Phase transitions in epitaxial (-110) BiFeO3 films from first principles

The effect of misfit strain on properties of epitaxial BiFeO3 films that are grown along the pseudocubic [110] direction, rather than along the usual [001] direction, is predicted from density-functional theory. These films adopt the monoclinic Cc space group for compressive misfit strains smaller in magnitude than ?1.6% and for any investigated tensile strain. In this Cc phase, both polarization and the axis about which antiphase oxygen octahedra tilt rotate within the epitaxial plane as the strain varies. Surprisingly and unlike in (001) films, for compressive strain larger in magnitude than ?1.6%, the polarization vanishes and two orthorhombic phases of Pnma and P2(1)2(1)2(1) symmetry successively emerge via strain-induced transitions. The Pnma-to-P2(1)2(1)2(1) transition is a rare example of a so-called pure gyrotropic phase transition, and the P2(1)2(1)2(1) phase exhibits original interpenetrated arrays of ferroelectric vortices and antivortices.     

Atomic-force microscopy of polarization domains in ferroelectric films

Single-crystal (001)-oriented PbZr_0.47Ti_0.53O₃ and polycrystalline (111)-oriented PbZr_0.47Ti_0.53O₃ thin ferroelectric films were studied using contact electrostatic force microscopy. Local electromechanical response measurements permitted study of the polarization vector distribution in natural and intentionally created polarization nanodomains in films. The principal components of an electromechanical response signal encountered in studies of ferroelectric films (the piezoelectric response and the additional capacitive contribution) were isolated and analyzed. The effect of tip-surface contact stiffness on the capacitive contribution to the electromechanical response signal was demonstrated experimentally and in terms of a model. It was shown that more accurate information on the distribution of the polarization vector in ferroelectric films can be gained by monitoring local variations in the tip-surface contact stiffness.     

Terahertz radiation by an ultrafast spontaneous polarization modulation of multiferroic BiFeO3 thin films

Terahertz (THz) radiation has been observed from multiferroic BiFeO3 thin films via ultrafast modulation of spontaneous polarization upon carrier excitation with illumination of femtosecond laser pulses. The radiated THz pulses from BiFeO3 thin films were clarified to directly reflect the spontaneous polarization state, giving rise to a memory effect in a unique style and enabling THz radiation even at zero-bias electric field. On the basis of our findings, we demonstrate potential approaches to ferroelectric nonvolatile random access memory with nondestructive readability and ferroelectric domain imaging microscopy using THz radiation as a sensitive probe.     

Strain dependence of polarization and piezoelectric response in epitaxial BiFeO3 thin films

Epitaxial strain has recently emerged as a powerful means to engineer the properties of ferroelectric thin films, for instance to enhance the ferroelectric Curie temperature (T(C)) in BaTiO(3). However, in multiferroic BiFeO(3) thin films an unanticipated strain-driven decrease of T(C) was reported and ascribed to the peculiar competition between polar and antiferrodistortive instabilities. Here, we report a systematic characterization of the room-temperature ferroelectric and piezoelectric properties for strain levels ranging between -2.5% and +1%. We find that polarization and the piezoelectric coefficient increase by about 20% and 250%, respectively, in this strain range. These trends are well reproduced by first-principles-based techniques.     

Atomic structure of highly strained BiFeO3 thin films

We determine the atomic structure of the pseudotetragonal T phase and the pseudorhombohedral R phase in highly strained multiferroic BiFeO(3) thin films by using a combination of atomic-resolution scanning transmission electron microscopy and electron energy-loss spectroscopy. The coordination of the Fe atoms and their displacement relative to the O and Bi positions are assessed by direct imaging. These observations allow us to interpret the electronic structure data derived from electron energy-loss spectroscopy and provide evidence for the giant spontaneous polarization in strained BiFeO(3) thin films.     

Stress-induced phase transformations in ⟨001⟩-oriented Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 crystals: Bilinear coupling of ferroelastic strain and ferroelectric polarization

The strain-stress ( k- σ) response characteristics of d001 -oriented crystals of Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 have been investigated. A stress-induced ferroelastic transformation has been observed under uniaxial stress. The k- σresponse was fitted with a Landau phenomenological model based upon a bilinear gradient coupling between ferroelastic strain and ferroelectric polarization.     

Field-Induced Spin-Modulated Transitions in Epitaxial (001) BiFeO3 Films

Physics of the Solid State - Alterations in the ground state of thin (001) films of a BiFeO3 (BFO)-type multiferroic in a magnetic field are studied theoretically, with changes in the energy of...     

不同生长条件下BiFeO_3薄膜的显微拉曼光谱研究

使用显微拉曼散射技术观察了用脉冲激光沉积方法(PLD)在不同的生长条件下制备的BiFeO3薄膜的拉曼光谱,并且研究了薄膜的结构以及不同的生长条件对薄膜的结构和微结构的影响。与BiFeO3粉末的拉曼光谱比较,我们发现在N2气氛中,在LaNiO3缓冲层上沉积的BiFeO3薄膜具有单一的三方相和最完整的晶格结构。     

Mn掺杂对多铁性BiFeO_3薄膜铁电性能以及漏电流的影响

本文利用脉冲激光沉积技术在(LaAlO3)0.3(Sr2AlTaO6)0.7(001)衬底上生长了BiFe1-xMnxO3(x=0～5%)外延薄膜,研究了Mn掺杂对BiFeO3(BFO)薄膜结构、铁电特性和漏电流的影响.XRD和SEM结果表明薄膜具有良好的结晶质量.漏电流测量显示Mn掺杂有效地减小了BFO薄膜的漏电流密度,因而在室温下5%Mn掺杂的BFO薄膜能够获得饱和的电滞回线.XPS分析证明,Mn掺杂改善BFO薄膜性能的可能原因在于其极大地减少了BFO薄膜中的Fe2+离子.     

SrTiO3(110) thin films grown directly on different oriented silicon substrates

We have grown (110)-oriented SrTiO₃ (STO) thin films on silicon without any buffer layer, by means of pulsed laser deposition technique. The crystal structures of the grown films were examined by X-ray diffraction analysis including θ–2θ scan and rocking curve as well as Laue diffraction methods. STO films with single (110) out-of-plane orientation were formed on all (100), (110) and (111)-oriented Si substrates. The in-plane alignments for the epitaxial STO films grown directly on Si (100) were found as STO[001]//Si[001] and STO[11̄0]//Si[010]. The results should be of interest for better understanding of the growth of perovskite oxide thin films on silicon wafers. PACS 77.55.+f; 68.55.JK; 81.15Fg     

Local weak ferromagnetism in single-crystalline ferroelectric BiFeO3

Polarized small-angle neutron scattering studies of single-crystalline multiferroic BiFeO(3) reveal a long-wavelength spin density wave generated by ～1° spin canting of the spins out of the rotation plane of the antiferromagnetic cycloidal order. This signifies weak ferromagnetism within mesoscopic regions of dimension 0.03 microns along [110], to several microns along [111], confirming a long-standing theoretical prediction. The average local magnetization is 0.06 μ(B)/Fe. Our results provide an indication of the intrinsic macroscopic magnetization to be expected in ferroelectric BiFeO(3) thin films under strain, where the magnetic cycloid is suppressed.     

Synthesis and characterization of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>–Ba<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>TiO<Subscript>3</Subscript> magnetoelectric composites with dielectric and magnetic properties

Composite structures consisting of (001)-oriented SrTiO₃ (STO)/La_0.7Sr_0.3MnO₃ (LSMO) films of 30 nm thickness, grown on an (001) Pb(Mg_1/3Nb_2/3)TiO₃– 28 mol.% PbTiO₃ piezoelectric relaxor-ferroelectric single-crystalline wafer were investigated by means of Wide-Angle X-ray Diffraction (WAXRD) in situ under influence of a d.c. electric field with strength E up to ±18 kV/cm. The WAXRD measurements of the films and substrate reflection profiles resulted in a determination of the strain s in the films and the substrate separately. The strained state of the STO/LSMO films is effectively controlled by a huge converse piezoelectric effect of the PMN-PT substrate. The coefficients of coupling between electric-field-induced out-of-plane strain in the films and in the substrate for the composite system STO/LSMO/PMN-PT are obtained.     

The crystalline orientation and ferroelectric properties of Bi3.25La0.75Ti3O12 thin films mediated by the intermediate layer of LaNiO3

c-Axis-oriented and (1 1 7)-oriented Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films are successfully controlled by the intermediate layer of LaNiO₃ (LNO) with chemical solution deposition (CSD), respectively. X-ray diffraction (XRD) demonstrates that the structure and orientation of LNO thin films have a strong effect on the orientation of BLT thin films. Scanning electron microscopy suggests that BLT thin films on LNO electrode exhibit crack-free, uniform size grains and dense microstructure. A crystalline orientation dependent remanent polarization is observed in BLT thin films, and it is found that the remanent polarization (2P_r) of (1 1 7)-oriented films is larger than that of c-axis-oriented films. Our research directly demonstrates that the vector of the main spontaneous polarization in these layered perovskite materials (BLT) is along a-axis.     

Tuning the growth orientation of NiFe2O4 films by appropriate underlayer selection

We have grown NiFe₂O₄ thin films on (001)-oriented SrTiO₃ and (001)-oriented Pt underlayers. Although these two templates present a similar lattice mismatch with NiFe₂O₄ (about -6%), the ferrite grows cube-on-cube with a (001) orientation on SrTiO₃ and (111)-textured on Pt, with four different in-plane variants. This evidences that the interface energy between NiFe₂O₄ and Pt or SrTiO₃ is the key parameter ruling the film texture, while the elastic energy appears as a second-order factor. We compare the structural and magnetic properties of these two films and discuss possible applications of our findings. PACS 68.55.Jk; 75.50.Gg     

Strain control of domain-wall stability in epitaxial BiFeO3 (110) films

We have studied the stability of domains and domain walls in multiferroic BiFeO3 thin films using a combination of piezoelectric force microscopy and phase-field simulations. We have discovered that a film-substrate misfit strain may result in a drastically different thermodynamic stability of two parallel domain walls with the same orientation. A fundamental understanding of the underlying physics, the stress distribution in a domain structure, leads to a novel approach to control the ferroelastic domain stability in the multiferroic BiFeO3 system.     

Pulsed laser deposition of ZnO as conductive buffer layer of (001)-LiNbO3 thin films

1 (LO) mode peak at 579 cm^-1, indicating oxygen deficiency in the films. Significant dependence of the electrical property for the films on substrate temperature was shown. The I–V relation of the films exhibited non-ohmic behavior. Whereas the films deposited above 500 °C showed a metal-like property, at a lower substrate temperature semiconducting thin films were achieved. The (001)-oriented LiNbO₃/ZnO heterostructure was successfully prepared on quartz fused and (001) sapphire plates. Received: 6 April 1998/Accepted: 26 May 1998     

Transition of crystalline orientation of yttria-stabilized zirconia films grown by pulsed laser deposition

We have synthesized 5 wt. %-yttria-stabilized zirconia (YSZ) films on stainless steel and glass substrates by pulsed laser deposition (PLD) at substrate temperatures between 60 and 600 °C and oxygen pressures from 0.1 to 1000 mTorr. While highly (101)-oriented tetragonal YSZ films are obtained at intermediate oxygen pressures, near complete (001)-oriented tetragonal films are achieved at lower oxygen pressures, with a full width at half maximum X-ray peak of only 0.4°. The results can be attributed to surface energy-induced orientation; that is, the lowest surface energy plane of the YSZ films changes from (101) plane at intermediate pressures to the (001) plane at lower oxygen pressures due to surface relaxation. PACS 81.15.Fg; 83.85.Hf; 68.55.jm     

Microstructure and strain analysis of GaN epitaxial films using in-plane grazing incidence x-ray diffraction

This paper investigates the major structural parameters,such as crystal quality and strain state of (001)-oriented GaN thin films grown on sapphire substrates by metalorganic chemical vapour deposition,using an in-plane grazing incidence x-ray diffraction technique.The results are analysed and compared with a complementary out-of-plane xray diffraction technique.The twist of the GaN mosaic structure is determined through the direct grazing incidence measurement of (100) reflection which agrees well with the result obtained by extrapolation method.The method for directly determining the in-plane lattice parameters of the GaN layers is also presented.Combined with the biaxial strain model,it derives the lattice parameters corresponding to fully relaxed GaN films.The GaN epilayers show an increasing residual compressive stress with increasing layer thickness when the two dimensional growth stage is established,reaching to a maximum level of-0.89 GPa.     

Tetragonal structure of thin nickel films on Cu(001)

The crystallographic structure of thin Ni films deposited on Cu(001) has been studied using Surface Extended X-ray Absorption Fine Structure (SEXAFS). Taking advantage of the linear polarization of the synchrotron radiation, we have shown that Ni adopts the Cu lattice parameter parallel to the interface. This lateral expansion induces a longitudinal compression of the unit cell, leading to a face centered tetragonal structure of the Ni films from 3 to 10 monolayers. The temperature dependence of the EXAFS oscillations has allowed to measure strain inside the Ni layers. Received 22 December 1998     

Oxygen deficiency dependence of magnetic and magnetoresistive properties for La0.5Sr0.5CoO3-x thin films prepared by pulsed laser deposition

The complete (001)-oriented thin films of La_0.5Sr_0.5CoO_3-x (LSCO) are deposited on (001) SrTiO₃ substrates by pulsed laser deposition under reduced oxygen pressure. It is revealed that the c axis of the film stretches with depleting oxygen. The magnetic, electrical, and magnetoresistive properties of the films are characterized by means of various techniques. Significant dependence of these properties on oxygen deficiency in the films is demonstrated, with enhanced magnetoresistance recorded for the samples deposited over a wide range of reduced oxygen pressure. Received: 9 July 1998 / Accepted: 15 January 1999 / Published online: 31 March 1999