Magnetic and dielectric properties of Yb(Mn1-xAlx)O3 thin films

Hexagonal RMnO(3) is known as one of the candidates for multiferroic materials. We have focused on YbMnO(3), which has ferroelectricity along the c axis below 1000 K and antiferromagnetic ordering below 80 K. We have reported that magnetic, field-induced ferromagnetism in YbMnO(3) epitaxial thin films. In this study, we investigate the substitutional effect of the nonmagnetic element, Al, for Mn ions with triangular spin ordering on the ferroelectric and magnetic properties of YbMnO(3).     

Characterization of (Fe'Zr,Ti - VO..). defect dipoles in (La,Fe)-codoped PZT 52.5/47.5 piezoelectric ceramics by multifrequency electron paramagnetic resonance spectroscopy

Ferroelectric 1 mol.% La(3)+ and 0.5 mol.% Fe(3)+ codoped Pb[Zr0(0.54)Ti0(0.46)]O(3) ceramics were studied by means of multifrequency electron paramagnetic resonance (EPR) spectroscopy. The obtained results prove that iron is incorporated at the [Zr,Ti]-site, acting as an acceptor and building a charged Fe'(Zr,Ti) - V(O)..)(.) defect dipole with a directly coordinated oxygen vacancy for partial charge compensation. This feature of the defect associates has hitherto been identified only in hard, exclusively Fe(3)+-doped PZT compounds. The present results show, however, that a similar defect association of the Fe3+ functional center with a V(O)..) also exists in soft, donor-acceptor (La(3)+,Fe(3)+)-codoped PZT. According to models developed by Arlt et al. electric dipoles from defect associates, such as the Fe'(Zr,Ti) - V(O)..)(.) defect associate, which may give rise to an internal bias field that is discussed being responsible for ferroelectric aging.     

Microscopic Origin of Electric and Magnetic Ordering in BiFeO<Subscript>3</Subscript>

The microscopic origin of electric and magnetic ordering in multiferroic materials is described. Multiferroic materials are systems with strong spin–orbit coupling. There is an electric mechanism for which ferroelectricity is generated by dynamic magnetism though vanishing of the electric current; this result is demonstrated in this paper. The multiferroic material BiFeO₃ is shown to be a Mott’s insulator. An expression describing the connection between the polarization, magnetization, and the spin–orbit coupling parameter is derived.     

Studies on the relaxor behavior of sol-gel derived Ba(Zr x Ti1− x )O3 (0.30≤x≤0.70) thin films

We have studied the relaxor behavior of sol-gel derived Ba(Zr _x Ti_{1− x} )O₃ (0.30≤ x≤0.70) thin films. The plausible mechanism of the relaxor behavior has been analyzed from the dielectric data and micro-Raman spectra. Substitution of Zr⁺⁴ for Ti⁺⁴ in BaTiO₃ lattice reduces its long-range polarization order yielding a diffused paraelectric to ferroelectric phase transition. The solid solution system is visualized as a mixture of Ti⁺⁴ rich polar region and Zr⁺⁴ rich regions and with the increase in Zr contents the volume fraction of the polar regions are progressively reduced. At about 25.0 at% Zr contents the polar regions exhibit typical relaxor behavior. The degree of relaxation increases with Zr content and maximizes at 40.0 at% Zr doped film. The frequency dependence of the polar regions follows Vogel-Fulcher relation with a characteristic cooperative freezing at freezing temperature (T _f). Below T_f, a long range polarization ordering was ascertained from the polarization hysteresis measurement.     

Multiferroicity in an organic charge-transfer salt that is suggestive of electric-dipole-driven magnetism

Multiferroics, showing simultaneous ordering of electrical and magnetic degrees of freedom, are remarkable materials as seen from both the academic and technological points of view. A prominent mechanism of multiferroicity is the spin-driven ferroelectricity, often found in frustrated antiferromagnets with helical spin order. There, as for conventional ferroelectrics, the electrical dipoles arise from an off-centre displacement of ions. However, recently a different mechanism, namely purely electronic ferroelectricity, where charge order breaks inversion symmetry, has attracted considerable interest. Here we provide evidence for ferroelectricity, accompanied by antiferromagnetic spin order, in a two-dimensional organic charge-transfer salt, thus representing a new class of multiferroics. We propose a charge-order-driven mechanism leading to electronic ferroelectricity in this material. Quite unexpectedly for electronic ferroelectrics, dipolar and spin order arise nearly simultaneously. This can be ascribed to the loss of spin frustration induced by the ferroelectric ordering. Hence, here the spin order is driven by the ferroelectricity, in marked contrast to the spin-driven ferroelectricity in helical magnets.     

Facile Control of Ferroelectricity Driven by Ingenious Interaction Engineering

Construction of ferroelectric and optimization of macroscopic polarization has attracted tremendous attention for next generation light weight and flexible devices, which brings fundamental vitality for molecular ferroelectrics. However, effective molecular tailoring toward cations makes ferroelectric synthesis and modification relatively elaborate. Here, the study proposes a facile method to realize triggering and optimization of ferroelectricity. The experimental and theoretical investigation reveals that orientation and alignment of polar cations, dominated factors in molecular ferroelectrics, can be controlled by easily processed anionic modification. In one respect, ferroelectricity is induced by strengthened intermolecular interaction. Moreover, ≈50% of microscopic polarization enhancement (from 8.07 to 11.68 µC cm⁻²) and doubling of equivalent polarization direction (from 4 to 8) are realized in resultant ferroelectric FEtQ2ZnBrI₃ (FEQZBI, FEtQ = N-fluoroethyl-quinuclidine). The work offers a totally novel platform for control of ferroelectricity in organic–inorganic hybrid ferroelectrics and a deep insight of structure–property correlations.     

Bond- versus site-centred ordering and possible ferroelectricity in manganites

Transition metal oxides with a perovskite-type structure constitute a large group of compounds with interesting properties. Among them are materials such as the prototypical ferroelectric system BaTiO(3), colossal magnetoresistance manganites and the high-T(c) superconductors. Hundreds of these compounds are magnetic, and hundreds of others are ferroelectric, but these properties very seldom coexist. Compounds with an interdependence of magnetism and ferroelectricity could be very useful: they would open up a plethora of new applications, such as switching of magnetic memory elements by electric fields. Here, we report on a possible way to avoid this incompatibility, and show that in charge-ordered and orbitally ordered perovskites it is possible to make use of the coupling between magnetic and charge ordering to obtain ferroelectric magnets. In particular, in manganites that are less than half doped there is a type of charge ordering that is intermediate between site-centred and bond-centred. Such a state breaks inversion symmetry and is predicted to be magnetic and ferroelectric.     

Ferroelectric ordering in ice nanotubes confined in carbon nanotubes

The ice nanotubes with odd number of side faces formed inside carbon nanotubes (CNTs) are found to exhibit spontaneous electric polarizations along their tube axes by means of molecular dynamics simulations. The physical mechanism underlying the quasi-one-dimensional (Q1D) ferroelectricity is an interplay between the Q1D geometrical confinement of CNTs and the distinct orientational ordering of the hydrogen bonds dictated by the "ice rule". This mechanism is fundamentally different from the conventional one seen in three-dimensional ferroelectric (FE) materials or in two-dimensional FE ice films. In addition, it is found that vacancies in the ice nanotubes can induce a net polarization normal to the tube axis.     

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。     

Role of Frustration in Quasi 1D Conductor – Charge Ordering and/or CDW in (R1,R2-DCNQI)2M (M=Ag, Li, Cu) System –

Synchrotron X-ray high-resolution measurements have been performed to investigate the nature of the insulating state of a quarter-filled quasi-one-dimensional conductor (DI-DCNQI)₂Ag and the π-d interacted conductor (DBr-DCNQI)2Cu. The two-fold structure in the ground state of (DI-DCNQI)₂Ag system consists of not only charge ordering columns but also monotonic charge dimerized columns caused by frustration among DCNQI columns. On the other hand, Cu salt in the insulating phase realizes a commensurate structure where the Cu charge ordering is coupled with the 3c-CDW in the insulator phase. The frustration among charged columns is restrained by the charge ordering of the Cu ions.     

Role of frustration in quasi 1D conductor — Charge ordering and/or CDW in (R1,R2-DCNQI)2M (M=Ag, Li, Cu) system —

Synchrotron X-ray high-resolution measurements have been performed to investigate the nature of the insulating state of a quarter-filled quasi-one-dimensional conductor (DI-DCNQI)₂Ag and the π-d interacted conductor (DBr-DCNQI)₂Cu. The two-fold structure in the ground state of (DI-DCNQI)₂Ag system consists of not only charge ordering columns but also monotonic charge dimerized columns caused by frustration among DCNQI columns. On the other hand, Cu salt in the insulating phase realizes a commensurate structure where the Cu charge ordering is coupled with the 3c-CDW in the insulator phase. The frustration among charged columns is restrained by the charge ordering of the Cu ions.     

Giant Polarization Sustainability in Ultrathin Ferroelectric Films Stabilized by Charge Transfer

Ferroelectricity is generally deteriorated or even vanishes when the ferroelectric films are downsized to unit cell scale. To maintain and enhance the polarization in nanoscale ferroelectrics are of scientific and technological importance. Here, giant polarization sustainability is reported in a series of ultrathin PbTiO₃ films scaled down to three unit cells grown on NdGaO₃(110) substrates with La_0.7Sr_0.3MnO₃ as bottom electrodes. Atomic mappings via aberration‐corrected scanning transmission electron microscopy demonstrate the robust ferroelectricity for the sub‐10 nm thick film. For the 1.2 nm thick film, the polarization reaches ≈50 µC cm^?2. The 2 nm thick film possesses a polarization as high as the bulk value. The films ranging from 10 to 35 nm display a giant elongation of out‐of‐plane lattice parameter, which corresponds to a polarization of 100 µC cm^?2, 20% larger than that of the bulk PbTiO₃. The giant enhancement of polarization in the present films is proposed to result from the charge transfer at the La_0.7Sr_0.3MnO₃/PbTiO₃ interface, as supported by the anomalous decrease of Mn valence measured from X‐ray photoelectron spectroscopy. These results reveal the significant role of charge transfer at interfaces in improving large polarizations in ultrathin ferroelectrics and are meaningful for the development of future electronic devices.     

聚偏氟乙烯电极化的动力学相变和铁电性

对聚偏氟乙烯进行高温电极化处理以改善半晶聚合物的铁电性,研究了其铁电极化值随着极化电场的连续非线性变化,实现了结晶相中非铁电相向铁电相的转变.用动力学平衡方法模拟了聚偏氟乙烯样品的电场极化相变过程,结果表明:聚偏氟乙烯中结晶区域的有效极化电场约为50 MV/m,与铁电聚合物的矫顽电场吻合;非晶无定型区域的等效电场与实验的相变开启电场接近,并得到了理论与实验符合较好的结果,从而证明动力学平衡方法模拟聚偏氟乙烯铁电相变的可行性.     

Direct Probing of Polarization Charge at Nanoscale Level

Ferroelectric materials possess spontaneous polarization that can be used for multiple applications. Owing to a long‐term development of reducing the sizes of devices, the preparation of ferroelectric materials and devices is entering the nanometer‐scale regime. Accordingly, to evaluate the ferroelectricity, there is a need to investigate the polarization charge at the nanoscale. Nonetheless, it is generally accepted that the detection of polarization charges using a conventional conductive atomic force microscopy (CAFM) without a top electrode is not feasible because the nanometer‐scale radius of an atomic force microscopy (AFM) tip yields a very low signal‐to‐noise ratio. However, the detection is unrelated to the radius of an AFM tip and, in fact, a matter of the switched area. In this work, the direct probing of the polarization charge at the nanoscale is demonstrated using the positive‐up‐negative‐down method based on the conventional CAFM approach without additional corrections or circuits to reduce the parasitic capacitance. The polarization charge densities of 73.7 and 119.0 µC cm^?2 are successfully probed in ferroelectric nanocapacitors and thin films, respectively. The obtained results show the feasibility of the evaluation of polarization charge at the nanoscale and provide a new guideline for evaluating the ferroelectricity at the nanoscale.     

铌钽酸钠钾锂-铟酸铋铁电压电性能的研究

采用传统固相合成法制备了(1-x)(0.97Na_0.5K_0.5NbO₃+0.03LiTaO₃)+xBiInO₃[(1-x)(KNN+LT)+xBI]无铅压电陶瓷, 研究了该体系的晶体结构、压电、铁电与介电性能。结果表明: (1-x)(KNN+LT)+xBI陶瓷的晶体结构在室温下为正交相和四方相两相共存, 随着BI含量的增加, 四方相含量逐渐增多, 居里温度T_c降低, 但是正交四方相转变温度T_o-t呈现升高趋势, 晶格参数逐渐增大。随着BI含量的继续增加, Bi在高温下的挥发导致氧空位的增多, 从而使材料性能下降。在x=0.0015时得到组分的最佳性能, 其居里温度T_c、相转变温度T_o-t、压电常数d₃₃、剩余极化P_r、矫顽场E_c分别达到400℃、126℃、160 pC/N、19.26 μC/cm2和7.75 kV/cm。     

Spin Liquid State in κ-(BEDT-TTF)2Cu2(CN)3 Studied by Muon Spin Relaxation Method

Magnetic properties of an organic Mott insulator with an approximately isotropic triangular lattice structure, κ-(ET)₂Cu₂(CN)₃, are investigated by the muon spin relaxation (μSR) method. No magnetic ordering is observed down to 20 mK in the zero-field (ZF) μSR measurement, suggesting the realization of the spin liquid ground state. An enhancement of the distribution of internal fields, corresponding to broadening of the linewidth reported in the ¹³C NMR study under magnetic fields, is not observed under ZF. A slight change in the spin dynamics is detected below 4 K. This change is quite small and different from the critical slow-down effect attributable to a magnetic transition.     

Ferroelectric and magnetic properties of multiferroic BiFeO(3)-based composite films

BiFeO(3)-based composite films were fabricated onto the Pt/Ti/SiO(2)/Si(100) substrates by a chemical solution deposition (CSD) method using the precursor solutions with various excess iron composition followed by annealing at 923 K for 30 minutes under oxygen gas flow. Coexistence of spontaneous magnetization and remanent polarization could be obtained in the BiFeO(3)-based composite films with high excess iron composition. The remanent magnetization of almost 20 emu/cm(3) and the magnetic coercive field of 1.5 kOe were obtained at the iron composition ratio of Fe/Bi = 1.25. In this specimen, the remanent polarization at 90 K was approximately 10 microC/cm(2) at the electric field of 1500 kV/cm. Structural analysis suggested that the remanent polarization has a possibility to increase by suppressing the formation of the secondary phases of Bi(2)Fe(4)O(9) and alpha-Fe(2)O(3), these are the nonferroelectric material as well as antiferromagnetic phase.     

Spontaneous Polarization and Magnetic Investigation Of BiXO3 (X=Co,Mn, Fe,V, Zn): First-Principle Study

Spontaneous polarization and magnetic properties of BiXO3 with X = Co, Fe, Mn, V, and Zn have been studied using the full-potential, linear augmented plane wave (FP-LAPW) method with GGA and mBJ implemented in the WIEN2K code. The investigation shows that BiXO3 with X = Co, Fe, Mn, and Zn is stable in the antiferromagnetic state while for BiVO3, the magnetic stability was shown in the ferromagnetic state. The calculation shows the existence of a charge transfer between p states of oxygen and d states of Co and Fe for X = Co and Fe. The existence of Co, Fe, Mn, and V makes BiXO3 a non-absorbent compound. However, for Zn, the structure behaves as an absorbent compound especially in the visible light. We note that the spontaneous polarization is calculated with Berry phase BI and was found very close and comparable with other works.     

Factors influencing the piezoelectric behaviour of PZT and other “morphotropic phase boundary” ferroelectrics

Recent studies of the mechanism of piezoelectricity in PZT and related materials are reviewed and complemented by new analyses based on the Landau-Ginzburg-Devonshire theory of ferroelectricity. Particular attention is given to the nature of the morphotropic phase boundary between the tetragonal and rhombohedral perovskite phases and the accompanying peak in the piezoelectric coefficient. The importance of the changes in angular dependence of single crystal piezoelectric coefficients as a function of composition is highlighted together with the concept of field-induced rotation of the polarization in the (110) plane. It is shown that introducing the tendency to form monoclinic phases enhances this phenomenon. The model that the monoclinic phase in PZT is due to the condensation of local disorder in the polar cation displacements from the macroscopic tetragonal and rhombohedral phases is examined in some detail using statistical analyses of the Zr/Ti conformation. Whilst the concept of monoclinic nano-domains is not inconsistent with statistically random distributions, it is argued that some ordering of the B-site cations may be required to enable the transformation to a macroscopically observable phase. The implications of this model on the contribution of polarization rotation to piezoelectricity in PZT are discussed.     

氮氢混合气氛对SrBi2Ta2O9铁电薄膜和粉末性能的影响

用金属有机物分解法分别制备了SrBi₂Ta₂O₉（SBT）薄膜和粉末样品. XRD和SEM结果显示SBT粉末经历氮氢混合气氛400℃退火处理后发生了还原反应,金属Bi和δ-Bi₂O₃析出,成针状结构聚集在表面,晶体结构没有被破坏. SBT薄膜在500℃退火处理时,表面出现Bi的球形及针状结构聚集体,相对于薄膜结构,SBT粉末中的Bi元素在较低温度时更容易被还原. Bi的大量缺失严重影响薄膜的铁电性能,当退火时间为5.5min时,SBT薄膜剩余极化强度Pr下降了约43%,但是在109极化反转后仍然保持了良好的抗疲劳特性;退火时间超过8.5min时,薄膜被击穿,铁电性能消失.