Soft-mode hardening in SrTiO3 thin films

Understanding the behaviour of the dielectric constant in ferroelectric thin films remains a challenging problem. These ferroelectric materials have high static dielectric constants, and so are important for their applications in high-storage-density capacitor structures such as dynamic random access memory (DRAM). But the dielectric constant tends to be significantly reduced in thin films, thereby limiting the potential benefit of ferroelectrics for memory devices. Extensive studies have shown that this phenomenon could be caused by a 'dead layer' of very low dielectric constant between the ferroeletric film and the electrode. And, although very few direct measurements are in fact available, it has been recognized that the lattice dynamical properties in the thin films should also play a key role in the reduction of the dielectric constant. Here we report far-infrared ellipsometry and low-frequency dielectric measurements in SrTiO3 thin films, which demonstrate that the Lyddane-Sachs-Teller relation between the optical-phonon eigenfrequencies and the dielectric constant is fully maintained, as is the case in the bulk material. This indicates that the dramatic reduction of the dielectric constant is a consequence of a profound change of the lattice dynamical properties, in particular of the reduced softening of its lowest optical-phonon mode. Our results therefore provide a better understanding of the fundamental limitations of the dielectric constant values in ferroelectric thin films.     

铁电纳米颗粒相变中的尺寸效应

由铁电纳米颗粒和介电质基体组成的复合材料具有很多功能特性,这些特性可广泛用于新型电子和能量收集装置.然而,铁电颗粒的形状、尺寸和在介电质基体中的取向对这些功能的影响还没有被研究清楚.利用一种中尺度建模方法,模拟了具有极性、弹性和热自由度的铁电纳米复合材料的性质,以及这些性质对尺寸、形状和取向的依赖性.基于时间依赖的朗道...     

Strong polarization enhancement in asymmetric three-component ferroelectric superlattices

Theoretical predictions--motivated by recent advances in epitaxial engineering--indicate a wealth of complex behaviour arising in superlattices of perovskite-type metal oxides. These include the enhancement of polarization by strain and the possibility of asymmetric properties in three-component superlattices. Here we fabricate superlattices consisting of barium titanate (BaTiO3), strontium titanate (SrTiO3) and calcium titanate (CaTiO3) with atomic-scale control by high-pressure pulsed laser deposition on conducting, atomically flat strontium ruthenate (SrRuO3) layers. The strain in BaTiO3 layers is fully maintained as long as the BaTiO3 thickness does not exceed the combined thicknesses of the CaTiO3 and SrTiO3 layers. By preserving full strain and combining heterointerfacial couplings, we find an overall 50% enhancement of the superlattice global polarization with respect to similarly grown pure BaTiO3, despite the fact that half the layers in the superlattice are nominally non-ferroelectric. We further show that even superlattices containing only single-unit-cell layers of BaTiO3 in a paraelectric matrix remain ferroelectric. Our data reveal that the specific interface structure and local asymmetries play an unexpected role in the polarization enhancement.     

Electric polarization reversal and memory in a multiferroic material induced by magnetic fields

Ferroelectric and magnetic materials are a time-honoured subject of study and have led to some of the most important technological advances to date. Magnetism and ferroelectricity are involved with local spins and off-centre structural distortions, respectively. These two seemingly unrelated phenomena can coexist in certain unusual materials, termed multiferroics. Despite the possible coexistence of ferroelectricity and magnetism, a pronounced interplay between these properties has rarely been observed. This has prevented the realization of multiferroic devices offering such functionality. Here, we report a striking interplay between ferroelectricity and magnetism in the multiferroic TbMn2O5, demonstrated by a highly reproducible electric polarization reversal and permanent polarization imprint that are both actuated by an applied magnetic field. Our results point to new device applications such as magnetically recorded ferroelectric memory.     

Nucleation and growth mechanism of ferroelectric domain-wall motion

The motion of domain walls is critical to many applications involving ferroelectric materials, such as fast high-density non-volatile random access memory. In memories of this sort, storing a data bit means increasing the size of one polar region at the expense of another, and hence the movement of a domain wall separating these regions. Experimental measurements of domain growth rates in the well-established ferroelectrics PbTiO3 and BaTiO3 have been performed, but the development of new materials has been hampered by a lack of microscopic understanding of how domain walls move. Despite some success in interpreting domain-wall motion in terms of classical nucleation and growth models, these models were formulated without insight from first-principles-based calculations, and they portray a picture of a large, triangular nucleus that leads to unrealistically large depolarization and nucleation energies. Here we use atomistic molecular dynamics and coarse-grained Monte Carlo simulations to analyse these processes, and demonstrate that the prevailing models are incorrect. Our multi-scale simulations reproduce experimental domain growth rates in PbTiO3 and reveal small, square critical nuclei with a diffuse interface. A simple analytic model is also proposed, relating bulk polarization and gradient energies to wall nucleation and growth, and thus rationalizing all experimental rate measurements in PbTiO3 and BaTiO3.     

钙钛矿型铁电体铁电相变时介电表现

铁电－铁电相变并不总是伴随介电峰值。ＢａＴｉＯ３出现峰值，ＰｂＺｒ１－ｘＴｉｘＯ３系统则不是，利用铁电热力学理论研究了出现介电峰值的条件，并将此与斜方相是否存在联系起来。     

Effects of SrTiO3 buffer layer on structural and electrical properties of Bi3.15Nd0.85Ti3O12 thin films prepared by a chemical method

Ferroelectric Bi3.15Nd0.85Ti3O12 (BNT) thin films have been grown on Pt/Ti/SiO2/Si substrates at 750 ℃ by a chemical solution deposition method using SrTiO3 (STO) as a buffer layer.The influence of STO...     

Relaxor ferroelectricity and colossal magnetocapacitive coupling in ferromagnetic CdCr2S4

Materials in which magnetic and electric order coexist--termed 'multiferroics' or 'magnetoelectrics'--have recently become the focus of much research. In particular, the simultaneous occurrence of ferromagnetism and ferroelectricity, combined with an intimate coupling of magnetization and polarization via magnetocapacitive effects, holds promise for new generations of electronic devices. Here we present measurements on a simple cubic spinel compound with unusual, and potentially useful, magnetic and electric properties: it shows ferromagnetic order coexisting with relaxor ferroelectricity (a ferroelectric cluster state with a smeared-out phase transition), both having sizable ordering temperatures and moments. Close to the ferromagnetic ordering temperature, the magnetocapacitive coupling (characterized by a variation of the dielectric constant in an external magnetic field) reaches colossal values, approaching 500 per cent. We attribute the relaxor properties to geometric frustration, which is well known for magnetic moments but here is found to impede long-range order of the structural degrees of freedom that drive the formation of the ferroelectric state.     

稀土Dy掺杂对BiFeO3陶瓷样品介电和铁电特性的影响

采用传统固相烧结法制备Bi1-xDyxFeO3(x=0, 0.05, 0.1, 0, 0.15)陶瓷样品.分别对样品的介电性能、铁电性进行了测量与分析.结果表明:Bi1-xDyxFeO3样品主衍射峰与纯相BiFeO3相吻合且具有良好的晶体结构;BiFeO3样品的介电常数、介电损耗均随Dy3+掺杂量的增加而逐渐增大.在测...     

铌镁酸铅类钙钛矿结构铁电多晶体的制备技术

铌镁酸铅(PMN)类化合物是一类弱铁电特性的ABO_3型复合钙钛矿结构铁电弛豫体。利用在A/B位固溶外来离子(例如,Sr、Ba和Ti等)的原理,改变该类化合物基固溶体的电负性差值φ和加权结构容差因子t值,达到合成全钙钛矿结构相的目的。根据电负性差值理论和钙钛矿结构容差因子理论,本文提出了铌镁酸铅类化合物钙钛矿结构相稳定性优值参数M的新概念。基于M值定义和PZN基固熔体的实验数据,推算出钙钛矿结构相稳定性临介优值Mc=1.67,并用以设计和优选PMN和PZN基多晶体最佳配方,优化材料性能。同时研究了钙钛矿结构相合成程度与MnO_2添加量和Mg偏离化学计量比程度的关系。研制出性能稳定、特性优良的新材料。     

大各向异性锑锰改性PbTiO3压电陶瓷制备工艺研究

实验研究了以（Ｍｎ１／３Ｓｂ２／３）＾４＋Ｂ位取代Ｔｉ＾４＋为特征的大各向异性改性ＰｂＴｉＯ３压电陶瓷制备工艺,通过详细研究工艺条件对改性ＰｂＴｉＯ３陶瓷性能的影响。确定了其较佳工艺参数,在优化的工艺条件下,该陶瓷具有高压电活性,大压电各向异性,小介电常数,低机械品质因素,在多种压电超声换能器的研制方面显出良好应用前景。     

Origin of the dielectric dead layer in nanoscale capacitors

Capacitors are a mainstay of electronic integrated circuits and devices, where they perform essential functions such as storing electrical charge, and blocking direct current while allowing alternating currents to propagate. Because they are often the largest components in circuits, extensive efforts are directed at reducing their size through the use of high-permittivity insulators such as perovskite-structure SrTiO3 (refs 1, 2), which should provide more capacitance per unit area of device. Unfortunately, most experiments on thin-film SrTiO3 capacitors have yielded capacitance values that are orders of magnitude smaller than expected. The microscopic origin of this reduced capacitance, which is often discussed in terms of a low-permittivity interfacial 'dead layer', is not well understood. Whether such a dead layer exists at all, and if so, whether it is an intrinsic property of an ideal metal-insulator interface or a result of processing issues such as defects and strains, are controversial questions. Here we present fully ab initio calculations of the dielectric properties of realistic SrRuO3/SrTiO3/SrRuO3 nanocapacitors, and show that the observed dramatic capacitance reduction is indeed an intrinsic effect. We demonstrate the existence of a dielectric dead layer by calculating the dielectric profile across the interface and analyse its origin by extracting the ionic and electronic contributions to the electrostatic screening. We establish a correspondence between the dead layer and the hardening of the collective SrTiO3 zone-centre polar modes, and determine the influence of the electrode by repeating our calculations for Pt/SrTiO3/Pt capacitors. Our results provide practical guidelines for minimizing the deleterious effects of the dielectric dead layer in nanoscale devices.     

Magnetic control of ferroelectric polarization

The magnetoelectric effect--the induction of magnetization by means of an electric field and induction of polarization by means of a magnetic field--was first presumed to exist by Pierre Curie, and subsequently attracted a great deal of interest in the 1960s and 1970s (refs 2-4). More recently, related studies on magnetic ferroelectrics have signalled a revival of interest in this phenomenon. From a technological point of view, the mutual control of electric and magnetic properties is an attractive possibility, but the number of candidate materials is limited and the effects are typically too small to be useful in applications. Here we report the discovery of ferroelectricity in a perovskite manganite, TbMnO3, where the effect of spin frustration causes sinusoidal antiferromagnetic ordering. The modulated magnetic structure is accompanied by a magnetoelastically induced lattice modulation, and with the emergence of a spontaneous polarization. In the magnetic ferroelectric TbMnO3, we found gigantic magnetoelectric and magnetocapacitance effects, which can be attributed to switching of the electric polarization induced by magnetic fields. Frustrated spin systems therefore provide a new area to search for magnetoelectric media.     

Critical thickness for ferroelectricity in perovskite ultrathin films

The integration of ferroelectric oxide films into microelectronic devices, combined with the size reduction constraints imposed by the semiconductor industry, have revived interest in the old question concerning the possible existence of a critical thickness for ferroelectricity. Current experimental techniques have allowed the detection of ferroelectricity in perovskite films down to a thickness of 40 A (ten unit cells), ref. 3. Recent atomistic simulations have confirmed the possibility of retaining the ferroelectric ground state at ultralow thicknesses, and suggest the absence of a critical size. Here we report first-principles calculations on a realistic ferroelectric-electrode interface. We show that, contrary to current thought, BaTiO3 thin films between two metallic SrRuO3 electrodes in short circuit lose their ferroelectric properties below a critical thickness of about six unit cells (approximately 24 A). A depolarizing electrostatic field, caused by dipoles at the ferroelectric-metal interfaces, is the reason for the disappearance of the ferroelectric instability. Our results suggest the existence of a lower limit for the thickness of useful ferroelectric layers in electronic devices.     

PbTiO3-CoFe2O4磁电材料样品库的同步辐射X射线衍射结构分析

用高分辨微区同步辐射X射线衍射研究了PbTiO3-CoFe2O4组合材料样品库晶嵌：结构随成分的变化，发现在80％PbTiO3成分附近存在着一个由外延应力引起的狭窄立方相区．分析表明，正是此相区的存在造成了该区域介电常数、非线性介电常数和磁电系数异常的增大．     

PbTiO_3-CoFe_2O_4磁电材料样品库的同步辐射X射线衍射结构分析

用高分辨微区同步辐射X射线衍射研究了PbTiO3-CoFe2O4组合材料样品库晶体结构随成分的变化,发现在80%PbTiO3成分附近存在着一个由外延应力引起的狭窄立方相区.分析表明,正是此相区的存在造成了该区域介电常数、非线性介电常数和磁电系数异常的增大.     

High Resolution Acoustic Microscopy with Low Frequency and Its Applications in Analysis of Ferroelectrics

Ferroelectrics have spontaneous polarization, which can be oriented along with electric field and show high dielectric constant, high switching speed, nonvolatile memory character and large capacity memories on a given silicon wafer. Large-scale manufacturing is presently underway to incorporate ferroelectric materials as storage and bypass capacitors in IC circuitry. Nondestructive observation of domain structure of ferroelectrics, dynamic behavior under external field and related phenomena is becoming sig...     

锡酸铅对铌镁酸铅-钛酸铅铁电陶瓷性能的影响

采用固相法合成（0．90-x）Pb（Mg1/3Nb2/3）O3-0．10PbSnO3-xPbTiO3（PMN—PSn-PT）铁电陶瓷．研究了（0．90-x）PMN0．10PSn-xPT铁电陶瓷的结构、介电、铁电和压电性能．在PMN-PT陶瓷中加入PSn后,由于Sn在合成过程中的变价行为导致了第二相Sn2Nb2O7的产生．根据XRD分析确定了（0．90-x）PMN-0．10PSn-xPT体系的准同型相界（MPB）位于x=0．36附近,进而结合介电温谱分析得到了（0．90-x）PMN-0．10PSn-xPT的部分低温相图．PSn加入到PMN-PT陶瓷中,降低了PMN-PT铁电陶瓷的压电活性,最大值位于0．52PMN-0．10PSn-0．38PT组分处（390pC／N）．极化样品的介电温谱上没有观察到三方四方相变温度点,说明PSn的加入使PMN-PT陶瓷的MPB变窄和竖直,因而PSn的加入可以提高PMN-PT铁电陶瓷的实际使用温度．     

PbTiO3多晶及薄膜材料强电场下的电热效应研究

基于Landau-Devonshire的热动力学模型,计算了PbTiO3块体及薄膜材料在铁电相变附近的电热效应。PbTiO3块体在769 K出现了一级铁电-顺电转变。700 K时其矫顽场为25MV·m-1。强的电场使得PbTiO3块体材料的一级相变逐渐转变为二级连续相变,且相变在更高的温度发生。PbTiO3薄膜材料的相变为二级相变,熵值减小,而熵变也相应减少。随着电场的增强,两者的比热容减小,但对应的温度向高温方向移动,2者的熵值却随着电场的增大而增强。对于PbTiO3薄膜来说,随着面内错配度的变化,张应力使相变温度降低,而压应力则相反。计算了PbTiO3块体与薄膜材料的绝热温变与制冷热容,其中块体的绝热温变ΔTad与制冷热容RC最大,分别为4.76 K与94.1 kJm-2K-1。     

Ferroelectricity from iron valence ordering in the charge-frustrated system LuFe2O4

Ferroelectric materials are widely used in modern electric devices such as memory elements, filtering devices and high-performance insulators. Ferroelectric crystals have a spontaneous electric polarization arising from the coherent arrangement of electric dipoles (specifically, a polar displacement of anions and cations). First-principles calculations and electron density analysis of ferroelectric materials have revealed that the covalent bond between the anions and cations, or the orbital hybridization of electrons on both ions, plays a key role in establishing the dipolar arrangement. However, an alternative model-electronic ferroelectricity-has been proposed in which the electric dipole depends on electron correlations, rather than the covalency. This would offer the attractive possibility of ferroelectric materials that could be controlled by the charge, spin and orbital degrees of freedom of the electron. Here we report experimental evidence for ferroelectricity arising from electron correlations in the triangular mixed valence oxide, LuFe(2)O(4). Using resonant X-ray scattering measurements, we determine the ordering of the Fe(2+) and Fe(3+) ions. They form a superstructure that supports an electric polarization consisting of distributed electrons of polar symmetry. The polar ordering arises from the repulsive property of electrons-electron correlations-acting on a frustrated geometry.