxide electronics 1,since it can pave theway to the development of new technologies and device concepts.Inparticular the possibility of controlling the magnetic properties of amaterial with an electric field would allow overcoming the actuallimitations of spintronic devices,by replacing current lines withelectrical writing of magnetic information.A relevant example of itsapplication are non volatile magnetic random access memories(NVMRAMs),consisting of magnetoresistive devices,such as magnetictunnelling junctions(MTJs).In perspective the magnetically storedinformation,which is electrically read thanks to the magnetoresistivebehaviour,could be also electrically written exploiting a suitablemagnetoelectriceffect.Anumberofdifferentrouteshavebeenexploredin the search for a magnetoelectric effect:single phase multiferroicmaterials,displayingmagneticandelectricpolarizationatthesametime2,3,3D composite materials made of a ferroelectric(FE)andferromagnetic(FM)materials,2D heterostructures and interfaces 4.The latter is particularly appealingbecause it is basedonwell knownFEand FM properties of materials while magnetoelectric coupling isconfined at the interface which can be controlled and engineered inordertomaximizetheeffect.Inthiscontextepitaxialinterfacesbetweenwell known FM and FE materials,like Fe and BaTiO3(BTO),are veryappealing.In fact the bulk properties of these materials fulfil the basicrequirements of applications,i.e.robustness,stability and RT operation.Furthermore the Fe/BTO interface has been predicted to display sizablemagnetoelectric effects.The first work,by Duan et al.5,reports on aME mechanism consisting of the variation of the interfacial Femagnetization,due to alteration of the hybridization between Fe andTi,connected to the displacement of Ti ions from the central position inthe octahedron after the application of external electric fields whichreverse the ferroelectric polarization.Another mechanism was pro-posed by Cai et al.6 and Duan et al.7 based on the spin dependentscreening of the ferroelectric polarization of BTO in the interfacial FMlayer.In both cases the application of an electric field with oppositepolarity is supposed to change the modulus of the magnetization.TheeffectofinterfacialelectricfieldsonmagneticanisotropiesintheFelayerhas been also investigated in Ref.7;they found that the electric fieldmodifies the magnetocrystalline anisotropy energy by virtue of changeof interfacial hybridization between Ti and Fe atoms.Experimentally,lots of magnetoelectric effects have been reported at the interfacebetween a FM film and a FE crystal substrate,but they are essentiallyascribed to FE domain formation.However they require high voltagesand the finite dimension of FE domains is an evident obstacle towardsthe miniaturization of devices.On the other hand,only few results arereportedaboutinterfacesmadeof FEfilms.Veryrecently Zhangetal.8reported a way for controlling the magnetization of a NiFe layer grownon BTO films with the application of tens of volts.The aim of this paper is to demonstrate the growth of epitaxial Fe/BTO interfaces on SrTiO3(001)(STO)substrates,which is a funda-mental achievement in view of the investigation of ME couplingbetween the two materials.2.Experimental detailsPulsed laser deposition technique(PLD)and molecular beamepitaxy have been used for the growth of Fe/BTO/STO heterostruc-tures in a cluster tool where the two deposition techniques areavailable in-situ 9.BTO films have been grown on STO and Nb:SrTiO3Thin Solid Films xxx(2011)xxxxxx Corresponding author.E-mail address:(S.Brivio).TSF-28473;No of Pages 40040-6090/$see front matter 2010 Published by Elsevier B.V.doi:10.1016/j.tsf.2010.12.193Contents lists available at ScienceDirectThin Solid Filmsjournal homepage: cite this article as:S.Brivio,et al.,Thin Solid Films(2011),doi:10.1016/j.tsf.2010.12.193(Nb:STO)substrates by PLD in a dedicated chamber.The growthparameters for BTO have been optimized in order to obtain highquality epitaxial films with low surface roughness.During the wholedeposition process the oxygen pressure has been kept at 2.67 Pa(0.02 torr).Before the growth,an annealing of the substrate up to1003 K is performed for cleaning and ordering the surface,thetemperature being controlled by a pyrometer.A quadrupled Q-SwitchedNd:YAG laser(266 nm),providing pulses an 7 ns longwith afluence of 5.6 J/cm2,has been operated at a repetition frequency of2 Hz to generate a plasma from a stoichiometric target placed in frontof the substrate at a distance of 30 mm.The deposition rate in theseconditions is 0.24/pulse,as deduced from RHEED oscillations andconfirmed by thickness measurements on different BTO films by X-Ray Reflectometry(XRR)using a Rigaku Rotaflex RU-200B diffrac-tometer.After deposition,a post-annealing in 1 atm of oxygen at873 K has been performed in order to improve the surface structuralquality.The crystal structure was studied by X-ray diffraction(XRD)using Cu K-alfa radiation.Theta-2theta scans around symmetricalreflections have been measured using a Siemens D500 diffractometer,and out-of-plane lattice parameters were calculated from the positionof the diffraction peaks.In-plane lattice parameters were determinedfrom reciprocal space maps(RSMs)around STO(103)and BTO(103)asymmetrical reflections,measured with a Rigaku D8 Advance areadetector.Fe films have been grown by Molecular Beam Epitaxy(MBE)in UHV conditions with the substrate kept at 373 K.The pressureduring the deposition process was always in the 1010torr range.Thedeposition rate for Fe was calibrated with a quartz microbalance andthen checked by X-ray photoemission spectroscopy(XPS)performedin-situ,which was also employed for investigating the interfacialchemistry.Al K-alpha and Mg K-alpha lines were employed forquantitative evaluation of the concentration of the chemical speciesand lineshape analysis,respectively.The magnetic characterizationhas been performed ex-situ with a standard Magneto Optical KerrEffect(MOKE)setup operating in longitudinal configuration,asdescribed in Ref 10.3.Results and discussionThe first step of this work has been the optimization of the PLDgrowth of BTO films on STO(001)substrates.The lattice mismatchbetween the two materials is quite high(2.3%)and consequently BTOfilms grow on STO under in-plane compressive strain in case of cube-on-cube epitaxial growth,with an elongation in the out of planedirection.This structural relation fixes the c axis of the BTO tetragon inthe direction perpendicular to the plane of the film,thus forcing theferroelectric polarization to stay perpendicular to the BTO surface,i.e.along the natural direction for the application of an electric field in thetypical structure of a vertical capacitor.Incidentally this should lead toan enhancement of the electric polarization of BTO according to Choiet al.11 and Neaton et al.12.In Fig.1 the results of XRD analysis ona 17 nm thick BTO film grown on STO(001)are shown.In panel(a)atheta-2theta scan is shown where only the(001)and(002)reflectionfrom the BTO film and the cubic STO substrate are visible.Thisindicates that BTO grows completely c-oriented on STO,withoutdetectable spurious phases,nor a-oriented domains.The asymmetric(103)reflection from substrate and film shown in Fig.1b reveals thatthe in-plane parameters of film and substrate are equivalent,indicating a perfect epitaxy,with a compression of the a parameterof BTO from the bulk value of 0.3995 nm to the value of the STO latticeparameter(0.3905 nm).The out-of-plane parameter is,instead,0.420.005 nm 13,i.e.larger than the BTO bulk value for the cparameter(0.403 nm)as expected for in plane compressive strain14.Preliminary C-V and PUND(Positive Up Negative Down)measurements show a hysteretic out of plane component of thepolarization in our BTO films(data not shown)thus indicating the FEnature of our films.Furthermore the RHEED pattern shown in panel(c)demonstrates that the crystalline quality of the film is preservedup to the very surface,which is a fundamental point in order to obtainepitaxial FE/FM interfaces.The presence of elongated streaks is a clearindication of reduced roughness,as confirmed by AFM analysis whichgives a RMS value of 0.2 nm on a 55 m2area.Note that the surfacequality is independent on the film thickness up to 300 nm,as checkedby RHEED and AFM(data not shown),while the lattice parameterschange as a result of progressive strain relaxation.When the thicknessis increased the films remain completely c-oriented and withoutspurious phases but they tend to relax progressively the compressivestrain.Indeed in Fig.2 we have summarized the evolution of thelattice parameters as a function of the BTO film thickness.Startingfromthetop,thec/a ratio,thecell parametersandtheunit cell volumeV1/3are shown.The red lines in the graphs,instead,represent thevalues of the corresponding quantities for a bulk BTO crystal.Thinfilms are epitaxial and perfectly match the substrate:the a parametercoincides with that of the STO substrate while the c parameter iscorrespondingly increased.This leads to an enhancement of thetetragonality,as expressed by the quantity c/a,which is larger thanthe bulk value also for thick films up to thickness in the micron rangeFig.1.(a)2 scan of a 16 nm BTO film on STO substrate;(b)asymmetric scan aroundthe(103)reflection of the substrate;(c)typical RHEED pattern of a BTO film.2S.Brivio et al./Thin Solid Films xxx(2011)xxxxxxPlease cite this article as:S.Brivio,et al.,Thin Solid Films(2011),doi:10.1016/j.tsf.2010.12.193(data not shown).The unit cell volume,instead,stays quite constantfor all the investigated thicknesses and slightly larger with respect tothe bulk value,probably due to some oxygen losses.Note that thesevalues are nicely in agreement with the previously reported data byZhao et al.15 and Suzuki et al.16.The high quality BTO surface is a good starting point for the growthof an epitaxial Fe/BTO interface.To this scope Fe films in the range of13 nm have been grown by MBE on BTO/STO(001),according to therecipe described above.Fe films appear to grow epitaxially with a welldefined cubic structure presenting a 45 rotation with respect to theBTO lattice.Indeed the inset of Fig.3 shows a LEED pattern taken withan electron energy of 50 eV on a 2 nm Fe film,where the off-centrespot corresponds to the first order diffraction peaks coming from theFe crystal lattice,resulting from the calibration of our LEED apparatuson single crystal Fe(001)films.The(10)and(10)spots rotated by45 with respect to the side of the STO substrate,coinciding with the100 direction of BTO,so that the epitaxial relationship between Feand BTO is Fe100/BTO110.Fig.4 shows the results of the XPS analysis on a Fe film withnominal thickness(as calibrated with the quartz microbalance)of2 nm.The Fe 2p,Ti 2p and Ba 4d peak intensities have been recordedat different collection angles with respect to the surface normal (0,30,45 and 60).In the case of two dimensional growth of a Fe filmwith thickness t the Fe peak intensity and Ti,Ba peak intensitiesshould follow a trend versus the collection angle given respectively byIFe(1exp(t/Fecos)and ITi,Baexp(t/Ti,Bacos)17,whereFe,Ti,Ba are the electron escape depths.The inset of Fig.4 shows thenormalized experimental intensities of Fe 2p,Ti 2p and Ba 4d peaks asa function of 1/cos(black squares,red circles and blue trianglesrespectively)together with the best fit with the functions reportedabove(continuous lines).By employingthevalues of theescape depthcalculated according to the empirical formula=0.54 K.E.18 wefind that the fits of Ti,Ba and Fe intensities give a value for thethickness t=2.2 nm0.4 nm,in nice agreement with the estimationof the quartz microbalance used for calibrating the effusion cell.Thisclearly indicates that 2 nm thick Fe films on BTO are 2D andcontinuous,as also confirmed by electrical measurements,withlimited interdiffusion between Ti,Ba and Fe.The oxidation of Fe during the interface formation can beinvestigated looking at the lineshape of Fe 2p peaks,which is verysensitivetotheFe oxidationstate.Fig.4 showsa typicaldeconvolutionof a Fe 2p spectrum taken on a 2 nm thick Fe film grown on BTO.Metallic Fe gives the characteristic doublet with 2p3/2and 2p1/2peaksat 719.9 eV and 706.7 eV respectively 19,while the fingerprint ofiron oxide formation is the presence of doublets shifted towardshigher binding energies:by 3.2 eV and 2.9 eV for 2p 3/2 and 2p 1/2 inthe case of FeO(Fe2+),4.3 eV and 4.4 eV in the case of Fe2O3(Fe3+).The Fe 2p deconvolution of Fig.4 shows an additional doublet withpeaks at 709.5 eV and 723.2 eV,i.e.coherent with the presence ofoxidized iron atoms with prevalent 2+oxidation state.The angleresolved analysis shows no sizable variation of the ratio between thesignal from oxidized and metallic atoms(0.3)when increasing thecollection angle and correspondingly the surface sensitivity,thusindicating that oxidation is not limited to the very interface.On theother hand when increasing the Fe thickness to 3 nm,the ratio takenat zero collection angle decreases to 0.2:this suggests the presence ofan interfacial oxidized layer whose thickness is lower than 3 nm.Themagnetic properties of the Fe overlayer have been checked withMOKE.In Fig.3 the Kerr hysteresis loops corresponding to a 1 nm-and2 nm-thick Fe film(black and red curves,respectively)are reported.The thinnerfilm displays a square loop with a very small coercive fieldFig.2.c/a ratio,a and c parameters and unit cell volume V1/3as a function of the BTOthickness.The horizontal red lines indicate the values of the same quantities for bulkBTO crystals.(For interpretation of the references to color in this figure legend,thereader is referred to the web version of this article.)Fig.3.MOKE hysteresis loops for 1 nm-and 2 nm-thick Fe films(black and red,respectively)grown on BTO.The inset shows a characteristic LEED pattern of the Feoverlayers.(For interpretation of the references to color in this figure legend,the readeris referred to the web version of this article.)Fig.4.Fe 2p spectra of a 2 nm-thick Fe film on BTO(black circles).The peak isdeconvoluted into a doublet coming from metallic Fe(red thin line)and a doubletcoming from oxidized species(thick blue line).In the inset Fe 2p and Ti 2p are peakintensities as a function of 1/cos,where is the electron collection angle.(Forinterpretation of the references to color in this figure legend,the reader is referred tothe web version of this article.)3S.Brivio et al./Thin Solid Films xxx(2011)xxxxxxPlease cite this article as:S.Brivio,et al.,Thin Solid Films(2011),doi:10.1016/j.tsf.2010.12.193(1 Oe),while for 2 nm thickness the coercive fields becomes 6.5 Oe,the shape of the loops remaining essentially square.Note that thesevalues are comparable to the coercive fields of thin epitaxial Fe layerson MgO,which represents the reference substrate for the growth ofhigh quality epitaxial Fe films.Analogously to Fe/MgO films,in ourcase magnetization switching takes place via domain wall propaga-tion in a film with reduced number of defects(pinning sites)characterized by low activation energy.4.ConclusionsIn this paper we demonstrated the epitaxial growth of Fe/BTO/STO(001)heterostructures grown by combined use of PLD and MBE.Fegrows on BTO with a 45 rotation of its cubic lattice with respect tothat of BTO,while cube on cube epitaxial growth of BTO on STO within plane compressive strain is observed.XRD,RHEED and LEED dataindicate the high degree of crystallinity of the Fe and BTO layers.Nocation(Ti,Ba)and Fe interdiffusion has been detected,while a sizableoxidation of interfacial Fe atoms occurs more or less uniformly withina thickness thinner than 3 nm.Preliminary ferroelectric characteriza-tion indicates remnant out of plane dielectric polarization of BTO,while the magnetic behaviour of Fe films is typical of high qualitysingle crystal thin films.These results represent a fundamental steptowards the investigation of magnetoelectric coupling at the Fe/BTOinterface.AcknowledgementsThe authors are deeply grateful to Marco Leone for his skillfultechnical assistance.This work was funded by Fondazione Cariplo viathe project MANDIS(Project No.2007.5095).Financial support by theSpanish Government(Projects:MAT2008-06761-C03 and NanoselectCSD2007-00041)is acknowledged.References1 M.Fiebig,J.Phys.D Appl.Phys.38(2005)R123R152.2 M.Azuma,K.Takata,T.Saito,S.Ishiwata,Y.Shimakawa,M.Takano,J.Am.Chem.Soc.127(2005)8889.3 N.A.Hill,J.Phys.Chem.B 104(2000)6694.4 H.Zheng,J.Wang,S.E.Lofland,Z.Ma,L.Mohaddes-Ardabili,T.Zhao,L.Salamanca-Riba,S.R.Shinde,S.B.Ogale,F.Bai,D.Viehland,Y.Jia,D.G.Schlom,M.Wutting,A.Roytburd,R.Ramesh,Science 303(2004)661.5 C.G.Duan,S.S.Jaswal,E.Y.Tsymbal,Phys.Rev.Lett.97(2006)047201.6 T.Cai,S.Ju,J.Lee,N.Sai,A.A.Demkov,Q.Niu,Q.Niu,Z.Li,J.Shi,E.Wang,Phys.Rev.B 80(2009)1404158(R).7 C.-G.Duan,J.P.Velev,R.F.Sabirianov,W.N.Mei,S.S.Jaswal,E.Y.Tsymbal,Appl.Phys.Lett.92(2008)122905.8 Y.Zhang,J.Liu,X.H.Xiao,T.C.Peng,C.Z.Jiang,Y.H.Lin,C.W.Nan,J.Phys.D Appl.Phys.43(2010)082002.9 R.Bertacco,M.Cantoni,M.Riva,A.Tagliaferri,F.Ciccacci,Appl.Surf.Sci.252(2005)1754.10 P.Vavassori,Appl.Phys.Lett.77(2000)1605.11 K.J.Choi,M.Biegalski,Y.L.Li,A.Sharan,J.Schubert,R.Uecker,P.Reiche,Y.B.Chen,X.Q.Pan,V.Gopalan,L.-Q.Chen,D.G.Schlom,C.B.Eom Sci.305(2004)1005.12 J.B.Neaton,C.-L.Hsueh,K.M.Rabe,arXiv:cond-mat/0204511v1.13 The low intensity of the peak prevents from a precise evaluation of the out of planeparameter for ultrathin films.14 Y.S.Kim,D.H.Kim,J.D.Kim,Y.J.Chang,T.W.Noha,J.H.Kong,K.Char,Y.D.Park,S.D.Bu,J.-G.Yoon,J.-S.Chung,Appl.Phys.Lett.86(2005)102907.15 T.Zhao,F.Chen,H.Lu,G.Yang,Z.Chen,J.Appl.Phys.87(2000)7442.16 T.Suzuki,Y.Nishi,M.Fujimoto,Phi.Mag A 79(1999)2461.17 D.Petti,R.Bertacco,S.Brivio,M.Cantoni,A.Cattoni,F.Ciccacci,J.Appl.Phys.103(2008)044903.18 M.P.Seah,W.A.Dench,Surf.Interface Anal.1(1979)2.19 S.J.Roosendaal,B.van Asselen,J.W.Elsenaar,A.M.Vredenberg,F.H.P.M.Habraken,Surf.Sci.442(1999)329.4S.Brivio et al./Thin Solid Films xxx(2011)xxxxxxPlease cite this article as:S.Brivio,et al.,Thin Solid Films(2011),doi:10.1016/j.tsf.2010.12.193
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