Ferroelectric domain structures and polarization switching characteristics of polycrystalline BiFeO3 thin films on glass substrates

We investigated ferroelectric characteristics of BiFeO₃ (BFO) thin films on SrRuO₃ (SRO)/yttria-stabilized zirconia (YSZ)/glass substrates grown by pulsed laser deposition. YSZ buffer layers were employed to grow highly crystallized BFO thin films as well as SRO bottom electrodes on glass substrates. The BFO thin films exhibited good ferroelectric properties with a remanent polarization of 2P_r = 59.6 μC/cm² and fast switching behavior within about 125 ns. Piezoelectric force microscopy (PFM) study revealed that the BFO thin films have much smaller mosaic ferroelectric domain patterns than epitaxial BFO thin films on Nb:SrTiO₃ substrates. Presumably these small domain widths which originated from smaller domain energy give rise to the faster electrical switching behavior in comparison with the epitaxial BFO thin films on Nb:SrTiO₃ substrates.     

多铁异质结构中逆磁电耦合效应的研究进展

电场调控磁性能够有效降低功耗,在未来低功耗多功能器件等方面具有巨大的潜在应用前景.铁磁/铁电多铁异质结构是实现电场调控磁性的有效途径,其中室温、磁电耦合效应大的应变媒介磁电耦合是最为活跃的研究领域之一.本文简要介绍在以Pb(Mg_(1/3)Nb_(2/3))_(0.7)Ti_(0.3)O_3为铁电材料的多铁异质结构中通过应变媒介磁电耦合效应对磁性、磁化翻转及磁性隧道结调控的研究进展.首先讨论了多铁异质结构中电场对磁性的调控;之后介绍了电场调控磁化翻转的研究进展及理论上实现的途径;然后简述了电场对磁性隧道结调控的相关结果;最后在此基础上,对多铁异质结构中电场调控磁性及磁性器件进行了总结和展望.     

Nanoscale imaging of ferroelectric domain and resistance switching in hybrid improper ferroelectric Ca3Ti2O7 thin films

Hybrid improper ferroelectrics have their electric polarization generated by two or more combined non-ferroelectric structural distortions such as the rotation and tilting of Ti-O octahedral in Ca₃Ti₂O₇ (CTO) family. In this work, we prepared different thickness CTO thin films on Pt substrates by pulsed laser deposition, and investigated their ferroelectric polarization reversal and the current transport properties by using the piezoresponse force microscopy and conducting atomic force microscopy, respectively. It is found that the CTO films exhibit clear ferroelectric domain switching and ferroelectric resistance switching behaviors, and the maximum resistive ratios of CTO film reaches ∼1750. These results demonstrate that hybrid improper ferroelectrics CTO films are promising materials for being employed in non-volatile memory and logic devices.     

基于PbMg_(1/3)Nb_(2/3)O_3-PbTiO_3压电单晶的磁电复合薄膜材料研究进展

电子信息技术的迅速发展对磁电功能器件的微型化、智能化、多功能化以及灵敏度、可靠性、低功耗等都提出了更高的需求,传统的块体磁电功能材料已日渐不能满足上述需求,而层状磁电复合薄膜材料同时具有铁电性、铁磁性和磁电耦合等多种特性,因此能满足上述需求且有望应用于新一代磁电功能器件.层状磁电复合材料不仅具有非常丰富的物理现象和效应,而且在弱磁探测器、多态存储器、电写磁读存储器、电场可调低功耗滤波器、移相器、天线等微波器件中也具有广阔的应用前景,因而受到材料科学家和物理学家广泛的关注和研究.在层状磁电复合材料中,功能薄膜/铁电单晶异质结因其制备简单、结构设计和材料选择灵活以及电场调控方便和有效,最近十余年引起了越来越多的研究人员的兴趣.目前,以具有优异铁电和压电性能的(1-x)PbMg_(1/3)Nb_(2/3)O_3-PbTiO_3(PMN-PT)单晶作为衬底,构建功能薄膜/PMN-PT异质结已成为国内外多铁性复合薄膜材料研究领域的重要方向之一.相比于其他国家,我国科学家无论在发表的文章数量还是在文章被引用次数方面都处于领先地位,表明我国在功能薄膜/PMN-PT单晶异质结方面的研究卓有成效.迄今为止,研究人员已构建了锰氧化合物/PMN-PT、铁氧体/PMN-PT、铁磁金属/PMN-PT、稀磁半导体/PMN-PT、发光材料/PMN-PT、二维材料/PMN-PT、多层薄膜/PMN-PT、超导薄膜/PMN-PT等多种类型的异质结,在理论研究和实验方面都取得了丰富的研究成果.本文对基于PMN-PT压电单晶的磁电复合薄膜材料的研究进展进行了总结:简要介绍了与功能薄膜/PMN-PT异质结相关的研究论文发表现状;介绍了PMN-PT单晶在准同型相界附近的相图和应变特性;按照功能薄膜材料所属的体系对异质结进行了分类,并选取部分代表性的研究成果,介绍了材料的磁电性能和内涵的物理机制;最后就目前有待解决的问题和未来可能的应用方向进行了总结和展望.     

In-situ strain effect on extrinsic transport of polycrystalline La0.7(Ca,Sr)0.3MnO3 films

The spin-polarised transport in ferromagnetic polycrystalline La_0.7(Sr,Ca)_0.3MnO₃ films on piezoelectric substrate has been investigated. The systematic study involved in finding the effect of in-situ strain on extrinsic electrical transport of various thick polycrystalline La_0.7(Sr,Ca)_0.3MnO₃ thin films. The in-situ strain in the manganite polycrystalline thin film is achieved by applying an electric field to the piezoelectric substrate 0.72 Pb(Mg_1/3Nb_2/3)O₃-0.28 PbTiO₃ (PMN-PT). A reversible strain of about 0.11% is acquired with an application of 10 kV/cm to the piezoelectric substrate. A typical drop in resistance at low magnetic fields has been found in all the polycrystalline manganite films. The effect of reversible strain versus the resultant strain gauges was discussed in all the polycrystalline films. At low temperatures, the effect of strain on low-field magnetoresistance and high-field magnetoresistance was found to be negligible. Further, the results are compared with the transport in manganite films deposited on step edge junctions.     

Direct observation of ferroelectric domain switching in varying electric field regimes using in situ TEM

In situ Transmission Electron Microscopy (TEM) techniques can potentially fill in gaps in the current understanding interfacial phenomena in complex oxides. Select multiferroic oxide materials, such as BiFeO(3) (BFO), exhibit ferroelectric and magnetic order, and the two order parameters are coupled through a quantum-mechanical exchange interaction. The magneto-electric coupling in BFO allows control of the ferroelectric and magnetic domain structures via applied electric fields. Because of these unique properties, BFO and other magneto-electric multiferroics constitute a promising class of materials for incorporation into devices such as high-density ferroelectric and magnetoresistive memories, spin valves, and magnetic field sensors. The magneto-electric coupling in BFO is mediated by volatile ferroelastically switched domains that make it difficult to incorporate this material into devices. To facilitate device integration, an understanding of the microstructural factors that affect ferroelastic relaxation and ferroelectric domain switching must be developed. In this article, a method of viewing ferroelectric (and ferroelastic) domain dynamics using in situ biasing in TEM is presented. The evolution of ferroelastically switched ferroelectric domains in BFO thin films during many switching cycles is investigated. Evidence of partial domain nucleation, propagation, and switching even at applied electric fields below the estimated coercive field is revealed. Our observations indicate that the occurrence of ferroelastic relaxation in switched domains and the stability of these domains is influenced the applied field as well as the BFO microstructure. These biasing experiments provide a real time view of the complex dynamics of domain switching and complement scanning probe techniques. Quantitative information about domain switching under bias in ferroelectric and multiferroic materials can be extracted from in situ TEM to provide a predictive tool for future device development.     

Structural and electrical properties of 0.7Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>–0.3PbTiO<Subscript>3</Subscript> thin films grown on Ir/MgO buffered Si(100) substrates

Thin films of 0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ (PMN-PT) have been grown on Ir/MgO buffered Si(100) substrates at different substrate temperatures by pulsed laser deposition. Crystalline phases as well as preferred orientations in the PMN-PT films were investigated by X-ray diffraction analysis (XRD). The microstructure, dielectric and ferroelectric properties of PMN-PT film prepared at 650 °C were studied. The results show that the film prepared at 650 °C exhibits pure perovskite phase and single c-axis orientation. The dielectric constant and dissipation factor of the single c-axis oriented film are 1000 and 0.04 at a frequency of 1 kHz, while the remnant polarization and coercive field are about 13.0 μC/cm² and 100 kV/cm under an electric field of 480 kV/cm, respectively. PACS 81.15.Fg; 77.80.-e; 77. 22.Ej; 77.55.+f; 85.50.Gk     

Temperature-dependent ferroelectric and dielectric properties of Bi3.25La0.75Ti3O12 thin films

To investigate temperature-dependent ferroelectric and dielectric properties of ferroelectric films, Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films were prepared on Pt-coated silicon substrates by pulsed laser deposition. The ferroelectric and dielectric behaviors have been studied in a wide temperature range from 80 K to room temperature. The saturated polarization (P_sat) decreases with decreasing temperature and decreasing electric field, whereas remnant polarization (P_r) shows a more complex temperature dependence. These results, which can be well explained based on a temperature-dependent charged defects-domain wall interaction model, might be helpful for further understanding the domain switching behavior. Based on these results, an alternative way to investigate temperature-dependent ferroelectric fatigue is proposed and experimentally carried out. The measured fatigue rate is found to be linearly dependent on temperature, consistent with the report on Pb(Zr,Ti)O₃ films. Temperature-dependent dielectric measurements of the films further confirm the above explanation.     

1-3型纳米多铁复合薄膜中电场诱导的磁化研究

运用Landau-Devonshire热力学唯像理论,考虑铁电相和铁磁相的电致伸缩、磁致伸缩效应以及产生于铁电/铁磁和薄膜/基底界面的弹性应力作用,两次重整介电和磁作用系数得到了这种多铁系统在Landau自由能函数下的本征二次方磁电耦合形式,从而研究了外延1-3型纳米多铁复合薄膜中极化、磁化随薄膜厚度、温度的变化以及该薄膜中外加电场诱导的磁化变化.结果表明薄膜平面内的应压力的弛豫使得磁化强度和极化强度随薄膜厚度的增加而减少,外加电场不仅能诱导铁电相极化场翻转,而且由于铁电和铁磁相界面竖直方向的弹性耦合导致 关键词： 多铁 磁电效应 磁致伸缩 薄膜     

溶胶-凝胶法制备Sr2Bi4Ti5O18薄膜及其铁电性能研究

采用溶胶-凝胶法,在氧气氛中和层层晶化的工艺条件下,成功地制备了沉积在Pt/Ti/SiO₂/Si(100)衬底上的铁电性能优良的Sr₂Bi₄Ti₅O₁₈ (SBTi)薄膜,并研究了SBTi薄膜的微结构、表面形貌、铁电性能和疲劳特性.研究表明：薄膜具有单一的层状钙钛矿结构，且为随机取向；薄膜表面光滑，无裂纹，厚度约为725nm；铁电性能测试显示较饱和、方形的电滞回线，当外电场强度为275kV/cm时 关键词： 溶胶-凝胶法 铁电薄膜 2Bi₄Ti₅O₁₈')" href="#">Sr₂Bi₄Ti₅O₁₈     

Magnetoelectric coupling induced multistate magnetization

Complex oxides provide an ideal playground for exploring the interplay among the fundamental degrees of freedom: structural (lattice), electronic (orbital and charge), and magnetic (spin). With trends toward device miniaturization, there is growing interest in combining electronic and magnetic properties into multifunctional thin-film materials for potential applications in novel versatile devices. Here, we report growth of ferromagnetic nanostructures on ferroelectric ($1?x$)[Pb(Mg_1/3Nb_2/3)O₃]???x[PbTiO₃] (PMN-PT) single crystals. With careful control of material composition and application of electric field direction and amplitude, we realized stabilization of both monoclinic and orthorhombic phases in the PMN-PT single crystals with the intrinsic rhombohedral phase. Multistate magnetization modulation has been realized through phase transformation control in the ferroelectric component. This finding promises multistate information recording through magnetoelectric effect.     

Investigation of domain structure of TGS single crystal after a transverse electric field by piezoresponse force microscopy

An investigation of the influence of electric field transverse to the ferroelectric axis b_HOP and parallel to c_HOP axis of triglycine sulfate (TGS) single crystal on ferroelectric domain structure was performed by piezoresponse force microscopy. To check if the applied electric field changed the dielectric properties and ferroelectric domain structure the hysteresis loop measurements were carried out as well as observations of domain structure by the liquid crystal technique. The investigation revealed existence of blocked domains in the crystal modified by the electric field TGS.     

Interrelatedness of Fe composition on structural and magnetic properties in Fe-doped SrRuO3 thin films

Fe-doping (up to 11 mole%) into SrRuO₃ (SRO) thin films on SrTiO₃ substrates decreased correlation lengths of both surface and interface. It turned out that Fe was doped in the valence state of 3+ without formation of the Fe₂O₃ phase, which caused orthorhombic distortion. T _C values decreased from 145 K to 97 K with increasing Fe concentration (C _Fe). High magnetic switching fields were observed for all Fe-doped SRO thin films and their strengths showed a linear relationship with C _Fe. Detail structural characterization using synchrotron X-ray diffraction and X-ray photoemission spectroscopy were used to understand its unique magnetic switching field properties.     

外加电场对铁电薄膜相变的影响

采用热力学非线性理论,研究了外加电场对立方基底Pb(Zr_0.3Ti_0.7)O₃(PZT)铁电薄膜相变的影响.通过数值计算,得到了"失配应变-外加电场"相图,及外加电场与极化强度的关系.当外加电场达到186 kV/cm时,能使生长在SrTiO₃ 基底上PZT铁电薄膜从单斜r相转变为c相.在实验上,采用扫描探针显微镜通过对PZT薄膜施加不同的极化电场来研究了它的电畴翻转.从得到的压电响应相图可以看出,绝大多数的电畴是清晰可 关键词： 铁电薄膜 相变 扫描探针显微镜 失配应变     

Phase compositions and ferroelectric properties of Pb(Zr0.52,Ti0.48)O3 thin films with different highly orientations prepared by a sol-gel route

It is shown that different highly oriented Pb(Zr_0.52,Ti_0.48)O₃ films can be obtained on Pt/Ti/SiO₂/Si substrate using a sol-gel technique. The effects of pyrolysis temperature on the orientation, phase composition and ferroelectric properties of the films are investigated. It is found the ferroelectric hysteresis loops of (1 1 1)-oriented film, (1 1 1) and (1 0 0) mix-oriented film can both be saturated when the external electric field is large enough, whereas the hysteresis loop of (1 0 0)-oriented film is difficult to saturate. The analysis of X-ray diffraction indicated the possibility of different phase composition in different oriented films under large film residual stress. Higher remnant polarization (53 μC/cm²) for (1 0 0)-oriented film can be attributed to its more tetragonal phase composition, which results in that the in-plane domain switching can continuously occur with external electric field increasing.     

Effect of bilayer structure and a SrRuO3 buffer layer on ferroelectric properties of BiFeO3 thin films

Effects of the BiFe_0.95Mn_0.05O₃ thickness and a SrRuO₃ (SRO) buffer layer on the microstructure and electrical properties of BiFeO₃/BiFe_0.95Mn_0.05O₃ (BFO/BFMO) bilayered thin films were investigated, where BFO/BFMO bilayered thin films were fabricated on the SRO/Pt/Ti/SiO₂/Si(100) substrate by a radio frequency sputtering. All thin films are of a pure perovskite structure with a mixture of (110) and (111) orientations regardless of the BFMO layer thickness. Dense microstructure is demonstrated in all thin films because of the introduction of BFMO layers. The SRO buffer layer can also further improve the ferroelectric properties of BFO/BFMO bilayered thin films as compared with those of these thin films without a SRO buffer layer. The BFO/BFMO bilayered thin film with a thickness ratio of 220/120 has an enhanced ferroelectric behavior of 2P _r??165.23???C/cm² and 2E _c??518.56?kV/cm, together with a good fatigue endurance. Therefore, it is an effective way to enhance the ferroelectric and fatigue properties of bismuth ferrite thin films by constructing such a bilayered structure and using a SRO buffer layer.     

Effects of switching pulse width and stress on properties of Bi3.25La0.75Ti3O12 thin films

Polycrystalline ferroelectric Bi3.25La0.75Ti3O12 thin films are prepared on Pt/Ti/SiO2/Si substrates by the conventional metalorganic decomposition method.It is observed that with the increase of switching pulse width,the remnant polarisation and the coercive field increase.A wider switching pulse can result in poorer fatigue properties,which comes from more charged defects diffusing to and being trapped on domain walls.On the other hand,when the compressive stress is applied to films,the fatigue properties can be improved.This phenomenon is due to the reorientation of domains under stress.     

Ferroelectric polarization reversal tuned by magnetic field in a ferroelectric BiFeO3/Nb-doped SrTiO3 heterojunction

Interfacial resistive switching of a ferroelectric semiconductor heterojunction is highly advantageous for the newly developed ferroelectric memristors. Moreover, the interfacial state in the ferroelectric semiconductor heterojunction can be gradually modified by polarization reversal, which may give rise to continuously tunable resistive switching behavior. In this work, the interfacial state of a ferroelectric BiFeO₃/Nb-doped SrTiO₃ junction was modulated by ferroelectric polarization reversal. The dynamics of surface screening charges on the BiFeO₃ layer was also investigated by surface potential measurements, and the decay of the surface potential could be speeded up by the magnetic field. Moreover, ferroelectric polarization reversal of the BiFeO₃ layer was tuned by the magnetic field. This finding could provide a method to enhance the ferroelectric and electrical properties of ferroelectric BiFeO₃ films by tuning the magnetic field.     

Strain-dependent magnetic configurations in manganite-titanate heterostructures probed with soft X-ray techniques

We present a detailed study on the strain-induced magnetic domain structure of a (La,Sr)MnO₃ thin film epitaxially grown on a BaTiO₃ substrate through the use of polarization-dependent X-ray photoemission electron microscopy and X-ray absorption spectroscopy. Angular-dependent measurements allow us to detect vector magnetization on a single-domain scale, and we relate the strain-induced changes in magnetic anisotropy of the ferromagnetic film to the ferroelectric domain structure of the underlying substrate using X-ray magnetic circular and linear dichroism spectro-microscopy. Comparisons to measurements on a nearly strain free film of (La,Sr)MnO₃ grown on a (La,Sr)(Al,Ta)O₃ substrate illustrate that the BaTiO₃ ferroelectric domain structure imprints specific domain sizes and wall orientations in the (La,Sr)MnO₃/BaTiO₃ artificial multiferroic heterostructure. Furthermore, a change of the BaTiO₃ ferroelectric domain structure either with temperature or with applied electric field results in a corresponding change in the (La,Sr)MnO₃ ferromagnetic domain structure, thus showing a possible route to obtain room-temperature electric field control of magnetic anisotropy at the nanoscale.     

Domain wall creep in epitaxial ferroelectric Pb(Zr(0.2)Ti(0.08)O(3) thin films

Ferroelectric switching and nanoscale domain dynamics were investigated using atomic force microscopy on monocrystalline Pb(Zr(0.2)Ti(0.8))O(3) thin films. Measurements of domain size versus writing time reveal a two-step domain growth mechanism, in which initial nucleation is followed by radial domain wall motion perpendicular to the polarization direction. The electric field dependence of the domain wall velocity demonstrates that domain wall motion in ferroelectric thin films is a creep process, with the critical exponent mu close to 1. The dimensionality of the films suggests that disorder is at the origin of the observed creep behavior.