Multiferroic BiFeO3 Thin Films Research Articles

Effect of substrates on the structure and optical properties of polycrystalline BiFeO3 thin films

Multiferroic BiFeO3 thin films were prepared on Pt/TiO2/SiO2/Si and ITO/glass substrates by sol–gel preparation method. Pure perovskite phase and uniform surface topography have been verified by X-ray diffraction and atomic force microscopy, respectively. In addition, optical property measured by spectroscopic ellipsometer indicated that the absorptivity of both BiFeO3 thin films was enhanced with increasing photo energy after an illumination among 250–1000nm. Tauc calculation indicated that the direct band gap was 2.92eV for the film on Pt/TiO2/SiO2/Si, which is about 0.4eV lower than that of the film on ITO/glass.

Grain size and stoichiometry control over RF-sputtered multiferroic BiFeO3 thin films on silicon substrates

This work reports the morphological and chemical characterization of multiferroic BiFeO3 polycrystalline thin films grown on Si(111) by RF-sputtering. Results are shown for a large set of samples and a wide array of experimental techniques, including imaging (atomic/piezoresponse force microscopy) and spectroscopic (μ-Raman, X-ray photoemission, X-ray diffraction) probes. Through growth and post-growth annealing treatment, a fine control over stoichiometry, grain size, grain orientation, crystal order and surface roughness is achieved. In particular, the grain size can be tailored from nanocrystals to large micrometric plates as a function of the annealing temperature. For the optimal stoichiometric sample, an additional X-ray absorption and magnetic circular dichroism analysis has been carried out, which provides high quality spectra comparable with epitaxial films and further proves the expected strong local antiferromagnetic order.

Microstructural and optical properties tuning of BiFeO3 thin films elaborated by magnetron sputtering

Multiferroic BiFeO3 thin films were deposited using a conventional RF magnetron sputtering technique on Si substrate. The influence of the microstructure of BiFeO3 thin films on their optical properties was investigated through an extensive characterization. Using X-ray diffraction and atomic force microscopy, the structure and crystalline quality of the films were studied with regard to the effect of oxygen partial pressure during the deposition and to post annealing treatment. Stoichiometric films with the crystallographic perovskite structure required for multiferroic behaviour were obtained. It is demonstrated that films annealed at 700 °C in oxygen at atmospheric pressure are highly (100) perovskite orientated and free of the undesired phases. This perovskite phase is observed only for a narrow range of oxygen partial pressure The refractive index and extinction coefficient were obtained using variable angle spectroscopic ellipsometry. The optical characteristics of the films are directly correlated to their microstructural properties. Ellipsometry analysis showed that we can tune the optical constants of BiFeO3 films by controlling the oxygen partial pressure during deposition.

Local Piezoresponse and Thermal Behavior of Ferroelastic Domains in Multiferroic BiFeO3 Thin Films by Scanning Piezo-Thermal Microscopy

A dual probe, i.e., high resolution scanning piezo-thermal microscopy, is developed and employed to characterize the local piezoresponse and thermal behaviors of ferroelastic domains in multiferroic BiFeO3 thin films. Highly inhomogeneous piezoelectric responses are found in the thin film. A remarkably local thermal transformation across ferroelastic domain walls is clearly demonstrated by the quantitative 3ω signals related to thermal conductivity. Different polarization oriented ferroelastic domains are found to exhibit different local thermal responses. The underlying mechanism is possibly associated with the inhomogeneous stress distribution across the ferroelastic domain walls, leading to different phonons scattering contributions in the BiFeO3 thin film.

Enhanced photoresponse in BiFeO3/SrRuO3 heterostructure

We report photovoltaic effect and photo response in multiferroic BiFeO3 (BFO) thin films. Pure phase of polycrystalline BFO thin films have been deposited on SRO/Pt/TiO2/SiO2/Si substrates by RF sputtering. The as grown BFO thin films showed good ferroelectric properties and antiferromagnetic characteristics at room temperature. The diode like characteristics at the interface of BFO/SRO thin films were observed through current–voltage measurements. The open circuit voltage (Voc) and short circuit current density (Jsc) were measured as ∼0.08V and∼−63μA/cm2 respectively with illumination of white light density ∼1kW/m2. The photo response at zero bias voltage of BFO capacitor showed a rapid increase of the photocurrent which saturated at a value∼−63μA/cm2 followed by a rapid decrease with negligible delay to reach its original state during periodic on and off state of the light source. This high sensitivity makes it possible for the BFO films to be applied in a photosensitive capacitor.

Optimization of the multiferroic BiFeO3 thin films by divalent ion (Mn, Ni) co-doping at B-sites

Multiferroic BiFe0.96−xMn0.04NixO3 (BFMNO) thin films were prepared on fluorine doped tin oxide (FTO) substrates by the chemical solution deposition (CSD) method. X-ray diffraction, Rietveld refinement and Raman spectra reveal the structural transition from rhombohedral (R3c:H) to the biphasic structure (R3c:H+R3m:R) with Mn/Ni co-doping. The X-ray photoelectron spectroscopy analysis confirms the coexistence of Fe3+ and Fe2+ ions in the thin films. The lower leakage current density (8.1×10−6A/cm2 at 300kV/cm) via co-doping is obtained. The two-phase coexistence of BFMN3O gives rise to the superior ferroelectric (2Pr=226.4μC/cm2) and the enhanced ferromagnetic properties (Ms=15.9emu/cm3).

Strain modulated optical properties in BiFeO3 thin films

Spectroscopic ellipsometry was used to investigate the strain-dependent optical properties of BiFeO3 thin films. At room temperature, the compressively strained BiFeO3/LaAlO3 thin films show the largest band gap of about 3.12 eV. It redshifts to 2.75 eV for the tensile strained BiFeO3/NdScO3 thin films. With increasing temperature, observable anomalies in the band gap for all strained thin films near 640 K indicate that antiferromagnetic transition temperature is independent of strain and close to its bulk value, which are in good agreement with the first-principles calculations. These results further suggest a complex nature of charge-spin coupling in multiferroic BiFeO3 thin films.

Epitaxial growth of multiferroic BiFeO3 thin films with (101) and (111) orientations on (100) Si substrates

Multiferroic (101)- and (111)-oriented BiFeO3 thin films were grown on heteroepitaxial substrates of (110) SrRuO3/(100) YSZ/(100) Si and (111) SrRuO3/(111) Pt/(100) YSZ/(100) Si, respectively, by pulsed laser deposition. X-ray diffraction θ–2θ scans, ϕ scans, and pole figures showed the well-developed out-of-plane and in-plane orientations of the epitaxial BiFeO3 films. The (111) BiFeO3 films showed ∼3/2 times higher remanent polarization (58.9 μC/cm2) than that of the (101) BiFeO3 films (44.3 μC/cm2), thus revealing the ferroelectric anisotropy. The (101) BiFeO3 films exhibited a good fatigue resistance up to 4.6 × 107 cycles, whereas the (111) BiFeO3 films proved to be vulnerable to fatigue, which is associated with the polarization switching path.

Raman spectroscopy of nanocrystalline Mn-doped BiFeO3 thin films

Enhancement of magnetic and electric properties in multiferroic Mn-substituted BiFeO3 thin films (BiFe1− x Mn x O3 with x = 0.00–0.05) grown via the low-temperature chemical solution deposition process on low-cost substrates like indium tin oxide (ITO)/glass and silicon is reported in this work. A detailed structural analysis by high-resolution X-ray diffraction and Raman spectroscopy indicates that the grown films exhibit rhombohedral distorted perovskite structure without any impure phase. Raman spectroscopic analysis shows all 13 Raman active (4A1 + 9E) phonon modes corresponding to distorted rhombohedral BFO R3c structure. BFO thin film with 5% Mn substitution showed maximum saturation and remnant magnetisation (M s = 16.70 emu/g and M r = 2.58 emu/g) and remarkably reduced leakage current due to distortion in the spin cycloid structure and suppression of oxygen vacancies due to partial replacement of Fe3+ ion by Mn3+ ion, respectively. These enhanced multifunctional properties highlight the potential applications of BFMO system in thin film devices which can couple magnetic and ferroelectric order parameters.

Ba and Ti co-doped BiFeO3 thin films via a modified chemical route with synchronous improvement in ferroelectric and magnetic behaviors

A modified chemical route via the layer-by-layer alternative deposition of BaTiO3 and BiFeO3 two compositions followed by the solution processing has been developed to prepare Ba and Ti co-doped multiferroic BiFeO3 thin films. The Ba and Ti co-doped BiFeO3 films crystallize in single perovskite phase and are composed of uniformly distributed grains of tens of nanometers as evidenced by X-ray diffraction and scanning electron microscopy images, respectively. But the films show local composition fluctuation with further high-resolution transmission electron microscope analysis, which leads to increased inner strain and thus a phase transition from R3c to P4mm occurs as Ba and Ti contents reach 50 mol. %. Accompanying the structural phase transition, a saturated P-E hysteresis loop with Pr ∼ 27 μC/cm2 and Ps ∼ 87 μC/cm2 and, moreover, a transition from antiferromagnetic to ferromagnetic state with Mr ∼ 1.46 emu/cm3 and Hc ∼ 1000 Oe have been observed. A strain induced structural change is proposed to explain for the observed synchronous enhancement in ferroelectric and magnetic properties, and elastic energy calculation is carried out to verify the viewpoint on magnetic behavior. The modified multiferroic behaviors of Ba, Ti co-doped BiFeO3 film imply an improved magnetoelectric coupling, which makes the material good candidate as memory devices and sensors.

Ultrafast Photostriction in Thin Film Bismuth Ferrite and its Correlation to Electronic Dynamics

ABSTRACTA series of laser pump, x-ray probe experiments show that above band gap photoexcitation can generate a large out-of-plane strain in multiferroic BiFeO3 thin films. The strain decays in a time scale that is the same as the photo-induced carriers measured in an optical transient absorption spectroscopy experiment. We attribute the strain to the piezoelectric effect due to screening of the depolarization field by laser induced carriers. A strong film thickness dependence of strain and carrier relaxation is also observed, revealing the role of the carrier transport in determining the structural and carrier dynamics in complex oxide thin films.

Polarization-tuned diode behaviour in multiferroic BiFeO3 thin films

Asymmetric rectifying I–V behaviour of multiferroic BiFeO3 (BFO) thin films grown on transparent ITO-coated glass was quantitatively studied as a function of ferroelectric polarization. Different polarized states were established by unipolar or bipolar poling with various applied electric fields. The effects of polarization relaxation and fatigue on the currents were also investigated. We found that the conduction currents and the associated rectifications were controlled by the amplitude and direction of the polarization. We clearly observed the linear dependence of the current on the polarization. It is suggested that the space-charge-limited conduction and the charge injection at the Schottky interface between the film and the electrodes dominate the current. The electrically controlled rectifying behaviour observed in this study may be useful in nonvolatile resistance memory devices or tunable diodes.

Effect of annealing atmosphere on microstructural and photoluminescence characteristics of multiferroic BiFeO3 thin films prepared by pulsed laser deposition technique

The effect of annealing atmosphere on microstructural and photoluminescence characteristics of multiferroic BiFeO3 (BFO) thin films deposited by pulsed laser deposition (PLD) technique is reported. The films annealed in oxygen environment showed improved microstructure and photoluminescence (PL) characteristics. The PL spectra of oxygen-annealed BFO thin films at room temperature show a strong emission in the blue region. A plot of (αE)2 vs. photon energy (E) (α-absorption coefficient) and the linear extrapolation to (αE)2=0 indicates a direct gap at 2.69±0.02 eV, which is in agreement with the previous reports. The results obtained in this study are accordingly expected to facilitate the understanding and optimization of BFO thin films for photovoltaic applications.

Domain structure and piezoelectric response in lanthanide rare earth-substituted multiferroic BiFeO3 thin films

The microstructure, domain structure and piezoelectric response in lanthanide rare-earth ion (Ln) substituted BiFeO3 (BFO) thin films are investigated systematically. Although most of the peaks in the x-ray patterns of the rare-earth-substituted BFO films are consistent with the rhombohedral crystal structure, the patterns of the Ce-, Eu- and Yb-doped samples indicate that the films contain a small amount of tetragonal impurity phase. The piezoelectric properties are enhanced due to the increased relative Bi–O displacement and the rotation of FeO6 octahedra. Meanwhile, the domain structures of BFO films become more homogeneous by inserting Ln ions. Furthermore, the piezoresponse properties are improved by introducing Ln ions, which is attributed to the variation of ionic strength of bands between Ln ions and O ions caused by chemical modification and a lower symmetry structure

A nonvolatile memory device made of a graphene nanoribbon and a multiferroic BiFeO3 gate dielectric layer

We demonstrate a field-effect nonvolatile random access memory (NVRAM) device made of a graphene nanoribbon (GNR) and a multiferroic epitaxial BiFeO3 thin film. The GNR and the source/drain electrodes were formed by position-controlled dip-pen nanolithography. The NVRAM device exhibited asymmetric hysteresis behavior originating from the combination of the p-type semiconducting behavior of the GNR and the ferroelectric hysteresis of the BiFeO3 layer. The memory window of the NVRAM device was significantly improved by a NH3 annealing process which changed the p-type GNR to n-type.

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 (TC) in BaTiO3. However, in multiferroic BiFeO3 thin films an unanticipated strain-driven decrease of TC 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 effect on the surface potential and nanoscale switching characteristics of multiferroic BiFeO3 thin films

The BiFeO3 films were deposited on the SrTiO3 (001) substrates via tuning the thickness of the SrRuO3 (SRO) bottom electrode by pulsed laser deposition. The macroscopic ferroelectric and dielectric properties were dramatically impacted by the various nanoscale domain structures for both films due to the tunable SRO thickness. The nanoscale domain switching behaviors for both films were investigated via piezoresponse force microscopy, and results suggest that the domain structure could be changed by tuning the strain state. The surface potential investigation indicates that strain helps increase data storage density and stability.

Nonlinearity in the high-electric-field piezoelectricity of epitaxial BiFeO3 on SrTiO3

The piezoelectricity of a multiferroic BiFeO3 thin film deviates from its low-field linear response in electric fields higher than 150 MV/m. Time-resolved synchrotron x-ray microdiffraction reveals a low-field piezoelectric coefficient of 55 pm/V and a steeper increase in strain at higher fields, with an effective piezoelectric coefficient of 86 pm/V. The strain reaches 2% at 281 MV/m, a factor of 1.3 higher than expected based on an extrapolation from low fields. The peak intensity of the BiFeO3 (002) Bragg reflection decreases throughout the high-electric-field regime, accompanied by increased diffuse scattering, consistent with lattice softening lattice near a field-induced phase transition.

Effect of Cr doping on the structure, optical and magnetic properties of multiferroic BiFeO3 thin films

Multiferroic BiCrxFe1−xO3 (BCFO) (0 ≤ x ≤ 0.12) thin films were fabricated on silicon substrates by sol–gel technique. The microstructure and properties of the films are characterized using X-ray diffraction, spectroscopic ellipsometry, micro-Raman spectrometry and a Modular Control system. The BCFO films are the rhombohedral structure with the Cr content up to 7%. Raman scattering spectra of the BCFO films demonstrate the transformation of structure with the Cr content exceeding 10%. The band gap of the BCFO films is from 2.53 to 2.82 eV with the Cr content being from 0 to 12%. The magnetization of the BCFO films is significantly enhanced with the increasing of the Cr content.

Thickness‐Dependent Dielectric, Ferroelectric, and Magnetodielectric Properties of BiFeO3 Thin Films Derived by Chemical Solution Deposition

Thickness is a very important parameter to control the microstructures, as well as physical properties in the multiferroic BiFeO3 thin films. In this article, BiFeO3 thin films with different thickness (from 210 to 830 nm) are fabricated by chemical solution deposition on the Pt/Ti/SiO2/Si (100) substrates to investigate the thickness effects systematically. The results show that the crystallization is improved, the dielectric constant is enhanced, and the leakage current is reduced with increasing thickness. On the other hand, the magnetization and magnetodielectric are decreased with thickness. The results show that the optimized thickness should be within the range of 400–600 nm to obtain optimized properties using chemical solution deposition processing, which will provide a useful guidance to prepare the multiferroic BiFeO3 thin films.