We report polarized neutron scattering and piezoresponse force microscopy studies of millimeter-sized single crystals of multiferroic BiFeO$_3$. The crystals, grown below the Curie temperature, consist of a single ferroelectric domain. Two unique electric polarization directions, as well as the populations of equivalent spiral magnetic domains, can be switched reversibly by an electric field. A ferroelectric monodomain with a single-$q$ single-helicity spin spiral can be obtained. This level of control, so far unachievable in thin films, makes single-crystal BiFeO$_3$ a promising object for multiferroics research.
Ferroelectric BiFeO3 thin films and artificial superlattices of (BiFeO3)m(SrTiO3)m (m~ 1 to 10 unit cells) were fabricated on (001)-oriented SrTiO3 substrates by pulsed laser ablation. The variation of leakage current and macroscopic polarization with periodicity was studied. Piezo force microscopy studies revealed the presence of large ferroelectric domains in the case of BiFeO3 thin films while a size reduction in ferroelectric domains was observed in the case of superlattice structures. The results show that the modification of ferroelectric domains through superlattice, could provide an ad…
We present a terahertz spectroscopic study of magnetic excitations in ferroelectric antiferromagnet BiFeO3. We interpret the observed spectrum of long-wavelength magnetic resonance modes in terms of the normal modes of the material's cycloidal antiferromagnetic structure. We find that the modulated Dzyaloshinski-Moriya interaction leads to a splitting of the out-of-plane resonance modes. We also assign one of the observed absorption lines to an electromagnon excitation that results from the magnetoelectric coupling between the ferroelectric polarization and the cycloidal magnetic structure of …
A detailed investigation using variable temperature powder neutron diffraction demonstrates that BiFeO3 undergoes a phase transition from the ferroelectric alpha phase (rhombohedral, R3c) to a paraelectric beta phase (orthorhombic, Pbnm) between 820 oC and 830 oC. Co-existence of both phases over a finite temperature interval, together with abrupt changes in key structural parameters, confirm that the transition is firstorder. The beta phase corresponds to the GdFeO3-type perovskite. The metastability of BiFeO3 relative to Bi2Fe4O9 above 810 oC precludes observation of a reported cubic gamma p…
First-principles calculations for pristine neutral ferroelectric domain walls in BiFeO$_3$ reveal that excess electrons are selectively trapped by the domain walls, while holes are only weakly attracted. Such trapped excess electrons may be responsible for the thermally activated electrical conductivity at domain walls observed in experiments. In the case of a periodic array of domain walls, the trapped excess electrons create a zigzag potential, whose amplitude depends on the electron concentration in the material and the domain-wall distance. The potential is asymmetric for 71° and 109° doma…
We detected broadband coherent terahertz (THz) emission from multiferroic BiFeO3 after illuminating a high-quality bulk single ferroelectric domain crystal with a ~100 fs optical pulse. The dependence of the emitted THz waveform on the energy and polarization of the optical pulse is consistent with the optical rectification mechanism of THz emission. The THz emission provides a sensitive probe of the electric polarization state of BiFeO3, enabling applications in ferroelectric memories and ferroelectric domain imaging. We also report room-temperature THz optical constants of BiFeO3.
Using an experimental setup designed to scan a submicron sized light spot and collect the photogenerated current through larger electrodes, we map the photovoltaic response in ferroelectric BiFeO3 single crystals. We study the effect produced by a unique 180{\textdegree} ferroelectric domain wall (DW) and show that the photocurrent maps are significantly affected by its presence and shape. The effect is large in its vicinity and in the Schottky barriers at the interface with the Au electrodes, but no extra photocurrent is observed when the illuminating spot touches the DW, indicating that this…
Over the years attempts have been made to compensate for the inherent weaknesses in the bulk state of the multiferroic BiFeO3, such as high leakage current and the absence of ferromagnetic correlation, and exploit its magnetoelectric potential by forming solid solutions with other perovskites. Studies in the recent few years have shown that alloying of BiFeO3 with BaTiO3, both with and without additives, can induce both ferroelectric and ferromagnetic switching. While the coexistence of both the ferroic orders is encouraging from the view point of technological applications, the origin of ferr…
We report electrically switchable polarization and ferroelectric domain scaling over a thickness range of 5-100 nm in BiFeO3 films deposited on [110] vicinal substrates. The BiFeO3 films of variable thickness were deposited with SrRuO3 bottom layer using pulsed laser deposition technique. These films have fractal domain patterns and the domain width scales closely with the square root of film thickness, in accordance with the Landau-Lifschitz-Kittel (LLK) law. The Switching Spectroscopy Piezo-response Force Microscopy provides clear evidence for the ferroelectric switching behavior in all the …
Using Landau-Ginzburg-Devonshire approach, we predict the intrinsic instability of the ferroelectric-ferroelastic domain walls in the multiferroic BiFeO3 emerging from the interplay between the gradient terms of the antiferrodistortive and ferroelectric order parameters at the walls. These instabilities are the interface analogue of the structural instabilities in the vicinity of phase coexistence in the bulk; and so they do not steam from incomplete polarization screening in thin films or its spatial confinement, electrostrictive or flexoelectric coupling. The effect of BiFeO3 material parame…
Thin films of (1-x)BiFeO<sub>3</sub>-xPbTiO<sub>3</sub> (BF-xPT) with x ~ 0.60 were fabricated on Pt/Si substrates by chemical solution deposition of precursor BF and PT layers alternately in three different multilayer configurations. These multilayer deposited precursor films upon annealing at 700°C in nitrogen show pure perovskite phase formation. In contrast to the equilibrium tetragonal structure for the overall molar composition of BF:PT::40:60, we find monoclinic structured BF-xPT phase of M<sub>A</sub> type. Piezo-force microscopy confirmed ferroelectric switching in the films and revea…
There is growing evidence that domain walls in ferroics can possess emergent properties that are absent in bulk materials. For example, 180 domain walls in the ferroelectric-antiferromagnetic BiFeO3 are particularly interesting because they have been predicted to possess a range of intriguing behaviors; including electronic conduction and enhanced magnetization. To date, however, ordered arrays of such domain structures have not been reported. Here, we report the observation of 180 stripe nanodomains in (110)-oriented BiFeO3 thin films grown on orthorhombic GdScO3 (010)O substrates, and their …
Ferroelectric domains were investigated using piezoresponse force microscopy in superlattices composed of multiferroic BiFeO3 and SrTiO3 layers. Compared to single BiFeO3 thin films, a reduction in the domains size and a suppression of the in-plane orientation of domains are observed in a superlattice of (BiFeO3)4(SrTiO3)8, suggesting a constrained ferroelectric domain orientation along the out-of-plane <001> direction. Such modification of domain size and orientation in BiFeO3-based heterostructures could play a vital role on engineering the domains and domain wall mediated functional propert…
We report a density functional study on the evolution of the electronic and lattice structure in BiFeO$_3$ with injected electrons and holes. First, the self-trapping of electrons and holes were investigated. We found that the injected electrons tend to be localized on Fe sites due to the local lattice expansion, the on-site Coulomb interaction of Fe $3d$ electrons, and the antiferromagnetic order in BiFeO$_3$. The injected holes tend to be delocalized if the on-site Coulomb interaction of O $2p$ is weak (in other words, $U_\mathrm{O}$ is small). Single center polarons and multi-center polaron…
We describe the directional growth of ferroelectric domains in a multiferroic BiFeO3 thin film, which was grown epitaxially on a vicinal (001) SrTiO3 substrate. A detailed structural analysis of the film shows that a strain gradient, which can create a symmetry breaking in a ferroelectric double well potential, causes ferroelectric domains to grow with preferred directionality under the influence of an electric field. Our results suggest the possibility of controlling the direction of domain growth with an electric field by imposing constraints on ferroelectric films, such as a strain gradient…
We propose a way to use electric-field to control the magnetic ordering of the tetragonal BiFeO3. Based on systematic first-principles studies of the epitaxial strain effect on the ferroelectric and magnetic properties of the tetragonal BiFeO3, we find that there exists a transition from C-type to G-type antiferromagnetic (AFM) phase at in-plane constant a ~ 3.905 Å when the ferroelectric polarization is along [001] direction. Such magnetic phase transition can be explained by the competition between the Heisenberg exchange constant J1c and J2c under the influence of biaxial strain. Interestin…
Bismuth ferrite, BiFeO3, is the only known room-temperature 'multiferroic' material. We demonstrate here, using neutron scattering measurements in high quality single crystals, that the antiferromagnetic and ferroelectric orders are intimately coupled. Initially in a single ferroelectric state, our crystals have a canted antiferromagnetic structure describing a unique cycloid. Under electrical poling, polarisation re-orientation induces a spin flop. We argue here that the coupling between the two orders may be stronger in the bulk than that observed in thin films where the cycloid is absent.
Artificial superlattices of (BiFeO3)m(SrTiO3)m (m= 1 to 10 unit cells) consisting of multiferroic BiFeO3 and insulating SrTiO3 layers were fabricated on (100)-oriented SrTiO3 substrates by pulsed laser ablation. The remnant polarization and leakage current behavior were studied varying the periodicity (8-80A) of the superlattice. The leakage current was reduced by few orders of magnitude on increase of periodicity compared to single layer BiFeO3 thin films. Reduced leakage and intrinsic polarization hysteresis was observed and was confirmed by PUND analysis for periodicities in the range 20-60…
In this paper we examine the role of crystal chemistry factors in creating conditions for formation of magnetoelectric ordering in BiFeO3. It is generally accepted that the main reason of the ferroelectric distortion in BiFeO3 is concerned with a stereochemical activity of the Bi lone pair. However, the lone pair is stereochemically active in the paraelectric orthorhombic beta-phase as well. We demonstrate that a crucial role in emerging of phase transitions of the metal-insulator, paraelectric-ferroelectric and magnetic disorder-order types belongs to the change of the degree of the lone pair…
Multiferroic BiFeO3 epitaxial films with thickness ranging from 40 nm to 960 nm were grown by pulsed laser deposition on SrTiO3 (001) substrates with SrRuO3 bottom electrodes. X-ray characterization shows that the structure evolves from angularly-distorted tetragonal with c/a ~ 1.04 to more bulk-like distorted rhombohedral (c/a ~ 1.01) as the strain relaxes with increasing thickness. Despite this significant structural evolution, the ferroelectric polarization along the body diagonal of the distorted pseudo-cubic unit cells, as calculated from measurements along the normal direction, barely ch…
Multiferroic BiFeO3 (BFO) thin film exhibiting desired ferroelectric and enhanced magnetic properties was grown on La0.67Sr0.33MnO3 (LSMO) buffered Pt/TiO2/SiO2/Si substrates by off-axis RF magnetic sputtering, where a highly (111)-oriented texture was obtained. The BFO/LSMO thin film exhibits excellent ferroelectric and dielectric behaviors (2Pr ~210.7 μC/cm2, 2Ec~435 kV/cm, εr ~116.8, and tanδ ~ 2.7% at 1 kHz), together with a long fatigue endurance up to 1010 switching cycles at amplitude of 300 kV/cm. An enhancement in magnetic behavior was also observed with Ms=89.5 emu/cm3, which is larg…
Ferroelectric domain walls are planes within an insulating material that can accumulate and conduct charge carriers, hence the interaction of the domain walls with the charge carriers can be important for photovoltaic and other electronic applications. By means of first principles calculations we predict a transition from a large two-dimensional electron polaron to a small polaron at the domain walls at a critical electron density, with polaron signatures in optical absorption and photoluminescence. We find that large and small polarons at the domain walls create different absorption peaks wit…
Bismuth ferrite is one of the most widely studied multiferroic materials because of its large ferroelectric polarisation coexisting with magnetic order at room temperature. Using density functional theory (DFT), we identify several previously unknown polar and non-polar structures within the low-energy phase space of perovskite-structure bismuth ferrite, BiFeO$_3$. Of particular interest is a series of non-centrosymmetric structures with polarisation along one lattice vector, combined with anti-polar distortions, reminiscent of ferroelectric domains, along a perpendicular direction. We discuss…
We study the electrical behavior of multiferroic BiFeO$_3$ by means of first-principles calculations. We do so by constraining a specific component of the electric displacement field along a variety of structural paths, and by monitoring the evolution of the relevant physical properties of the crystal along the way. We find a complex interplay of ferroelectric, antiferroelectric and antiferrodistortive degrees of freedom that leads to an unusually rich electrical phase diagram, which strongly departs from the paradigmatic double-well model of simpler ferroelectric materials. In particular, we …
A new class of superparamagnetism was found in relaxor ferroelectric 2/3BiFeO$_{3}$-1/3BaTiO$_{3}$. The size of the magnetic particle, estimated from the superparamagnetic magnetization curve, coincides with the size of the polar nanoregion (PNR), which governs the relaxor ferroelectric property. This suggests that the magnetic domain is identical to the PNR. The temperature variations in the sizes of the magnetic domains and PNRs estimated by our neutron diffraction measurements support this picture. Since the same domain provides both electric and magnetic properties, strong coupling between…
The spin-driven component of electric polarization in a single crystal of multiferroic BiFeO$_{3}$ was experimentally investigated in pulsed high magnetic fields up to 41 T. Sequential measurements of electric polarization for various magnetic field directions provide clear evidence of electric polarization normal to the hexagonal $c$ axis (${\bf P}_{\rm t}$) in not only the cycloidal phase, but also the field-induced canted antiferromagnetic phase. The direction of ${\bf P}_{\rm t}$ is directly coupled with the ferromagnetic moment in the canted antiferromagnetic phase, and thus controlled by…
In this work, the BiFeO3 (BFO)/SrRuO3 (SRO) heterostructure was fabricated and the anomalous Hall effect (AHE) was investigated the in BFO/SRO. It is found the nonmonotonic anomalous Hall resistivity behavior in BFO/SRO is originated from the inhomogeneous SRO layer instead of the topological Hall effect. It is surprised that the AHE in BFO/SRO structure can be manipulated by ferroelectric polarization of BFO. Moreover, an inhomogeneous phenomenological model has been applied on those structure. Furthermore, the modification of band structure in SRO under ferroelectric polarization was discuss…
BiFeO$_3$ is the most famous multiferroic material, but it has no strong spontaneous magnetization due to its antiferromagnetism. Here we show that giant ferroelectric polarization and strong spontaneous magnetization can be both realized in double perovskite Bi$_2$FeMoO$_6$ with R3 (\#146) space group based on BiFeO$_3$. Our first-principles phonon spectra establishes that this multiferroic R3 phase is stable. Our systematic calculations show that it is a spin-polarized semiconductor with gap reaching to 0.54 eV and has a strong ferroelectric polarization of 85$μ$C/cm$^2$. This ferroelctricit…
We investigate the variation of the spontaneous ferroelectric polarization with epitaxial strain for BaTiO$_3$, PbTiO$_3$, LiNbO$_3$, and BiFeO$_3$ using first principles calculations. We find that while the strain dependence of the polarization is very strong in the simple perovskite systems BaTiO$_3$ and PbTiO$_3$ it is only weak in LiNbO$_3$ and BiFeO$_3$. We show that this different behavior can be understood purely in terms of the piezoelectric and elastic constants of the unstrained bulk material, and we discuss several factors that determine the strain behavior of a certain material.
Multilayers based on quantum materials (complex oxides, topological insulators, transition-metal dichalcogenides, etc) have enabled the design of devices that could revolutionize microelectronics and optoelectronics. However, heterostructures incorporating quantum materials from different families remain scarce, while they would immensely broaden the range of possible applications. Here we demonstrate the large-scale integration of compounds from two highly-multifunctional families: perovskite oxides and transition-metal dichalcogenides (TMDs). We couple BiFeO$_3$, a room-temperature multiferr…
Epitaxial Mn-doped BiFeO3 (MBFO) thin films were grown on GaAs (001) substrate with SrTiO3 (STO) buffer layer by pulsed laser deposition. X-ray diffraction results demonstrate that the films show pure (00l) orientation, and MBFO(100)//STO(100), whereas STO (100)//GaAs (110). Piezoresponse force microscopy images and polarization versus electric field loops indicate that the MBFO films grown on GaAs have an effective ferroelectric switching. The MBFO films exhibit good ferroelectric behavior (2Pr ~ 92 μC/cm2 and 2EC ~ 372 kV/cm). Ferromagnetic property with saturated magnetization of 6.5 emu/cm…
From an experimental point of view, room temperature ferroelectricity in BiFeO3 is raising many questions. Electric measurements made a long time ago on solid-solutions of BiFeO3 with Pb(Ti,Zr)O3 indicate that a spontaneous electric polarization exists in BiFeO3 below the Curie temperature TC=1143K. Yet in most reported works, the synthesised samples are too conductive at room temperature to get a clear polarization loop in the bulk without any effects of extrinsic physical or chemical parameters. Surprisingly, up to now there has been no report of a P(E) (polarization versus electric field) l…
In bismuth ferrite (BiFeO3), antiferromagnetic and ferroelectric order coexist at room temperature, making it of particular interest for studying magnetoelectric coupling. The mutual control of magnetic and electric properties is very useful for a wide variety of applications. This has led to an enormous amount of research into the properties of BiFeO$_3$. Nonetheless, one of the most fundamental aspects of this material, namely the symmetries of the lattice vibrations, remains controversial.We present a comprehensive Raman study of BiFeO$_3$ single crystals with the approach of monitoring the…
In a nanocomposite of reduced graphene oxide (RGO) and BiFeO$_3$ (BFO), the remanent ferroelectric polarization is found to follow nonmonotonic magnetic field dependence at room temperature as the applied magnetic field is swept across 0-20 kOe on a pristine sample. The remanent ferroelectric polarization is determined both from direct electrical measurements on an assembly of nanoparticles and powder neutron diffraction patterns recorded under 0-20 kOe field. The nanosized ($\sim$20 nm) particles of BFO are anchored onto the graphene sheets of RGO via Fe-C bonds with concomitant rise in coval…
The magnetoelectric behavior of BiFeO$_3$ has been explored on the basis of accurate density functional calculations. The structural, electronic, magnetic, and ferroelectric properties of BiFeO$_3$ are predicted correctly without including strong correlation effect in the calculation. Moreover, the experimentally-observed elongation of cubic perovskite-like lattice along the [111] direction is correctly reproduced. At high pressure we predicted a pressure-induced structural transition and the total energy calculations at expanded lattice show two lower energy ferroelectric phases, closer in en…
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.
Defect engineering has been a powerful tool to enable the creation of exotic phases and the discovery of intriguing phenomena in ferroelectric oxides. However, accurate control the concentration of defects remains a big challenge. In this work, ion implantation, that can provide controllable point defects, allows us the ability to produce a controlled defect-driven true super-tetragonal (T) phase with enhanced tetragonality in ferroelectric BiFeO3 thin films. This point defect engineering is found to drive the phase transition from the as-grown mixed rhombohedral-like (R) and tetragonal-like (…
Terahertz (THz) radiation has been observed from multiferroic BiFeO$_3$ thin films via ultrafast modulation of spontaneous polarization upon carrier excitation with illumination of femtosecond laser pulses. The radiated THz pulses from BiFeO$_3$ thin films were clarified to directly reflect the spontaneous polarization state, giving rise to a memory effect in a unique style and enabling THz radiation even at zero-bias electric field. On the basis of our findings, we demonstrate potential approaches to ferroelectric nonvolatile random access memory with nondestructive readability and ferroelect…
Multiferroic materials, in which ferroelectric and magnetic ordering coexist, are of fundamental interest for the development of novel memory devices that allow for electrical writing and non-destructive magnetic readout operation. The great challenge is to create room temperature multiferroic materials with strongly coupled ferroelectric and ferromagnetic (or ferrimagnetic) orderings. BiFeO3 has been the most heavily investigated single-phase multiferroic to date due to the coexistence of its magnetic order and ferroelectric order at room temperature. However, there is no net magnetic moment …
We present an ab initio study of the BFCO solid solution formed by multiferroics BiFeO3 (BFO) and BiCoO3 (BCO). We find that BFCO presents a strongly discontinuous morphotropic transition between BFO-like and BCO-like ferroelectric phases. Further, for all compositions such phases remain (meta)stable and retain well-differentiated properties. Our results thus suggest that an electric field can be used to switch between these structures, and show that such a switching involves large phase-change effects of various types, including piezoelectric, electric, and magnetoelectric ones.
Piezoelectrics interconvert mechanical energy and electric charge and are widely used in actuators and sensors. The best performing materials are ferroelectrics at a morphotropic phase boundary (MPB), where several phases can intimately coexist. Switching between these phases by electric field produces a large electromechanical response. In the ferroelectric BiFeO$_3$, strain can be used to create an MPB-like phase mixture and thus to generate large electric field dependent strains. However, this enhanced response occurs at localized, randomly positioned regions of the film, which potentially …
We have analyzed the morphology of ferroelectric domains in very thin films of multiferroic BiFeO3. Unlike the more common stripe domains observed in thicker films BiFeO3 or in other ferroics, the domains tend not to be straight, but irregular in shape, with significant domain wall roughening leading to a fractal dimensionality. Also contrary to what is usually observed in other ferroics, the domain size appears not to scale as the square root of the film thickness. A model is proposed in which the observed domain size as a function of film thickness can be directly linked to the fractal dimen…
The control of optical fields is usually achieved through the electro-optic or acousto-optic effect in single-crystal ferroelectric or polar compounds such as LiNbO3 or quartz. In recent years, tremendous progress has been made in ferroelectric oxide thin film technology - a field which is now a strong driving force in areas such as electronics, spintronics and photovoltaics. Here, we apply epitaxial strain engineering to tune the optical response of BiFeO3 thin films, and find a very large variation of the optical index with strain, corresponding to an effective elasto-optic coefficient large…
Charge transport across the interfaces in complex oxides attracts a lot of attention because it allows creating novel functionalities useful for device applications. In particular, it has been observed that movable domain walls in epitaxial BiFeO3 films possess enhanced conductivity that can be used for read out in ferroelectric-based memories. In this work, the relation between the polarization and conductivity in sol-gel BiFeO3 films with special emphasis on grain boundaries as natural interfaces in polycrystalline ferroelectrics is investigated. The grains exhibit self-organized domain stru…
We report a comprehensive study of ultrafast carrier dynamics in single crystals of multiferroic BiFeO$_{3}$. Using femtosecond optical pump-probe spectroscopy, we find that the photoexcited electrons relax to the conduction band minimum through electron-phonon coupling with a $\sim$1 picosecond time constant that does not significantly change across the antiferromagnetic transition. Photoexcited electrons subsequently leave the conduction band and primarily decay via radiative recombination, which is supported by photoluminescence measurements. We find that despite the coexisting ferroelectri…
The ground-state structural and electronic properties of ferroelectric BiFeO$_3$ are calculated using density functional theory within the local spin-density approximation and the LSDA+U method. The crystal structure is computed to be rhombohedral with space group $R3c$, and the electronic structure is found to be insulating and antiferromagnetic, both in excellent agreement with available experiments. A large ferroelectric polarization of 90-100 $μ$C/cm$^2$ is predicted, consistent with the large atomic displacements in the ferroelectric phase and with recent experimental reports, but differi…
Using the model system of ferroelectric domain walls, we explore the effects of long-range dipolar interactions and periodic ordering on the behavior of pinned elastic interfaces. In piezoresponse force microscopy studies of the characteristic roughening of intrinsic 71° stripe domains in BiFeO$_3$ thin films, we find unexpectedly high values of the roughness exponent ζ = 0.74 $\pm$ 0.10, significantly different from those obtained for artificially written domain walls in this and other ferroelectric materials. The large value of the exponent suggests that a random field-dominated pinning, com…
Multiferroic BiFeO3 ceramics have been doped with Ca. The smaller ionic size of Ca compared with Bi means that doping acts as a proxy for hydrostatic pressure, at a rate of 1%Ca=0.3GPa. It is also found that the magnetic Neel temperature (TNeel) increases as Ca concentration increases, at a rate of 0.66K per 1%Ca (molar). Based on the effect of chemical pressure on TNeel, we argue that applying hydrostatic pressure to pure BiFeO3 can be expected to increase its magnetic transition temperature at a rate around ~2.2K/GPa. The results also suggest that pressure (chemical or hydrostatic) could be …
