Coupling In BiFeO3 Research Articles

Ptychographic Nanoscale Imaging of the Magnetoelectric Coupling in Freestanding BiFeO3.

Understanding the magnetic and ferroelectric ordering of magnetoelectric multiferroic materials at the nanoscale necessitates a versatile imaging method with high spatial resolution. Here, soft X-ray ptychography is employed to simultaneously image the ferroelectric and antiferromagnetic domains in an 80nm thin freestanding film of the room-temperature multiferroic BiFeO3 (BFO). The antiferromagnetic spin cycloid of period 64nm is resolved by reconstructing the corresponding resonant elastic X-ray scattering in real space and visualized together with mosaic-like ferroelectric domains in a linear dichroic contrast image at the Fe L3 edge. The measurements reveal a near perfect coupling between the antiferromagnetic and ferroelectric ordering by which the propagation direction of the spin cycloid is locked orthogonally to the ferroelectric polarization. In addition, the study evinces both a preference for in-plane propagation of the spin cycloid and changes of the ferroelectric polarization by 71°between multiferroic domains in the epitaxial strain-free, freestanding BFO film. The results provide a direct visualization of the strong magnetoelectric coupling in BFO and of its fine multiferroic domain structure, emphasizing the potential of ptychographic imaging for the study of multiferroics and non-collinear magnetic materials with softX-rays.

Tunability of magnetoelectric coupling in BiFeO3–PbZr0.52Ti0.48O3–CoFe2O4 tri-phase composites for ultra-fast switching applications

Magnetoelectric coupling an exquisite feature of multiferroics make them suitable for advanced technological applications like magnetic sensors, spintronic devices, and fast-switching devices, respectively. In the present work, a series of tri-phase composites (1-x)(0.7BiFeO3 +0.3PbZr0.52Ti0.48O3) + xCoFe2O4 (x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10) is synthesized through sol–gel auto-combustion method followed by the solid-state ball milling route. The analysis of X-ray diffraction patterns confirms the development of the rhombohedral perovskite phase of BiFeO3, the tetragonal phase of PbZr0.52Ti0.48O3, and the cubic spinel structure of CoFe2O4. The micrographs clearly show an increase in the average grain size by enhancing the CoFe2O4 contents having sharp and distinct grain boundaries. The energy-dispersive X-ray study assures the exact formation of a composite recipe through elemental mapping. The ferroelectric analysis comprising bipolar polarization versus electric field loops exhibits a significant decrease in maximum polarization with an increase in remanent polarization by enhancing the CoFe2O4 contents in the composite. The positive up and negative down sequence validates the presence of leakage current in all the samples with precise measurement of remanent polarization with and without switching polarization. The gradual increase of CoFe2O4 contents causes an increase in the magnetic response of the composites. Meanwhile, the magnetoelectric coupling phenomenon for all the composition shows a linear response whereas for x = 0.1 there is a slight reversibility observed, making this tri-phase composite a robust candidate for fast-switching devices.

Recent advances on magnetoelectric coupling in BiFeO3: Technological achievements and challenges

The control on magnetization via applied electric field is the centre of attention in multiferroic materials for the achievement of technological device applications. The bismuth ferrite (BiFeO3) is best multiferroic material which has the ferroelectric as well as weak magnetic nature simultaneously in the single phase near the room temperature. There are many unsolved problems in the path of the actual device applications of multiferroic BiFeO3 (BFO) due to the transformation of several scientific progresses into technical innovations. So, there is a need to solve these complications via focused research movements, which may convert this useful material into actual device applications. This review article is mainly concentrated on look over the recent advances on technological achievements and challenges in BFO via the magnetoelectric coupling and presents the latest devices achievements.

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.

Symmetry Modulation and Enhanced Multiferroic Characteristics in Bi1‐xNdxFeO3 Ceramics

AbstractBiFeO3 is recognized as the most important room temperature single phase multiferroic material. However, the weak magnetoelectric (ME) coupling remains as a key issue, which obstructs its applications. Since the magnetoelectric coupling in BiFeO3 is essentially hindered by the cycloidal spin structure, here efforts to improve the magnetoelectric coupling by destroying the cycloidal state and switching to the weak ferromagnetic state through symmetry modulation are reported. The structure is tuned from polar R3c to polar Pna21, and finally to nonpolar Pbnm by forming Bi1‐xNdxFeO3 solid solutions, where two morphotropic phase boundaries (MPBs) are detected. Greatly enhanced ferroelectric polarization is obtained together with the desired weak ferromagnetic characteristics in Bi1‐xNdxFeO3 ceramics at the compositions near MPBs. The change of magnetic state from antiferromagnetic (cycloidal state) to ferromagnetic (canted antiferromagnetic) is confirmed by the observation of magnetic domains using magnetic force microscopy. More interestingly, combining experiments and first‐principles‐based simulations, an electric field‐induced structural and magnetic transition from Pna21 back to R3c is demonstrated, providing a great opportunity for electric field‐controlled magnetism, and this transition is shown to be reversible with additional thermal treatment.

Deterministic and robust room-temperature exchange coupling in monodomain multiferroic BiFeO3 heterostructures

Exploiting multiferroic BiFeO3 thin films in spintronic devices requires deterministic and robust control of both internal magnetoelectric coupling in BiFeO3, as well as exchange coupling of its antiferromagnetic order to a ferromagnetic overlayer. Previous reports utilized approaches based on multi-step ferroelectric switching with multiple ferroelectric domains. Because domain walls can be responsible for fatigue, contain localized charges intrinsically or via defects, and present problems for device reproducibility and scaling, an alternative approach using a monodomain magnetoelectric state with single-step switching is desirable. Here we demonstrate room temperature, deterministic and robust, exchange coupling between monodomain BiFeO3 films and Co overlayer that is intrinsic (i.e., not dependent on domain walls). Direct coupling between BiFeO3 antiferromagnetic order and Co magnetization is observed, with ~ 90° in-plane Co moment rotation upon single-step switching that is reproducible for hundreds of cycles. This has important consequences for practical, low power non-volatile magnetoelectric devices utilizing BiFeO3.

Enhancement in electrical and magnetodielectric properties of Ca‐ and Ba‐doped BiFeO3 polycrystalline ceramics

AbstractWe carried out a comparative study on the electrical and magnetodielectric properties of polycrystalline BiFeO3, Bi0.9Ca0.1FeO2.95, Bi0.9Ba0.05Ca0.05FeO2.95, and Bi0.9Ba0.1FeO2.95 ceramics. The two dielectric anomalies, near 25 K and 281 K, are observed for BiFeO3. Interestingly, the anomaly near 25 K shifts towards a higher temperature above 60 K with Ca and/or Ba doping, attributed to the doping induced chemical pressure. In addition, the room temperature switchable magnetodielectric effect is witnessed for the doped BiFeO3 compounds, due to the quadratic magnetoelectric coupling. This indicates the improved magnetoelectric coupling in BiFeO3 with the Ca and Ba doping. This is essentially due to the enhanced magnetic ordering and reduced leakage current in BiFeO3 after the doping.

Electrical Switching of Magnetic Polarity in a Multiferroic BiFeO3 Device at Room Temperature

We have directly imaged reversible electrical switching of the cycloidal rotation direction (magnetic polarity) in a (111)pc-BiFeO3 epitaxial-film device at room temperature by non-resonant x-ray magnetic scattering. Consistent with previous reports, fully relaxed (111)pc-BiFeO3 epitaxial films consisting of a single ferroelectric domain were found to comprise a sub-micron-scale mosaic of magneto-elastic domains, all sharing a common direction of the magnetic polarity, which was found to switch reversibly upon reversal of the ferroelectric polarization without any measurable change of the magneto-elastic domain population. A real-space polarimetry map of our device clearly distinguished between regions of the sample electrically addressed into the two magnetic states with a resolution of a few tens of micron. Contrary to the general belief that the magneto-electric coupling in BiFeO3 is weak, we find that electrical switching has a dramatic effect on the magnetic structure, with the magnetic moments rotating on average by 90 degrees at every cycle.

Readdressing of Magnetoelectric Effect in Bulk BiFeO3

After decades of study, BiFeO3 is still the most promising single‐phase multiferroic material due to its large polarization and high operating temperature, drawing much attention. As a typical type‐I multiferroic material, the magnetoelectric coupling in BiFeO3 is deemed to be weak due to the different origins of its ferroelectricity and magnetism. Here, the magnetoelectric effect in bulk BiFeO3 is readdressed both theoretically and experimentally. Based on the Dzyaloshinsky–Moriya interaction scenario, the magnetoelectric effect in BiFeO3 is actually strong, with a coupling energy of about 1.25 meV and a magnetism‐coupled parasitic polarization comparable to that of the type‐II multiferroics. However, such strong magnetoelectric coupling also causes the cycloidal spin structure, which inhibits the observation of linear magnetoelectric coupling in bulk BiFeO3. To resolve this contradiction, Sm‐substitution is utilized to suppress the magnetoelectric effect and unlocks the weak ferromagnetism. At an optimized composition, such a weak ferromagnetic state can be switched back to the cycloidal state by an electric field, thus realizing electrical control of the magnetism. It has been argued that field‐controlled phase transition is a promising path to colossal magnetoelectric effect. It is of pioneering significance for further investigations down this road.

Magnetic, dielectric and magnetoelectric properties of BiFeO3-CoFe2O4 nanocomposites

Nanocomposites (NCs) comprising BiFeO3 (BFO) and CoFe2O4 (CFO) with nominal composition, x CoFe2O4 - (1-x) BiFeO3 (x = 0.2, 0.3 and 0.4) have been synthesized adopting a sol-gel route. XRD patterns of the prepared samples exhibit presence of both BFO and CFO crystalline phases. TEM images of the NCs show that particles are mostly spherical with size in the range 15–25 nm. Room temperature Mössbauer spectra of the pristine samples and NCs reveal presence of Fe3+ ions at different inequivalent sites of BFO and CFO. Room temperature dc magnetization measurements show significant enhancement in magnetization of the NCs compared to that of the pristine BFO. Variation of dielectric constant of the NCs with frequency shows a dispersive behavior in low frequency region which is attributed to grain boundary relaxation in the NCs. εr – T curves of all samples exhibit an anomaly near the antiferromagnetic transition temperature of BFO indicating presence of magnetoelectric (ME) coupling. Variation of capacitance of the NCs in presence of external magnetic field confirms an improvement of ME coupling in BFO after formation of the NCs.

Electrically enhanced magnetization in highly strained BiFeO3 films

The control of magnetism via an electric field has attracted substantial attention because of potential applications in magnetoelectronics, spintronics and high-frequency devices. In this study, we demonstrate a new approach to enhance and control the magnetization of multiferroic thin film by an electric stimulus. First, to reduce the strength of the antiferromagnetic superexchange interaction in BiFeO3, we applied strain engineering to stabilize a highly strained phase. Second, the direction of the ferroelectric polarization was controlled by an electric field to enhance the Dzyaloshinskii–Moriya interaction in the highly strained BiFeO3 phase. Because of the magnetoelectric coupling in BiFeO3, a strong correlation between the modulated ferroelectricity and enhanced magnetization was observed. The tunability of this strong correlation by an electric field provides an intriguing route to control ferromagnetism in a single-phase multiferroic.

Structural transformation and multiferroic properties of Ba–Mn co-doped BiFeO3

Pure BiFeO3 and Bi1−xBaxFe1−xMnxO3 (x=0.10,0.20) fine ceramics were synthesized using mechano-synthesis route. The influence of co-doping (Ba–Mn) on structural and multiferroic properties of BiFeO3 has been studied in different experimental conditions. X-ray diffraction patterns, Rietveld structural refinement of XRD patterns and Fourier transform infrared (FTIR) spectra reveal the structural transition from rhombohedral (R3c) to the biphasic structure (R3c + P4mm) on co-doping. The co-doping improves surface morphology and also reduces the particle size. The room temperature M–H loops of all samples showed antiferromagnetic/weak ferromagnetic behavior. Magnetoelectric coupling coefficient determination is carried out to reveal extent of intimate interaction between electric and magnetic dipoles interaction in the samples. Room temperature occurrence of ferromagnetism, ferroelectricity and magnetoelectric effect supports the observation of multiferroism and magnetoelectric coupling in BiFeO3. Thus, co-doping at Bi- and Fe-sites of BiFeO3 can improve multiferroic properties of BiFeO3 for various applications.

Effect of strain on voltage-controlled magnetism in BiFeO3-based heterostructures

Voltage-modulated magnetism in magnetic/BiFeO3 heterostructures can be driven by a combination of the intrinsic ferroelectric-antiferromagnetic coupling in BiFeO3 and the antiferromagnetic-ferromagnetic exchange interaction across the heterointerface. However, ferroelectric BiFeO3 film is also ferroelastic, thus it is possible to generate voltage-induced strain in BiFeO3 that could be applied onto the magnetic layer across the heterointerface and modulate magnetism through magnetoelastic coupling. Here, we investigated, using phase-field simulations, the role of strain in voltage-controlled magnetism for these BiFeO3-based heterostructures. It is predicted, under certain condition, coexistence of strain and exchange interaction will result in a pure voltage-driven 180° magnetization reversal in BiFeO3-based heterostructures.

Interfacial magnetism and exchange coupling in BiFeO3–CuO nanocomposite

Ferromagnetic BiFeO3 nanocrystals of average size 9 nm were used to form a composite with antiferromagnetic CuO nanosheets, with the composition (x)BiFeO3/(100−x)CuO, x = 0, 20, 40, 50, 60, 80 and 100. The dispersion of BiFeO3 nanocrystals into the CuO matrix was confirmed by x-ray diffraction and transmission electron microscopy. The ferromagnetic ordering as observed in pure BiFeO3 occurs mainly due to the reduction in the particle size as compared to the wavelength (62 nm) of the spiral modulated spin structure of the bulk BiFeO3. Surface spin disorder of BiFeO3 nanocrystals gives rise to an exponential behavior of magnetization with temperature. Strong magnetic exchange coupling between the BiFeO3 nanocrystal and the CuO matrix induces an interfacial superparamagnetic phase with a blocking temperature of about 80 K. Zero field and field cooled magnetizations are analyzed by a ferromagnetic core and disordered spin shell model. The temperature dependence of the calculated saturation magnetization exhibits three magnetic contributions in three temperature regimes. The BiFeO3/CuO nanocomposites reveal an exchange bias effect below 170 K. The maximum exchange bias field HEB is 1841 Oe for x = 50 at 5 K under field cooling of 50 kOe. The exchange bias coupling results in an increase of coercivity of 1934 Oe at 5 K. Blocked spins within an interfacial region give rise to a remarkable exchange bias effect in the nanocomposite due to strong magnetic exchange coupling between the BiFeO3 nanocrystals and the CuO nanosheets.

Investigation of spin phonon coupling in BiFeO3 based system by Fourier transform infrared spectroscopy

In the present work, the low temperature infrared absorption spectra of BiFeO3 (BFO) are measured to explore the spin-phonon coupling in this compound. At 303 K, 4 weak transverse optic (TO) IR-active phonon modes E(TO6), E(TO7), E(TO8), and E(TO9) are observed. First two modes are corresponded to the Fe3+ cations caused by the internal vibration of FeO6 octahedra, E(TO8) is correlated to Fe-O bending vibration and E(TO9) is assigned to Fe-O stretching vibrations, respectively. At 213 K, two new modes E(TO5) and A1(TO3) are emerging out. Both are assigned to Fe3+ cations caused by the internal vibration of FeO6 octahedra. These modes get stronger and stronger with lowering the temperature due to the lattice contraction. When the temperatures decreases to T ≤ 213 K, an additional phonon mode is start appearing at around 638 cm−1 suggesting local lattice distortion of FeO6 octahedra. The temperature is corresponding with the FC and zero field cooled bifurcation temperature, which is related to the onset of spin glass behaviour. The occurrence of this additional phonon mode at this particular temperature suggests that there is strong spin-phonon coupling in BFO. This argument is further supported by the temperature dependence of this additional phonon peak. It shows anomaly around 124 K, which is related to spin reorientation of Fe3+ ions. This result clearly indicates that spin glass state and spin reorientation of Fe3+ is accompanied with the local structure distortion of FeO6 octahedra, providing evidence for the strong spin-phonon coupling in the BFO.

Evidences of magneto-electric coupling in BFO–BT solid solutions

Abstract Multiferroic properties viz antiferromagnetism and ferroelectricity of BiFeO3 (BFO) has been reported to be enhanced by forming solid solution with BaTiO3 (BT) designated by BFO–BT. In the present investigations a few compositions in the solid solution (1−x)BiFeO3–xBaTiO3 (x = 0.10, 0.15 and 0.30) have been synthesized by solid state ceramic route. Structural, magnetic and electrical properties of single phase solid solution compositions of BFO–BT have been studied in detail to understand their multiferroic properties. Powder X-ray diffraction confirms the formation of solid solution. All the samples show ferromagnetic behavior at room temperature having low coercivity and high saturation magnetization. Substitution of BaTiO3 into BiFeO3 enhances dielectric and ferromagnetic responses and reduces electrical leakage. It suggests a coupling between ferroelectric and magnetic parameters near the antiferromagnetic–paramagnetic transition, which is responsible for the broad frequency dependent dielectric maxima. The impedance spectroscopy also confirms the origin of the structure.

Accessing intermediate ferroelectric switching regimes with time-resolved transmission electron microscopy

BiFeO3 (BFO) is one of the most widely studied magneto-electric multiferroics. The magneto-electric coupling in BiFeO3, which allows for the control of the ferroelectric and magnetic domain structures via applied electric fields, can be used to incorporate BiFeO3 into novel spintronics devices and sensors. Before BiFeO3 can be integrated into such devices, however, a better understanding of the dynamics of ferroelectric switching, particularly in the vicinity of extended defects, is needed. We use in situ transmission electron microscopy (TEM) to investigate the response of ferroelectric domains within BiFeO3 thin films to applied electric fields at high temporal and spatial resolution. This technique is well suited to imaging the observed intermediate ferroelectric switching regimes, which occur on a time- and length-scale that are too fine to study via conventional scanning-probe techniques. Additionally, the spatial resolution of transmission electron microscopy allows for the direct study of the dynamics of domain nucleation and propagation in the presence of structural defects. In this article, we show how this high resolution technique captures transient ferroelectric structures forming during biasing, and how defects can both pin domains and act as a nucleation source. The observation of continuing domain coalescence over a range of times qualitatively agrees with the nucleation-limited-switching model proposed by Tagantsev et al. We demonstrate that our in situ transmission electron microscopy technique is well-suited to studying the dynamics of ferroelectric domains in BiFeO3 and other ferroelectric materials. These biasing experiments provide a real-time view of the complex dynamics of domain switching and complement scanning-probe techniques.

Non-linear thermal evolution of the crystal structure and phase transitions of LaFeO3 investigated by high temperature X-ray diffraction

The crystal structure, anisotropic thermal expansion and structural phase transition of the perovskite LaFeO3 has been studied by high-temperature X-ray diffraction from room temperature to 1533K. The structural evolution of the orthorhombic phase with space group Pbnm and the rhombohedral phase with R3¯c structure of LaFeO3 is reported in terms of lattice parameters, thermal expansion coefficients, atomic positions, octahedral rotations and polyhedral volumes. Non-linear lattice expansion across the antiferromagnetic to paramagnetic transition of LaFeO3 at TN=735K was compared to the corresponding behavior of the ferroelectric antiferromagnet BiFeO3 to gain insight to the magnetoelectric coupling in BiFeO3, which is also multiferroic. The first order phase transition of LaFeO3 from Pbnm to R3¯c was observed at 1228±9K, and a subsequent transition to Pm3¯m was extrapolated to occur at 2140±30K. The stability of the Pbnm and R3¯c polymorphs of LaFeO3 is discussed in terms of the competing enthalpy and entropy of the two crystal polymorphs and the thermal evolution of the polyhedral volume ratio VA/VB.

Direct observation of ferroelectric domain switching in varying electric field regimes using in situ TEM

In situ Transmission Electron Microscopy (TEM) techniques can potentially fill in gaps in the current understanding interfacial phenomena in complex oxides. Select multiferroic oxide materials, such as BiFeO3 (BFO), exhibit ferroelectric and magnetic order, and the two order parameters are coupled through a quantum–mechanical exchange interaction. The magneto-electric coupling in BFO allows control of the ferroelectric and magnetic domain structures via applied electric fields. Because of these unique properties, BFO and other magneto-electric multiferroics constitute a promising class of materials for incorporation into devices such as high-density ferroelectric and magnetoresistive memories, spin valves, and magnetic field sensors. The magneto-electric coupling in BFO is mediated by volatile ferroelastically switched domains that make it difficult to incorporate this material into devices.To facilitate device integration, an understanding of the microstructural factors that affect ferroelastic relaxation and ferroelectric domain switching must be developed. In this article, a method of viewing ferroelectric (and ferroelastic) domain dynamics using in situ biasing in TEM is presented. The evolution of ferroelastically switched ferroelectric domains in BFO thin films during many switching cycles is investigated. Evidence of partial domain nucleation, propagation, and switching even at applied electric fields below the estimated coercive field is revealed. Our observations indicate that the occurrence of ferroelastic relaxation in switched domains and the stability of these domains is influenced the applied field as well as the BFO microstructure. These biasing experiments provide a real time view of the complex dynamics of domain switching and complement scanning probe techniques. Quantitative information about domain switching under bias in ferroelectric and multiferroic materials can be extracted from in situ TEM to provide a predictive tool for future device development.

Effect of rare earth doping on BiFeO3 magnetic and structural properties (La, Gd)

In order to remove the incommensurate cycloid spin coupling in BiFeO3 that inhibits this multiferroic compound of showing magnetoelectric effect, we doped this perovskite on Bi site with rare-earth ions (La, Gd), showing that the antiferromagnetic ordering with space modulated coupling of spins is broken. A correlation between the structure and the magnetic properties is found through Rietveld Refinement of X-Ray diffraction patterns, magnetic hysteresis curves and the hyperfine parameters obtained by Mossbauer Spectroscopy at room temperature, justified by the doping ion size that induces a magnetic structure distortion reflected in an increase of the coercive field and the remanent magnetization as well as in the hyperfine parameters behavior, linked with the structural phase transitions caused by the removal of the Bi 6s2 lone pair that induces structural distortions. It is also shown that the symmetries used are polar, that in principle can let the compounds keep a ferroelectric behavior.