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.     

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.     

Strain-induced polarization rotation in epitaxial (001) BiFeO3 thin films

Direct measurement of the remanent polarization of high quality (001)-oriented epitaxial BiFeO3 thin films shows a strong strain dependence, even larger than conventional (001)-oriented PbTiO3 films. Thermodynamic analysis reveals that a strain-induced polarization rotation mechanism is responsible for the large change in the out-of-plane polarization of (001) BiFeO3 with biaxial strain while the spontaneous polarization itself remains almost constant.     

Optical properties of BiFeO3 epitaxial thin films

The properties of neodymium-doped BiFeO₃ nanosized films on magnesium oxide single-crystal substrates are studied. The films are obtained using high-frequency sputtering with the aid of layered growth. The structural perfection of the films is analyzed using the X-ray diffraction. The transmission of the films with different thicknesses is studied in the wavelength interval 200–1100 nm. The spectra are processed with the aid of a dispersion formula for permittivity of a sum of oscillators with allowance for damping, so that direct and indirect transitions can be revealed. The absorption edges are estimated to be 2.81 and 2.78 eV for the direct transitions of the films with thicknesses of 14 and 60 nm, respectively.     

Optical properties of epitaxial BiFeO3 thin films

Optical properties of epitaxial BiFeO₃ thin films grown via pulsed-laser deposition on (110) DyScO₃ substrates have been investigated. Their near-normal spectroscopic reflectivity was measured in the spectral range 2 to 14 eV at room temperature, while spectroscopic ellipsometry in the spectral range 1–6 eV was measured in the temperature range from 300 to 775 K. The optical response functions have been calculated and a direct optical gap was determined varying from 2.75 to 2.70 eV in this temperature range.     

Strain effects on multiferroic BiFeO3 films

The field of multiferroics has experienced a rapid progress resulting in the discovery of many new physical phenomena. BiFeO₃ (BFO) compound, which is one of the few room-temperature single-phase multiferroics, has contributed subsequently to this progress. As a result, significant review articles have been devoted specifically to this famous system. This chapter is dedicated to the strain effects on the structure stability and property changes of BFO thin films. It is a short and non-exhaustive topical overview that may be seen as an invitation for interested readers to go beyond. There is a very active and prolific research in this field and we apologize to the authors whose relevant work is not cited here. After a short introduction, we will thus review the effect of strain on BFO films by describing the consequences on the structure and the phase transitions as well as on polar, magnetic and magnetoelectric properties.     

Periodic lattice distortions in epitaxial films of Fe(110) on W(110)

The epitaxial growth of Fe(110) on W(110) at 500 K is analyzed using LEED and AES. Frank-van der Merwe growth is established by AES. According to LEED, pseudomorphism occurs up to θ = 1.64, where every W atom of two topmost W layers is just covered by exactly one Fe atom. For 2?θ?9, characteristic reflection-multiplets are observed, symmetric about the basic Fe(110) reflections, which are interpreted in terms of periodic lattice distortions. The latter are caused by interaction with the misfitting W substrate.     

Mechanisms of exchange bias with multiferroic BiFeO3 epitaxial thin films

We have combined neutron scattering and piezoresponse force microscopy to show that the exchange field in CoFeB/BiFeO_{3} heterostructures scales with the inverse of the ferroelectric and antiferromagnetic domain size of the BiFeO3 films, as expected from Malozemoff's model of exchange bias extended to multiferroics. Accordingly, polarized neutron reflectometry reveals the presence of uncompensated spins in the BiFeO3 film at the interface with CoFeB. In view of these results, we discuss possible strategies to switch the magnetization of a ferromagnet by an electric field using BiFeO3.     

Strain analysis of epitaxial ultrathin films on Pt(111)

X-ray photoelectron diffraction (XPD) measurements have demonstrated that Ni ultrathin films on Pt(111) exhibit a single domain fcc stacking pseudomorphic to the substrate, with a consequent trigonal distortion of the Ni unit cell in order to accommodate the in-plane expansion of the Ni lattice parameter of about 11% with respect to its bulk value. We show that the amount of interlayer contraction and the strain energy resulting form the trigonal distortion are very well predicted by a strain analysis in the framework of simple linear elasticity theory. Strain analysis also allows to discuss in some detail the controversial case of Co growth on the same substrate. Finally, we discuss the dependence of the strain energy of the overlayer on the substrate crystal orientation and its effects on chemisorption experiments performed on heteroepitaxial monolayers (HML).     

Biaxial stress‐induced giant bandgap shift in BiFeO3 epitaxial films

Thickness‐dependent stress relaxation and its influence on the bandgap of BiFeO₃ have been investigated. BiFeO₃ films with 15 at% Bi excess allow the control of lattice mismatch‐induced biaxial stress up to 10.8 GPa on SrTiO₃(001) substrates. Heteroepitaxy of such films follows the Stranski–Krastanov growth mode, as suggested by island formation on the wetting layer accompanied with stress relaxation. The bandgap is linearly decreased with the in‐plane compressive stress, and a large stress coefficient of 67 meV/GPa was extracted. A 0.7 eV red‐shift of the bandgap was observed in the fully epitaxially strained film. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Raman spectroscopic investigations of epitaxial BiFeO3 thin films on rare earth scandate substrates

BiFeO₃ epitaxial thin films fabricated by pulsed laser deposition on different scandate substrates were investigated by means of Raman spectroscopy. We observed periodic changes in Raman position, full width at half maximum and intensity for some phonon modes as a function of the azimuthal angle ϕ. Further analysis revealed the possibility to assign the so far controversial discussed Raman modes at low wavenumbers (<250 cm⁻¹) through rotational Raman measurements at different azimuthal angles, which show high sensitivity to the aforementioned parameters. Furthermore, the ferroelectric/ferroelastic domain structure shown by piezo‐response force microscopy investigations of the samples was confirmed. Our results are supported by symmetry‐based calculations including the analysis of Raman scattering geometry as well as the dielectric function of BiFeO₃ in the infrared range. Copyright © 2015 John Wiley & Sons, Ltd.     

Lattice deformation in thin BiFeO3 and Ba0.8Sr0.2TiO3 epitaxial films on (001)MgO single crystalline substrates

Bi_0.98Nd_0.02FeO₃ (BNFO) and Ba_0.8Sr_0.2TiO₃ (BST) epitaxial films produced by rf cathode sputtering on (001)MgO substrates are studied by means of XRD. The thickness dependences of uniform lattice deformations and linear dislocation density, generated due to epitaxial stress relaxation, are plotted.     

Growth,structure, and thermal stability of epitaxial BaTiO3 films on Pt(111)

The growth of epitaxial ultrathin BaTiO₃ films upon rf magnetron sputter deposition on a Pt(111) substrate has been studied by scanning tunnelling microscopy, low-energy electron diffraction, and X-ray photoelectron spectroscopy. The BaTiO₃ films have been characterized from the initial stages of growth up to a film thickness of 4 unit cells. The deposited films develop a long-range order upon annealing at 1050 K in UHV. In the submonolayer regime a wetting layer is formed on Pt(111). Thicker films reveal a Stranski–Krastanov-like structure as observed with STM. By XPS a good agreement of the thin film stoichiometry with BaTiO₃ single crystal data is determined. Due to annealing at 1150 K BaTiO₃ forms large two-dimensional islands on the Pt(111) substrate. Different surface structures develop on the islands depending on the O₂ partial pressure during annealing.     

A simple method for the determination of strains in epitaxial films: application to Eu(110) deposited on a Nb(110) buffer

In order to determine the strain tensor in a 375 nm thick Eu(110) epitaxial thin film, we have developed a new method, based on the accurate determination of the lattice vectors by high resolution X-ray diffraction. We show that a biaxial strain model gives a good representation of the state of the strains field in the film.     

Distinct ferroelectric domain switching dynamics in epitaxial BiFeO3 (001) capacitors depending on the bias polarity

Understanding ferroelectric domain switching dynamics at the nanoscale is a great of importance in the viewpoints of fundamental physics and technological applications. Here, we investigated the intriguing polarity-dependent switching dynamics of ferroelectric domains in epitaxial BiFeO₃ (001) capacitors using transient switching current measurement and piezoresponse force microscopy. We observed the distinct behavior of nucleation and domain wall motion depending on the polarity of external electric bias. When applying the negative bias to the top electrode, the sideways domain wall motion initiated by only few nuclei was dominant to polarization switching. However, when applying the positive bias, most of domains started to grow from the pre-existed pinned domains and their growth velocity was much smaller. We suggest that the observed two distinct domain switching behavior is ascribed to the interfacial defect layer.     

High-quality epitaxial LaNiO3 thin films on SrTiO3(100) and LaAlO3(100)

LaNiO₃ thin films have been deposited by pulsed laser deposition on SrTiO₃(100) and LaAlO₃(100) substrates. The processing conditions have been investigated in order to optimize electrical resistivity, crystal quality, and surface morphology. Excellent properties are achieved at moderate substrate temperature and relatively low oxygen pressure, without the need for annealing. Thickness exerts an important influence on electrical transport, as the electrical resistivity increases quickly in films thicker than a few tens of nanometer. The surface of the films on LaAlO₃ is very flat in all the studied thickness range, but the films on SrTiO₃ develop a pattern of boundaries and even cracks as the thickness is higher. Below the critical thickness, high-quality epitaxial films with very smooth surface and low electrical resistivity are obtained under the optimum conditions of substrate temperature and oxygen pressure. The optimum processing conditions are different depending on the substrate, and control is especially critical in films deposited on SrTiO₃.