BiFeO3 Single Crystals Research Articles

Neutron diffraction study of theBiFeO3 spin cycloid at low temperature

The reported observation of two anomalies in the intensity of the magnon Raman peaks ofBiFeO3 at 140 and 200 K (Singh et al 2008 J. Phys.: Condens. Mater 20 252203; Cazayous et al 2008 Phys.Rev. Lett. 101 037601) led to the hypothesis that such anomalies might originate from a spinreorientation transition. In order to test this hypothesis, we have used temperature-dependentneutron diffraction to track the evolution of the magnetic configuration in single crystals ofBiFeO3. Our results indicate that there is no average reorientation of the spins. This suggests thatthe magnon anomalies may instead be related to the freezing of modes that do not alter theaverage projection of the spins over the plane of the cycloid, as also reported for multiferroicTbMnO3 (Senff et al 2006 J. Phys.: Condens. Mater 18 2069).

High-field study of multiferroic BiFeO3

We studied the magnetization (M) and electric polarization (P) in single crystals of BiFeO3 with a unipolar ferroelectric domain in pulsed high magnetic fields up to 55 T. At low temperatures, the application of magnetic fields causes sharp changes in M and P at around 18 T, which can be ascribed to the magnetic transition from a cycloidal to a canted antiferromagnetic state. Measurements of P up to 600 K suggest that this transition disappears as the temperature approaches the Náel temperature. These results indicate that the field-induced change in P originates from the P component generated by the spiral magnetic ordering in BiFeO3. Furthermore, we found that the microscopic magnetoelectric coupling constant of BiFeO3 is greater than those of typical multiferroic materials such as TbMnO3. This fact indicates strong coupling between the magnetic and dielectric properties of BiFeO3.

Second harmonic generation microscopic observations of a multiferroic BiFeO3 single crystal

Anisotropy of the optical second harmonic generation (SHG) in a multiferroic BiFeO3 single crystal is observed by an SHG microscope. Polarization dependences of SH intensities are measured in several polarization configurations of the incident and SH waves to determine the magnitude and sign of polar i-type SHG tensor components. To obtain proper values of these SHG tensors, the effects of the interference term and wave number mismatch are taken into account in these analyses. d24 component is 15 times larger than d32 and this result is essentially different from the thin film case.

Possible interplay between a two phonon mode and high energy magnetic excitations in BiFeO3

We have performed Raman measurements on high energy excitations in BiFeO3 single crystals as a function of both temperature and laser excitation lines. A strong feature observed at 1250 cm-1 in the Raman spectra has been previously assigned to two phonon overtone. This peak exhibits an unusual frequency shift with the laser lines and the temperature dependence of its Fano lineshape shows two singularities at 150 K and 200 K which can be related to magnetic excitations. In the same energy range, we have also identified the two-magnon excitation with a temperature dependence very similar to the one measured for the one-magnon modes.

Centimeter-size BiFeO3 single crystals grown by flux method

A narrow part of Fe2O3–Bi2O3 phase diagram was re-investigated in order to elaborate single crystals of the multiferroic BiFeO3 (BFO). Centimeter-size single crystals were successfully obtained by flux method, and present a preferred growth direction. X-ray diffraction studied have highlighted that the growth direction is along the polar axis [111] r of the structure. The stability of BFO versus temperature (reversible ferroelectric transition followed by multiple irreversible decompositions) is discussed in the light of Differential Scanning Calorimetry (DSC) analysis performed between 25 and 1400°C.

Crystallographic, magnetic, and ferroelectric structures of bulklike BiFeO3 thin films

Bi Fe O 3 thin films deposited on (111)-SrTiO3 substrates present both structural (rhombohedral) and multiferroic properties similar to bulk. The structural relaxation occurring when thickness increases and the low critical thickness possibly indicate different defect types and distribution as compared to (001)-BiFeO3, which may influence multiferroic domain dynamics. The same G-type antiferromagnetic order as in bulk is observed by neutron diffraction, with a superimposed weak ferromagnetic moment (10emu∕cm3) showing a planar anisotropy of 5kJ∕m3. When doped with 5% Mn these films also exhibit a polarization of 95μC∕cm2 along the [111] direction at room temperature, similar to high-quality BiFeO3 single crystals.

Electric field control of the magnetic state in BiFeO3 single crystals

Single crystals of multiferroic BiFeO3 were investigated using neutron scattering. Application of an electric field reversibly switches ferroelastic domains, inducing changes in the magnetic structure which follows rotation of the structural domains. In addition, electric fields can be used to control the populations of the equivalent magnetic domains within a single ferroelastic domain, possibly via field-induced strain.

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.

10.1007/s11700-008-1001-5

BiFeO3 thin oriented films are grown on SrTiO3(001) and ZrO2(Y2O3)(001) substrates by metalloorganic chemical vapor deposition (MOCVD) in the temperature range T= 500–800°C. Iron dipivaloylmethanate Fe(thd)3 and triphenylbismuth Bi(C6H5)3 are used as volatile precursors. It is shown that the high thermal stability of Bi(C6H5)3 leads to significant deviations of the Bi: Fe ratio in the film from their ratio in the precursor flow. An increase in the residence time of the precursor flow in the reactor makes this ratio close to unity. The film-substrate epitaxial relations are determined. It is found that the film orientation on the ZrO2(Y2O3)(001) substrate changes with increasing temperature. Optical second harmonic generation (SHG) measurements show the presence of ferroelectric ordering in the films. A significant decrease in the Curie temperature due to epitaxial stress is found. At the same time, the magnetization noticeably increases as compared to a BiFeO3 single crystal.

Two-phonon coupling to the antiferromagnetic phase transition in multiferroic BiFeO3

A prominent band centered at ∼1000–1300cm−1 and associated with resonant enhancement of two-phonon Raman scattering is reported in multiferroic BiFeO3 thin films and single crystals. A strong anomaly in this band occurs at the antiferromagnetic Neel temperature, TN∼375°C. This band is composed of three peaks, assigned to 2A4, 2E8, and 2E9 Raman modes. While all three peaks were found to be sensitive to the antiferromagnetic phase transition, the 2E8 mode, in particular, nearly disappears at TN on heating, indicating a strong spin-two-phonon coupling in BiFeO3.

Hydrothermal synthesis of perovskite-type BiFeO3

Single crystals of BiFeO3 were prepared by hydrothermal synthesis using hydrate sodium bismuth oxide (NaBiO3·nH2O) and iron nitrates (Fe(NO3)3·9H2O) in the potassium hydroxide solutions. The size of BiFeO3 single crystals increased with the reaction temperature and duration. Single crystals of sillenite-type Bi12.5Fe0.5O19.5 and Bi2Fe4O9 were also obtained depending on the conditions. From the chemical analysis, the chemical composition of the crystals was deduced to be Bi0.96K0.03FeO3. Although the crystals contained a small amount of potassium, the lattice parameters of the crystals agreed well with the pure BiFeO3 and the Curie temperature was not affected by this replacement.

Infrared phonon dynamics of a multiferroicBiFeO3single crystal

We discuss the first infrared reflectivity measurement on a BiFeO3 single crystal between 5 K and room temperature. The 9 predicted ab-plane E phonon modes are fully and unambiguously determined. The frequencies of the 4 A1 c-axis phonons are found. These results settle issues between theory and data on ceramics. Our findings show that the softening of the lowest frequency E mode is responsible for the temperature dependence of the dielectric constant, indicating that the ferroelectric transition in BiFeO3 is soft-mode driven.

Electric field effect on BiFeO3 single crystal investigated by Raman spectroscopy

Micro-Raman spectroscopy has been used to study lattice dynamics associated with the ferroelectric domains of a BiFeO3 single crystal at low temperature. The phonon assignment shows a large frequency splitting between the transverse and longitudinal components of the A1 phonon mode related to the Bi–O bonds in contrast with thin films where the splitting is negligible. Applying an external electric field induces frequency shifts of the low energy modes related to the Bi–O bonds. These softenings are due to a tensile stress via the piezoelectric effect. The authors give estimates of the phonon deformation potentials.

Very large spontaneous electric polarization in BiFeO3 single crystals at room temperature and its evolution under cycling fields

Electric polarization loops are measured at room temperature on highly pure BiFeO3 single crystals synthesized by a flux growth method. Because the crystals have a high electrical resistivity, the resulting low leakage currents allow the authors to measure a large spontaneous polarization in excess of 100μCcm−2, a value never reported in the bulk. During electric cycling, the slow degradation of the material leads to an evolution of the hysteresis curves eventually preventing full saturation of the crystals.

Features of the spontaneous polarization of BiFeO3 single crystals

Features of the spontaneous polarization of BiFeO3 single crystals

Discovery of the linear magnetoelectric effect in magnetic ferroelectric BiFeo3in a strong magnetic field

Abstract The discovery of a new magnetic field induced phase transition between the space-modulated spin structure (SMSS) and the homogeneous antiferromagnetic state (HAFS) is reported. This phase transition is accompanied by large magnetoelectric (ME) anomalies. It is well known that BiFeO, shows the ferroelectric ordering (T c = 1083 K) and antiferromagnetic ordering (T N = 673 K). ME properties of BiFeO3 so far remain a challenging experimental and theoretical problem. While the crystal symmetry of BiFeO3 allows a linear ME effect, it cannot be observed because the antiferromagnetic order of BiFeO3 is SMSS (cycloid). We show that there is a Lifshits invariant, which decreases the ground state energy and induces the SMSS. In a high magnetic field the energy of the HAFS decreases more rapidly than the energy of the SMSS. Hence the field induced phase transition between SMSS and HAFS is possible. We stress that a large variation of the ME effect is expected at the transition due to a rising linear ME contribution. To study the breaking of the SMSS by a magnetic field we measured the ME effect induced by a pulsed field up to 280 kOe at T = 10–180 K. Electric polarization components were measured with the help of electrodes attached to the faces of a BiFeO3 single crystal. The measured dependence of P(H) is quadratic at H < H, and has a jump at H = H c when the SMSS is evidently destroyed. The critical field H c is of the order 200 kOe. These conclusions agree well with the theoretical views.