BiFeO3 Single Crystals Research Articles

Significant phonon assignments in multiferroic BiFeO3 single crystal

In the present work, the technique of Back Scattering Micro-Raman was employed at low and room temperatures in order to examine all the possible phonon vibration modes present in a crystal of BiFeO3(BFO). The observation & identification of all E (TO), E (LO), A1 (TO), and A1 (LO) normal modes was sucessfully accomplished. The low temperature Raman spectroscopy measurements showed two one-magnon branches, σ-magnon at 20.06 cm−1 and γ-magnon at 26.93 cm−1. The spectral characteristics of the three A1 (LO) modes were observed at roughly 176.63 cm−1, 225.93 cm−1 and 505.53 cm−1, exhibiting low intensity longitudinal optical modes. The observed peaks at 145.34 cm−1, 223.92 cm−1, 292.54 cm−1, and 552.18 cm−1 were identified and categorized as A1 (TO) modes. E (TO) peaks were observed at 73.30 cm-1, 131.94 cm-1, 239.35 cm-1, 265.04 cm-1, 278.45 cm-1, 349.95 cm-1, 374.69 cm-1, 441.04 cm-1, 522.99 cm-1. In addition, E(LO) peaks were observed at 80.76 cm-1, 175.03 cm-1, 241.59 cm-1, 275.42 cm-1, 346.92 cm-1, 371.34 cm-1, 419.38 cm-1, and 469.65 cm-1.

3D Reconstruction of Sawtooth 180° Tail‐to‐Tail Domain Walls in Single Crystal BiFeO3

AbstractPrevious studies of single crystal BiFeO3 have found a dense domain structure with alternating sawtooth and flat domain walls (DWs). The nature of these domains and their 3D structure has remained elusive to date. Herein, several sections taken at different orientations are used to examine the structure in detail, concentrating here on the sawtooth DWs using diffraction contrast transmission electron microscopy, electron diffraction, and aberration‐corrected scanning transmission electron microscopy (STEM). All DWs are found to be 180° type; the flat walls have head‐to‐head polarity while the sawtooth DWs are tail‐to‐tail with peaks elongated along the polar [111] axis, formed by neutral () DW facets and slightly charged facets with orientations close to () and (). The neutral DW facets are Ising type and very abrupt, while the charged DW facets have mixed Néel/Bloch/Ising character with a chiral nature and a width of about 2 nm.

High-quality ferroelectric Bi0.5K0.5TiO3‒BiFeO3 solid-solution single crystals grown under high-pressure oxygen atmosphere

Ferroelectric (Bi0.5K0.5)TiO3–BiFeO3 solid solutions are promising materials for high-temperature piezoelectric applications because of their high depolarization temperature. In this study, we demonstrate the successful growth of high-quality (Bi0.5K0.5)TiO3–BiFeO3 single crystals near the morphotropic phase boundary by a flux method. Crystal growth at a high oxygen pressure of 0.9 MPa drastically reduces leakage currents, which enables ferroelectric polarization switching and a piezoelectric response associated with a rotation of polar nanosized domains. This study contributes to further detailed analyses of the relationship between crystal structures and ferroelectric properties of the (Bi0.5K0.5)TiO3–BiFeO3 solid-solution system.

Boosting Piezoelectricity under Illumination via the Bulk Photovoltaic Effect and the Schottky Barrier Effect in BiFeO3.

Piezoelectricity is a key functionality induced by conversion between mechanical and electrical energy. Enhancement of piezoelectricity in ferroelectrics often has been realized by complicated synthetical approaches to host unique structural boundaries, so-called morphotropic phase boundaries. While structural approaches are well-known, enhancing piezoelectricity by external stimuli has yet to be clearly explored, despite their advantages of offering not only simple and in situ control without any prior processing requirement, but compatibility with other functionalities. Here, it is shown that light is a powerful control parameter to enhance the piezoelectric property of BiFeO3 single crystals. A series of measurements based on piezoresponse force microscopy and conductive atomic force microscopy, under illumination, reveal a locally enhanced effective piezoelectric coefficient, dzz , eventually showing almost a sevenfold increase. This phenomenon is explained with theoretical models by introducing the two main underlying mechanisms attributed to the bulk photovoltaic effect and Schottky barrier effect, involving the role of open-circuit voltage and photocharge carrier density. These results provide key insights to light-induced piezoelectricity enhancement, offering its potential for multifunctional optoelectronic devices.

Study of spin-phonon coupling in multiferroic BiFeO3 through Raman spectroscopy

Spin-phonon coupling in multiferroic BiFeO3 single crystal is studied through temperature dependant polarized Raman spectroscopy. The phonon hardening appears at 200 K and becomes stronger below ~135 K. Clear anomalous temperature dependence of the phonon hardening near the magnetic transitions evidence the presence of a spin-phonon coupling in BiFeO3. Different phonon modes show very distinct spin-phonon coupling behaviors. Possible interpretations of the anomalous of the phonon hardening are discussed. We suggest that the phonon hardening would be correlated to spin reorientation and partially influenced by strains of Bi vacancies.

Temperature dependent terahertz giant anisotropy and cycloidal spin wave modes in BiFeO3 single crystal**Project supported by the National Natural Science Foundation of China (Grant Nos. 61975110, 11674213, 61735010, and 11604202), the 111 Project, China (Grant No. D18014), the International Joint Lab Program supported by Science and Technology Commission Shanghai Municipality, China (Grant No. 17590750300), the Key Project supported by Science

THz time-domain spectroscopy (THz-TDS) is used to study the THz-optical properties of a single crystal bismuth ferrite BiFeO3 (BFO). It can be found that the anisotropy of BiFeO3 is strongly dependent on the temperature. A giant birefringence up to around 3.6 is observed at 1 THz. The presence of a spatially modulated cycloidal antiferromagnetic structure leads to spin cycloid resonances (SCR) ψ and Φ, corresponding to the out-of-plane and in-plane modes of the spin cycloid, respectively. We distinguish the SCR with respect to their response to orthogonal polarizations of the electric fields of the incident THz beam. In addition, we observe a resonance appearing below 140 K, which might be interpreted as an electromagnon mode and related to a spin reorientation transition. Our present observations present that the temperature and polarization, as the external control parameters, can be used to modulate the THz optical properties of BFO single crystal.

Multiferroic behavior from synergetic response of multiple ordering parameters in BiFeO3 single crystal under high magnetic field up to 50 Tesla

BiFeO3, a typical multiferroic compound, owns complicated interactions among its charge, lattice, and spin degrees, while largely remaining a mystery. In this work, the BiFeO3 single crystal has been investigated in pulsed high magnetic fields up to 50 T. The lattice, spin, and charge orderings are found to couple with each other strongly. High magnetic field could drive the spin cycloid phase to a canted antiferromagnetic phase while accompanied by a clear magnetostriction response and weakening of ferroelectric polarization. The critical magnetic field for the magnetic phase transition was shifted to the lower magnetic field region by an external electric field over a broad temperature range, which indicates the complex interaction among magnetization, electrical polarization, and crystal lattice. This work has provided a clear picture to show the synergetic response of multiple ordering parameters during the magnetic phase transition in BiFeO3, which could benefit the design of BiFeO3 based spintronic devices and novel multiferroic materials with strong magnetoelectric coupling.

Antiferromagnetic states of BiFeO3 single crystal multiferroic in external magnetic field

Room-temperature multiferroics take special attention due to possibilities for applications in technology, but they are also interesting from fundamental point of view. Bismuth ferrite is the most studied multiferroic demonstrating high ordering temperatures and fascinating properties in thin films and could be used as a functional element in magnetoelectric memory. For better utilization of bismuth ferrite in spintronic devices the detailed understanding of its properties is of great importance. In this work we apply the developed thermodynamic model for description of magnetic states and magnetic field driven transformation of magnetic structure of single crystal BiFeO3. Presence of magnetic field may lead to a situation, when the transition from inhomogeneous magnetic state having cycloidal structure into homogeneous state proceeds through emergence of intermediate magnetic state with conical-like structure. Magnetic field transitions emerging under influence of magnetic field should somehow lead to alternation of magnetization. For revealing this influence the magnetization curves were calculated for three directions of magnetic field and the comparison with experimental results was carried out. Good agreement with experimental curves allows us to say, that peculiarities of magnetization curves may be caused by the emergence of intermediate conical state.

Temperature-dependent photo-response in multiferroic BiFeO3 revealed by transmission measurements

We studied the light-induced effects in BiFeO3 single crystals as a function of temperature by means of optical spectroscopy. Here, we report the observation of several light-induced absorption features, which are discussed in terms of the photostriction effect and are interpreted in terms of excitons. The temperature dependence of their energy position suggests a possible coupling between the excitons and the lattice vibrations. Moreover, there are hints for anomalies in the temperature evolution of the excitonic features, which might be related to the temperature-induced magnetic phase transitions in BiFeO3. Our findings suggest a coupling between light-induced excitons and the lattice and spin degrees of freedom, which might be relevant for the observed ultrafast photostriction effect in multiferroic BiFeO3.

In situ study of electric‐field‐induced ferroelectric and antiferromagnetic domain switching in polycrystalline BiFeO3

AbstractAntiferromagnetic domain switching induced by ferroelectric polarization switching has previously been observed in situ in both multiferroic BiFeO3 single crystals and thin films. Despite a number of reports on macroscopic magnetoelectric measurements on polycrystalline BiFeO3, direct in situ observation of electric‐field‐induced antiferromagnetic domain switching in this material has not been addressed due to the lack of high‐quality samples capable of electrical poling. Here, the electric field control of antiferromagnetic domain texture is identified in polycrystalline BiFeO3 using in situ neutron diffraction, showing the resultant magnetic domain reorientation induced by an electric field. An antiferromagnetic domain reorientation to a value of 2.2‐2.5 multiples of a random distribution (MRD) is found to be induced by an electric field that provides a non‐180° ferroelectric‐ferroelastic domain texture of 2.2‐2.5 MRD along the field direction. The current results show well‐controlled coupling of multiferroic domain texturing in single‐phase polycrystalline BiFeO3.

Single-Crystal BiFeO3 Nanoplates with Robust Antiferromagnetism.

Freestanding and single-crystal BiFeO3 (BFO) nanoplates have been successfully synthesized by a fluoride ion-assisted hydrothermal method, and the thickness of the nanoplates can be effectively tailored from 80 to 380 nm by the concentration of fluoride ions. It is revealed that BFO nanoplates grew via an oriented attachment of layer by layer, giving rise to the formation of the inner interface within the nanoplates. In particular, antiferromagnetic (AFM) phase-transition temperature (Néel temperature, TN) of the BFO nanoplates is significantly enhanced from typical 370 to ∼512 °C, whereas the Curie temperature (TC) of the BFO nanoplates is determined to be ∼830 °C, in good agreement with a bulk value. The combination of scanning transmission electron microscopy, electron energy loss spectroscopy, and the first-principle calculations reveals that the interfacial tensile strain remarkably improves the stability of AFM ordering, accounting for the significant enhancement in TN of BFO plates. Correspondingly, the tensile strain induced the polarization and oxygen octahedral tilting has been observed near the interface. The findings presented here suggest that single-crystal BFO nanoplate is an ideal system for exploring an intrinsic magnetoelectric property, where a tensile strain can be a very promising approach to tailor AFM ordering and polarization rotation for an enhanced coupling effect.

Electric-Field-Induced Reorientation of the Magnetic Easy Plane in a Co-Substituted BiFeO3 Single Crystal.

Single crystals of BiFe0.9Co0.1O3 and BiFe0.892Mn0.008Co0.1O3, room temperature ferroelectric ferromagnets, were successfully grown by a flux method at a high pressure of 3 GPa. Remanent magnetization measurements along 18 crystallographic directions revealed the existence of a magnetic easy plane perpendicular to the electric polarization. Reorientation of the magnetic easy plane occurred in connection with 71° ferroelectric switching by applying an electric field. This is the first demonstration of an electric field affecting the local magnetic moment of Co-substituted BiFeO3.

Spin-lattice coupling phase transition and phonon anomalies in bismuth ferrite BiFeO3

Phase transition, phonon vibration, and magnetization in (100) BiFeO3 (BFO) multiferroic single crystals have been characterized as functions of temperature. The BFO single crystal exhibits a weak ferromagnetic hysteresis loop possibly due to appearance of ferromagnetic FeOFe orderings and a modulation of the FeOFe bond. Zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibilities suggest a weak spin-glass behavior below ∼200 K. High-resolution transmission electron microscopy (HRTEM) suggests a rhombohedral R3c symmetry (with tilted FeO6 octahedra) and lattice distortion in the surface layer. The BFO crystal undergoes a structural transition sequence of rhombohedral−tetragonal−orthorhombic− cubic at ∼620–640, ∼1000, and ∼1100 K, respectively. The spin-lattice coupling accounts for the rhombohedraltetragonal transition upon approaching the Néel temperature (TN). The low-frequency Raman-active modes ψ4,II, E(1), A1(1), and A1(2) reveal clear softening anomalies in frequency shifts and broad peaks in full-width-at-half-maximum (FWHM) around 450 K, which is attributed to the FeOFe ordering transition without change of structural symmetry.

Resistive memory effects in BiFeO3 single crystals controlled by transverse electric fields

The effects of electric fields perpendicular to the c-axis of the trigonal cell in single crystals of BiFeO3 are investigated through magnetization and resistance measurements. Magnetization and resistance exhibit hysteretic changes under applied electric fields, which can be ascribed to the reorientation of the magnetoelectric domains. Samples are repetitively switched between high- and low-resistance states by changing the polarity of the applied electric fields over 20 000 cycles at room temperature. These results demonstrate the potential of BiFeO3 for use in non-volatile memory devices.

Electronic Origin and Tailoring of Photovoltaic Effect in BiFeO3 Single Crystals

The sub-band levels situated around 2.2 eV within the band gap are at the electronic origin of the anomalous photo­voltaic effect in BiFeO3 single crystals. The photovoltaic effect can be effectively tailored by controlling the activity of the sub-band levels via thermal and electrical treatments. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

A Raman scattering study of the structural ordering in Bi1–x La x FeO3 ceramic ferroelectromagnetics

Based on Raman spectra, we have studied structural ordering processes in ceramics of ferroelectromagnetics Bi1–xLaxFeO3 (x = 0.075–0.20). It has been found that the structure of Bi1–xLaxFeO3 is close to the structure of the crystal BiFeO3. However, lines in Raman spectra of Bi1–xLaxFeO3 are considerably broadened compared to lines in the Raman spectrum of the BiFeO3 single crystal, which indicates that the structure of solid solutions is much more disordered. In Raman spectra of Bi1–xLaxFeO3, in the range of librational vibrations of octahedra as a whole (50–90 cm–1), several groups of lines are observed in frequency ranges 59–69, 72–77, and 86–92 cm–1 (depending on the composition of solid solution). This confirms X-ray data that examined solid solutions are not single-phase. At a La content x = 0.120, Raman lines in the low-frequency spectral range narrow, which indicates that the ordering of structural units in cationic sublattices somewhat increases. Upon an increase in the content of La in the Bi1–xLaxFeO3 structure, no unambiguous dependence of parameters of spectral lines is observed. It is likely that this is explained by the fact that, as the value of x increases, the character of the incorporation of La into the structure of the solid solution changes.

Neutron inelastic scattering measurements of low-energy phonons in the multiferroicBiFeO3

We present neutron inelastic scattering measurements of the low-energy phonons in single crystal BiFeO3. The dispersions of the three acoustic phonon modes (LA along [100], TA1 along [010] and TA2 along [110]) and two low energy optic phonon modes (LO and TO1) have been mapped out between 300 K and 700 K. Elastic constants are extracted from the phonon measurements. The energy linewidths of both TA phonons at the zone boundary clearly broaden when the system is warmed toward the magnetic ordering temperature TN = 640 K. This suggests that the magnetic and low-energy lattice dynamics in this multiferroic material are coupled.

High field studies on BiFeO3 single crystals grown by the laser-diode heating floating zone method

Magnetic field dependence of magnetization and electric polarization in mono-domain crystals of BiFeO3 were studied in pulsed magnetic fields up to 56T. The observed anisotropic transition fields were reasonably explained by Ginzburg–Landau theory at low temperatures, whereas their temperature dependence could not be in the simple model. The electric polarization measurements provide matrix elements of the linear magnetoelectric tensor |α31|∼|α32|=1.5±0.2ps/m.

Nanodomains and nanometer-scale disorder in multiferroic bismuth ferrite single crystals

We report on an investigation of state-of-the-art flux-grown multiferroic bismuth ferrite (BiFeO3; BFO) single crystals by transmission electron microscopy and electron diffraction. The crystals were pre-characterized by piezoresponse force microscopy, electrical resistance and superconducting quantum interference device magnetization measurements. The structurally highly perfect crystals show a ferroelectric stripe domain structure characterized by a domain width of 55nm. Inside these domains an additional contiguous nanodomain substructure occurs, consisting of 180° related domains, giving rise to satellite reflections at 121212-type positions along 〈110〉 directions in the electron diffraction pattern corresponding to a characteristic length in real space of 15.5nm. Furthermore, we present the first atomic-resolution study on the short-range order by aberration-corrected transmission electron microscopy in which all atoms including oxygen are imaged directly. By measuring the –Fe–O–Fe– atom topology, bond angles and atomic distances we derive the electrical dipole moment as well as the magnitude of the magnetic moment on the unit-cell level. The results evidence substantial atomic- to nano-scale disorder. Both the nanodomain substructure as well as the disorder should affect the subtle magnetoelectric interactions in this material and thereby impede the formation of long-range cycloidal spin ordering which up to now was considered an intrinsic feature of the magnetic properties of BiFeO3 single crystals. By Monte Carlo simulation on the basis of a state-of-the-art effective Hamiltonian we scrutinize certain aspects of the phase formation behavior in the BFO system forming the background of single-crystal growth. This study reveals a very sluggish phase evolution behavior, which should make it invariably difficult to obtain structurally fully equilibrated single crystals.

Pillar shape modulation in epitaxial BiFeO3–CoFe2O4 vertical nanocomposite films

Self-assembled epitaxial CoFe2O4-BiFeO3 nanocomposite films, in which pillars of CoFe2O4 grow within a single crystal BiFeO3 matrix, show both ferrimagnetism and ferroelectricity. The pillars typically have a uniform cross-section, but here two methods are demonstrated to produce a width modulation during growth by pulsed laser deposition. This was achieved by growing a blocking layer of BiFeO3 to produce layers of separated pillars or pillars with constrictions, or by changing the temperature during growth to produce bowling-pin shaped pillars. Modulated nanocomposites showed changes in their magnetic anisotropy compared to nanocomposites with uniform width. The magnetic anisotropy was interpreted as a result of magnetoelastic and shape anisotropies.