Polycrystalline BiFeO3 Films Research Articles

Dependence of ferroelectric and magnetic properties on measuring temperatures for polycrystalline BiFeO3 films

A multiferroic BiFeO(3) film was fabricated on a Pt/Ti/SiO(3)/Si(100) substrate by a chemical solution deposition (CSD) method, and this was followed by postdeposition annealing at 923 K for 10 min in air. X-ray diffraction analysis indicated the formation of the polycrystalline single phase of the BiFeO(3) film. A high remanent polarization of 89 microC/cm(2) was observed at 90 K together with a relatively low electric coercive field of 0.32 MV/cm, although the ferroelectric hysteresis loops could not be observed at room temperature due to a high leakage current density. The temperature dependence of the ferroelectric hysteresis loops indicated that these hysteresis loops lose their shape above 165 K, and the nominal remanent polarization drastically increased due to the leakage current. Magnetic measurements indicated that the saturation magnetization was less than 1 emu/cm(3) at room temperature and increased to approximately 2 emu/cm(3) at 100 K, although the spontaneous magnetization could not appear. The magnetization curves of polycrystalline BiFeO3 film were nonlinear at both temperatures, which is different with BiFeO3 single crystal.

Enhancement of ferroelectric and magnetic properties in BiFeO3 films by small amount of cobalt addition

Both the ferroelectric and magnetic properties of polycrystalline BiFeO3 films fabricated by chemical solution deposition were enhanced by adding small amounts of cobalt. Addition of 3at.% cobalt to BiFeO3 films increased the remanent polarization from 49to72μC∕cm2 and decreased the electric coercive field from 0.54to0.44MV∕cm. The ferroelectricity degraded when the cobalt concentration exceeded 9at.% due to the formation of the secondary phases of Bi2Pt. The saturation magnetization was drastically enhanced by the addition of cobalt up to 12at.%. This is because the magnetic moments are not canceled locally since the differences of magnetic moment between B-sites. The saturation magnetization decreased when the cobalt content exceeded 15at.%, thereby attributing to the formation of a nonmagnetic secondary phase of Bi2Pt. It is concluded that both ferroelectric and magnetic properties were enhanced, provided only small amount of cobalt were added to the films.

Annealing temperature effect on ferroelectric and magnetic properties in Mn-added polycrystalline BiFeO3 films

We fabricated 5 at.% Mn-added polycrystalline BiFeO3 films and investigated the annealing temperature effect on structural, ferroelectric and magnetic properties. In the x-ray diffraction patterns, only the diffraction peaks due to the BiFeO3 structure were observed and no secondary phase could be observed at annealing temperatures between 773 and 923 K. Adding Mn suppressed the leakage current density in the high electric field region when compared to pure BiFeO3 films. The conduction mechanism of the Mn-added BiFeO3 films was dominated by Ohmic conduction. Remanent polarization of the Mn-added polycrystalline BiFeO3 films for an applied electric field of approximately 1.5 mV/cm was 63 μC/cm2 for the specimen annealed at 773 K and 46 μC/cm2 for the specimen annealed at 923 K, although the remanent polarization still exhibited a tendency to increase with an increase in the electric field. Spontaneous magnetization was obtained at high annealing specimens. This study revealed that the annealing temperature strongly affected the ferroelectric and magnetic properties in Mn-added polycrystalline BiFeO3 films. In addition, by optimizing the annealing temperature, we realized multiferroics coexistent with spontaneous magnetization and spontaneous polarization at room temperature in the Mn-added polycrystalline BiFeO3 film.

LEAKAGE CURRENT MECHANISM OF POLYCRYSTALLINE BiFeO3 FILMS WITH PT ELECTRODE

ABSTRACT Leakage current mechanism of the polycrystalline BiFeO3 film annealing at 923 K with Pt electrodes was discussed based on Schottky-emission conduction, Poole-Frenkel trap limited conduction, Fowler-Nordheim tunneling conduction and space charge limited current (SCLC). The leakage current mechanism at room temperature is as follows; Schottky-emission or Poole-Frenkel was the candidate leakage current mechanism at low electric field and then leakage current mechanism was changed to the SCLC at high electric field. When decreasing the measuring temperature, the leakage current mechanism at low electric field was Poole-Frenkel trap limited conduction, though the electric field region of the PF trap limited conduction was much broader than that of the room temperature.

Anomalous ferromagnetism in spray pyrolysis deposited multiferroic BiFeO3 films

Oriented polycrystalline BiFeO3 (BFO) films are deposited on single crystal LaAlO3 (001) substrates by nebulized spray pyrolysis technique at low temperature of ∼300°C. Annealing of the as deposited films in air and oxygen at ∼550°C for 1h is found to produce crystallization and BFO phase formation. X-ray diffraction results show rhombohedral disordered perovskite structure with space group R3m. As revealed by atomic force microscopy, grain size increases with oxygen annealing. Magnetic measurements show well-defined hysteretic loops at room temperature for both the films. However, oxygenation leads to dramatic increase in room temperature ferromagnetism as evidenced by large saturation magnetization of ∼250emu∕cm3. This enhanced magnetization is attributed to the presence of iron oxide rich nanoclusters due to oxygen annealing.

Application of weak ferromagnetic BiFeO3 films as the photoelectrode material under visible-light irradiation

Bi Fe O 3 films prepared by pulsed laser deposition on Pt∕TiO2∕SiO2∕Si substrates were studied as photoelectrode for water splitting. Under visible-light irradiation, the photocurrent intensity of the polycrystalline BiFeO3 film was found to double that of the amorphous one in a three-electrode cell. The incident photon to current conversion efficiency for the polycrystalline BiFeO3 electrode was approximately 16% at 350nm and 7% at 530nm at 1.5V (versus saturated calomel electrode). The ferromagnetism of the amorphous BiFeO3 film was an order of magnitude weaker than that of the polycrystalline one, supporting the “size effect” explanation for magnetic origin.

Structural, magnetic, and ferroelectric properties of multiferroic BiFeO3 film fabricated by chemical solution deposition

Polycrystalline BiFeO3 film has been fabricated by a chemical solution deposition on Pt∕Ti∕SiO2∕Si(100) substrates. A ferroelectric hysteresis loop showed a high remanent polarization of 47μC∕cm2 at room temperature. Leakage current density was on the order of 10−1A∕cm2 at 100kV∕cm, indicating the high leakage current density in the present BiFeO3 film. The leakage current mechanism could be considered as follows: Ohmic conduction at low electric field and Poole-Frenkel trap-assisted conduction appeared as the electric field increased, and space-charge-limited current started at a high electric field. Weak ferromagnetism was observed at room temperature, and magnetic coercivity increased to 0.5kOe with small remanent magnetization of 2emu∕cm3 at 10K. In order to investigate the magnetoelectric effect of the BiFeO3 film, the ferroelectric hysteresis loop was measured under the magnetic field of 5kG at room temperature.