Multiferroic Bismuth Ferrite Research Articles

Magnetic properties and oxygen migration induced resistive switching effect in Y substituted multiferroic bismuth ferrite

Multiferroic Y-doped BiFeO3 (BY10FO) thin films were deposited on FTO coated glass substrate using sol-gel spin coating technique. Y-doping causes structural distortion without changing the structure of the parent BiFeO3 (rhombohedral: R3c). The M-H hysteresis curve reveals that the BY10FO film exhibits saturation magnetization at a low-coercive field by suppressing the spiral spin modulated structure. The bipolar resistive switching behavior has been investigated on a Ag/BY10FO/FTO hetero-structure through conventional I-V curve measurements and the device can produce an ON/OFF ratio of around 12 over 30 complete testing cycles. The space charge limited current and Schottky barrier emission conduction mechanism play a crucial role in switching the states between HRS and LRS. The impedance spectroscopy analysis at HRS and LRS confirms the significant degradation of resistance from MΩ to kΩ. The switching mechanism in the hetero-structure is due to migration and recombination of oxygen vacancies present in the film. The non-degradation of the Ag/BY10FO/FTO device after several testing cycles confirms that the switching of resistance between ON and OFF states is reproducible, reversible and controllable to be used for possible future non-volatile resistive random access memory application.

Effect of Ho doping on the crystal structure, surface morphology and magnetic property of BiFeO3 thin films prepared via the sol-gel technology

Multiferroic material bismuth ferrite (BiFeO3 = BFO) and Ho-substitution (Bi1-xHoxFeO3, x = 0.05, 0.10, 0.15, 0.20) thin films were successfully fabricated on silicon (110) substrates by the sol-gel approach. The doping-Ho effects on crystal structure, surface morphology and magnetic property were investigated. The X-ray diffraction (XRD) revealed that the Ho-substitution thin films underwent a phase transition from the rhombohedral to the orthorhombic structure. The changes in Ho-substitution and pure BFO surface appearance were analyzed using the Field Emission Scanning Electron Microscopy (SEM). Meanwhile, the saturation magnetization (Ms) of Ho-substitution samples obtained remarkable augment with the increase in Ho-doping due to the enhancement of superexchange interactions between Fe-O-Fe and spatial modulation for helical structure destruction. The heightening of superexchange interactions is attributed to the reduction of oxygen vacancies because of Ho3+ ions in a stable status.

Investigations of magnetic and ferroelectric properties of multiferroic Sr-doped bismuth ferrite

Strontium substituted bismuth ferrite (Bi1−xSrxFeO3 with x = 0.05, 0.15 and 0.25) multiferroic nanoparticles were successfully synthesized by a simple sol–gel auto combustion technique to study the effect of Sr2+ varying with a concentration on their structural and multiferroic properties. X-ray diffraction patterns confirmed the phase purity of Bi1−xSrxFeO3 samples having a hexagonal structure with R3c space group. The lattice parameters ‘a’ and ‘c’ decrease, whereas the unit cell volume contracts with increasing Sr2+ concentration in BFO. Fourier transform infrared spectroscopy analysis was employed to study the chemical bonds present. Field-emission scanning electron microscopy images of samples show agglomerated nanoparticles with varying average grain sizes 61 and 32 nm. The elemental composition was confirmed using energy-dispersive spectrum analysis. Vibrating sample magnetometer analysis at room temperature showed a significant change in magnetization on Sr2+ substituted BFO with increased saturation magnetization value (Ms = 5.55 emu/g) at x = 0.25 composition. P–E loop tracer used to study the ferroelectric properties and the observed diverse properties of Sr2+ substituted BFO show applicability for multifunctional devices like actuators.

Largely enhanced coercivity of cobalt adjacent to straight-stripe mixed-phase bismuth ferrites

Exchange coupling at the interface of an antiferromagnet and a ferromagnet is the major route toward the future magnetoelectric applications of multiferroic bismuth ferrite at room temperature. By using angle-resolved longitudinal magneto-optic Kerr effect microscopy, we have investigated the exchange anisotropy in a ferromagnetic Co layer adjacent to electrically aligned straight-stripe mixed-phase-boundary (MPB) regions of La-5%-doped $\mathrm{BiFe}{\mathrm{O}}_{3}$. We have found that the magnetic easy axis of the exchange-coupled Co layer becomes parallel to the in-plane crystallographic axis nearest to the adjacent MPB elongation axis. The coercive field of the exchange-coupled Co layer along the magnetic easy axis has prominently increased by $\ensuremath{\sim}66$ Oe, i.e., 30 times larger than that of a control sample (2.3 Oe), wherein a 5-nm-thick nonmagnetic Ta spacer is placed between the antiferromagnetic and ferromagnetic layers and thus it is intended to have only the shape anisotropy arising from the surface nanostructure of the MPB regions. The finding opens a promising avenue for magnetoelectric applications at room temperature.

Preparation parameters optimization and structure investigation of multiferroic bismuth ferrite

Multiferroic bismuth ferrite, BiFeO3 (BFO), was prepared using conventional solid state reaction method. The obtained powders were thermally treated at different temperatures (600–850 °C) and times (15 min–15 h) with two heating regimes (slow and fast) to investigate the optimum synthesis conditions. Rietveld refinement was applied to the X-ray diffraction data and Raman spectroscopy has been used to verify the phase purity. Both the temperature and time of the heat treatment were critical parameters to obtain the high BFO phase content. Sample of 95 wt% of BFO phase has been achieved by using fast heating/cooling regime of 30 min at 800 °C and leaching with HNO3 to eliminate the impurity phases Bi2Fe4O9 and Bi25FeO40. Upper and lower calcinations temperatures and times result in higher content of secondary phases. The Raman active modes confirmed the presence of the single phase of BFO. Additional physical measurement (electrical and magnetic) prove the multiferroic feature of BFO. The estimated high activation energy (0.89 eV) indicates the antiferromagnetic order. The magnetic hysteresis loops gives indication for the existence of a weak canting moment.

Effect of Nd Substitution on Properties of Multiferroic Bismuth Ferrite Synthesized by Sol-Gel Auto-combustion Method

In this work, pure and Nd-doped bismuth ferrite nanocrystals with the general formula of Bi1−x Nd x FeO3 (x = 0, 0.05, 0.1, 0.15, 0.2) were synthesized via the sol-gel auto-combustion method at two different calcination temperatures (550 and 650∘C). Phase analysis and investigating structural, thermal, and magnetic properties and measuring the size of nanoparticles have been done by using X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric and differential thermal analysis (TG/DTA), and vibrating sample magnetometry (VSM). For undoped BiFeO3 (BFO), increasing calcination temperature will increase the average size of nanocrystals. On the other hand, the average size of nanoparticles is decreased by increasing the concentration of neodymium in the structure of BFO. By entering neodymium to the structure of bismuth ferrite at the calcination temperature of 550∘C, the ferromagnetic properties decline. However, at 650∘C, ferromagnetic properties are improved which is due to a fall in the number of Fe2+ ions. In this case, by decreasing the Fe2+ ions, remanent and saturation magnetization increase.

Real-space imaging of non-collinear antiferromagnetic order with a single-spin magnetometer.

Although ferromagnets have many applications, their large magnetization and the resulting energy cost for switching magnetic moments bring into question their suitability for reliable low-power spintronic devices. Non-collinear antiferromagnetic systems do not suffer from this problem, and often have extra functionalities: non-collinear spin order may break space-inversion symmetry and thus allow electric-field control of magnetism, or may produce emergent spin-orbit effects that enable efficient spin-charge interconversion. To harness these traits for next-generation spintronics, the nanoscale control and imaging capabilities that are now routine for ferromagnets must be developed for antiferromagnetic systems. Here, using a non-invasive, scanning single-spin magnetometer based on a nitrogen-vacancy defect in diamond, we demonstrate real-space visualization of non-collinear antiferromagnetic order in a magnetic thin film at room temperature. We image the spin cycloid of a multiferroic bismuth ferrite (BiFeO3) thin film and extract a period of about 70 nanometres, consistent with values determined by macroscopic diffraction. In addition, we take advantage of the magnetoelectric coupling present in BiFeO3 to manipulate the cycloid propagation direction by an electric field. Besides highlighting the potential of nitrogen-vacancy magnetometry for imaging complex antiferromagnetic orders at the nanoscale, these results demonstrate how BiFeO3 can be used in the design of reconfigurable nanoscale spin textures.

Simple preparation and characterization of bismuth ferrites nanoparticles by thermal treatment method

In the present work, nanoparticles of multiferroic bismuth ferrites (BiFeO3) were synthesized via a simple thermal treatment method. BiFeO3 was prepared from an aqueous solution containing bismuth nitrate and iron nitrate as starting materials, polyvinyl pyrrolidone (PVP) as a capping agent and nitric acid to dissolve the bismuth nitrate, respectively. It is followed by thermal treatment at various calcination temperatures at 350, 450 and 550 °C. The samples were characterized by thermogravimetric analysis, X-ray diffractometer (XRD), transmission electron microscope (TEM), vibrating sample magnetometer and electron spin resonance (ESR) spectroscopy. XRD results indicate that the samples, calcined at 350, 450 and 550 °C, crystalized in rhombohedral crystal structure (space group R3c). The crystallinity of samples increased with increasing calcination temperature. Morphology study using TEM confirmed the growth of BiFeO3 nanoparticles with the average particle’s size increases from ~30 nm up to ~80 nm with the increasing of calcination temperature from 350 to 550 °C. Magnetic saturation Ms, of samples decreased from 2.15 to 0.25 emu/g while the coercivity Hc, increased from 54.41 to 272 G when the calcination temperature increased from 350 to 550 °C. ESR revealed increment of g-factor value from 2.14 to 2.64 and peak-to-peak linewidth from 129.33 to 201.61 Oe with the increasing of calcination temperature from 350 to 550 °C. The results demonstrate that by using thermal treatment method, the BiFeO3 nanoparticles can be obtained at low temperature, i.e. 350 °C.

Enhanced ferroelectric and piezoelectric properties of La x Bi (1−x) FeO 3 ceramics studied by impedance spectroscopy

Abstract Multiferroic bismuth ferrite (BiFeO 3 , BFO) is one of the most promising high-temperature ferroelectric and piezoelectric materials due to a high Curie temperature ( T C ∼830 °C) if the enhancement of ferroelectricity and piezoelectricity can be realized. In this work, we introduced La ( x =0.05–0.30) to BFO to change its crystal structure and obtained an enhanced Rhombohedral structure. The high pure La x Bi (1− x ) FeO 3 (LBFO) powders with refined grain size were prepared by microwave hydrothermal method, then the ceramics were sintered under microwave condition. The enhanced piezoelectric property results (d 33 =55–62pC·N −1 ) showed that La 3+ could not only stable the Rhombohedral structure and form Metastable phase contributing to the piezoelectric effects at a special range of doping contents. The figure of dielectric property versus frequency indicated La 3+ doped BFO ceramics could increase dielectric constant and reduce the loss. The values of bulk resistance (R b ) obtained from the intercept of semicircular arcs on the real axis (Z) at different temperatures decrease with the increasing of the doping contents. The P r increased which could be ascribed to that the introduction of La could lead to the distortion of the crystal structure of BFO ceramics and the formation of more dipoles or short-range charge carriers. Finally, the M-H loops suggested that the Saturation magnetization and Coercive magnetic field LBFO ceramics was improved with increasing La contents.

Thermodynamic potential and phase diagram for multiferroic bismuth ferrite (BiFeO3)

We construct a Landau–Ginzburg thermodynamic potential, and the corresponding phase diagram for pristine and slightly doped bismuth ferrite, a ferroelectric antiferromagnet at room temperature. The potential is developed based on new X-ray and neutron diffraction experiments complementing available data. We demonstrate that a strong biquadratic antiferrodistortive-type coupling is the key to a quantitative description of Bi1−xLaxFeO3 multiferroic phase diagram including the temperature stability of the antiferromagnetic, ferroelectric, and antiferrodistortive phases, as well as for the prediction of novel intermediate structural phases. Furthermore, we show that “rotomagnetic” antiferrodistortive–antiferromagnetic coupling is very important to describe the ferroelectric polarization and antiferrodistortive tilt behavior in the R3c phase of BiFeO3. The Landau–Ginzburg thermodynamic potential is able to describe the sequence of serial and trigger-type phase transitions, the temperature-dependent behavior of the order parameters, and the corresponding susceptibilities to external stimuli. It can also be employed to predict the corresponding ferroelectric and antiferrodistortive properties of Bi1−xLaxFeO3 thin films and nanoparticles by incorporating the gradient and surface energy terms that are strongly dependent on the shape, size, and preparation method.

An experimental study of the local electronic structure of B-site gallium doped bismuth ferrite powders

The un-doped and gallium (Ga) doped multiferroic bismuth ferrite (BiFeO3) compounds were successfully synthesized by the hydrothermal technique. This is then followed by a series of detailed investigations into the influence of Ga doping on the structural, morphological, optical and magnetic properties of BiFe1−xGaxO3(0≤x≤0.15). X-ray diffraction (XRD) analyses reveal that B-site doping of Ga may lead to the transformation of its crystal structure from a rhombohedral with a space group of R3c to an orthorhombic with a space group of Pbnm phase. The scanning electron microscope (SEM) images show that doping with Ga causes a significant reduction in particle size, when compared to un-doped BiFeO3. X-ray absorption near-edge structure (XANES) spectra have also shown that as the substitution of Ga increases, the Fe K and Bi LIII-edge absorption spectra shift towards higher energy site. It also implies that doping of Ga affects the local structures of both Bi and Fe atoms. Distinct transmission band fluctuations, as shown by Fourier transform infrared (FT-IR) spectra, can provide significant evidence for a transformation in crystal structure, following the increase of Ga concentration. The magnetization hysteresis measurements show that the magnetization decreases when Ga content is x≥0.05. Such a phenomenon mainly stems from the substitution of Fe for an excessive amount of Ga, resulting in reduction of both valence fluctuations and the magnetization in the BiFe1−xGaxO3 system.

Mechanically assisted synthesis of multiferroic BiFeO 3 : Effect of synthesis parameters

Abstract We report the influence of synthesis parameters on the crystal structure and the electronic and magnetic behavior of multiferroic bismuth ferrites, BiFeO 3 , synthesized by high-energy ball milling followed by annealing at low temperatures. Mixtures of Fe 2 O 3 and Bi 2 O 3 , in a stoichiometric ratio, were mixed and milled for different durations from 0 to 13 h in an ambient air atmosphere and annealed for 2 h at different temperatures, from 350 to 750 °C in order to obtain BiFeO 3 . X-ray diffraction and Rietveld refinement were used to elucidate the phase transformation as a function of the synthesis parameters. The results showed that BiFeO 3 particles with mostly a single phase (∼98 wt%) were fabricated by the assisted high-energy ball milling process, after 5 h of milling followed by heat treatment at 650 °C; higher milling durations and annealing temperatures promote perovskite decomposition. The obtained powder shows a weak ferromagnetic behavior, which changes to an antiferromagnetic order when the powder is pressed at 900 MPa and sintered at 800 °C for 2h. The magnetic and dielectric results confirm the multiferroic behavior of the obtained material.

Dielectric dispersion and impedance spectroscopy of yttrium doped BiFeO3-PbTiO3 electronic system

The dielectric dispersion and impedance spectroscopy of multiferroic bismuth ferrite (BFO; BiFeO3) and lead titanate (PT; PbTiO3) electro ceramics have been altered by doping chemically with Yttrium (Y3+). The distinctive structural study including morphology as well as electrical properties of Y3+ doped BFO-PT solid composite [(Pb0.9Bi.05Y0.05) (Fe0.1Ti0.9)O3] depicts several interesting results on structure-properties correlation. The development of single phase compound with tetragonal crystal coordination is predicted from X-ray diffraction (XRD). The micrograph substantiates the development of dense grains. The chemical content and composition of the prepared solid solution is determined using the energy dispersive X-ray (EDX) technique. The impedance spectroscopy shows the evidence of grain as well as grain boundary effects leading to the subsistence of negative temperature coefficient of resistance (NTCR) in this processed material. At lower temperature region, the ac conductivity of this compound rises with the frequency. The processed electronic compound illustrates non-Debye type electrical relaxation behaviour. The preliminary study of the prepared electronic material may provide some useful information for devise of electronic components as well as gadget.

Modification of Relaxor and Impedance Spectroscopy Properties of Lead Magnesium Niobate by Bismuth Ferrite

The relaxor and impedance characteristics of classic or traditional lead magnesium niobate (PbMg1/3Nb2/3O3; PMN) ferroelectric relaxor material have been modified by chemically synthesizing with multiferroic bismuth ferrite (BiFeO3; BFO). Detailed studies of structural, morphological and electrical properties of PMN-BFO-prepared solid solutions [((Pb1−xBix) (Mg0.33(1−x)Nb0.66(1−x)Fex) O3) with x = 0.1, 0.2, 0.3 and 0.4] reveal some interesting findings on structure–properties relationships. The formation of single phase material of each compound in orthorhombic crystal system is identified from x-ray diffraction. The microstructure analyses reveal that the grain size of PMN-BFO increases for increasing BFO weight percent with PMN. The increase of BFO concentration not only improves the dielectric response of PMN-BFO but also modifies the nature of attained phase transition from a typical relaxor to a normal ferroelectric. The impedance spectroscopy studies exhibit the presence of grain and grain boundary effects, and the existence of a positive temperature coefficient of resistance (PTCR) in the material. The ac conductivity increases with the increase in frequency in the low-temperature region for larger content of BFO in the solid solutions. It is observed that the prepared electronic materials obey the non-Debye-type of conductivity relaxation behavior.

An in situ resonant photoemission and x-ray absorption study of the BiFeO3 thin film

Multiferroic bismuth ferrite (BiFeO3) thin films were prepared using the pulsed laser deposition (PLD) technique. The electronic structure of the film was studied in situ using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). Both the Fe 2p PES and XAS spectra show that the Fe ions were initially in a+3 valence state. The Fe 2p and O K edge XAS spectra indicate that the oxygen octahedral crystal ligand field divides the unoccupied Fe 3d state into t2g↓and eg↓states. Valence band Fe 2p–3d resonant photoemission results indicate that hybridization between Fe 3d and O 2p plays an important role in the multiferroic properties of BiFeO3 thin films.

Ferroelectric and Magnetic Domain Walls in High Temperature Multiferroic Films and Heterostructures

In the last decade, considerable attention has been focused on the search of new multiferroic materials and the ways of improvement of their magnetoelectric properties. In this short review, we survey the progress in study of multiferroics focusing the high temperature multiferroic bismuth ferrite and rare earth iron garnets. We discuss the recent results of investigation of domain walls in multiferroics, concentrating the most important magnetoelectric manifestations (electric polarization and magnetization), and the pinning effect appearing as clamping of ferroelectric and magnetic domain walls.

Dielectric behavior and magnetical response for porous BFO thin films with various thicknesses over Pt/Ti/SiO2/Si substrate

A novel sol–gel method has been developed to deposit multiferroic nanocrystalline bismuth ferrite (BFO) thin films over Pt/Ti/SiO2/Si substrate by spin-coating technique with various thicknesses. It is found that the deposition parameters significantly influence the quality and the thickness of BiFeO3 films. The films are all uniform and adherent to Pt/Ti/SiO2/Si substrate. The spin-coated films are characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM), Atomic force microscope (AFM), photoluminescence spectroscopy (PL) and Fourier transform infrared spectroscopy (FTIR). Rhombohedral structure of BFO is confirmed from the XRD and FT-IR studies. The SEM image shows a porous structure formation of BFO over Pt/Ti/SiO2/Si substrate. The surface outgrowth for the films at various thicknesses is measured from root mean square (RMS) and surface roughness through AFM. The step height and the RMS are found to be high for the film at 500nm in comparison with thickness of 200nm. The influence of the dielectric properties of the porous BFO at different thicknesses is studied using LCRQ meter. Finally, the magnetic behavior of film is compared with M–H hysteresis loop and Magnetoresistance (MR) studies.

Synthesis and characterization of low density polyethylene with multiferroic bismuth ferrite nanocomposite

A novel low density polyethylene (LDPE) with multiferroic bismuth ferrite (BiFeO3) nano-dielectric was successfully prepared via a solution-melt blending method. The structure, thermal, magnetic, as well as dielectric properties of the nano-dielectric were examined. The microstructures of LDPE/BiFeO3 were characterized by Fourier transform infrared, X-ray diffraction, and scanning electron microscopy. Microstructural analysis indicated that BiFeO3 nanoparticles were well dispersed in the LDPE matrix and the structure of LDPE was unchanged even after doping with BiFeO3 nanoparticles. The differential scanning calorimetry data of the thermal properties revealed that melting temperature has decreased and crystallinity has increased. The reason was that amount of nucleation agents was increased and crystal propagation rate was changed. The corresponding magnetic and dielectric properties were measured using vibrating sample magnetometer and broadband dielectric spectrometer. The results showed that the magnetization behaviour of diamagnetic LDPE was transformed due to the antiferromagnetism of BiFeO3. In addition, the dielectric permittivity and loss of LDPE/BiFeO3 increased with increasing BiFeO3 filler concentrations in the LDPE matrix due to different response mechanisms.

Impedance and AC conductivity study of nano crystalline, fine grained multiferroic bismuth ferrite (BiFeO3), synthesized by microwave sintering

In this paper, major reduction in sintering time,temperautre and significant improvement over final density of sitnered sample is reported for the microwave sintered nanocrystalline BiFeO3 (BFO) ceramic. Also, different sintering time and temperatures have been used to tailor the grain size and the final density of the resulting BFO ceramics synthesized from phase pure BFO nanoparticles (d̄ ≈10 nm). Microwave sintering resulted in reducing the sintering time substantially (by 1h), and has resulted in submicron sized grains and high resistivity ∼1.8 GΩ-cm. The AC conductivity is seen to follow the Jonscher’s power law behavior, suggesting correlated barrier hopping (CBH) mechanism in the sample. The role of oxygen vacancies at high temperature, due to volatility of bismuth, in dielectric and conductivity behavior is also discussed. Further, the sample displayed dielectric anomaly near magnetic transition temperature (∼180 °C) indicating bearing of magnetic moments on the dielectric properties. Using Impedance Spectroscopy (IS) we have established, the electrical heterogeneity of the ceramic BFO reavealing semiconducting nature of grains and insulating nature of grain boundary. This, formation of network of insulating grain boundaries and semiconducting grains could lead to formation of internal barrier layer capacitance (IBLC) leading to high dielectric constant in microwave sintered BFO.

Ultrafast near-infrared nonlinear absorption in a multiferroic single crystal of bismuth ferrite

We studied the ultrafast third-order optical nonlinearity in a single crystal of multiferroic bismuth ferrite (BiFeO3) in the near-infrared range of 0.5–1.0 eV, where the material is fundamentally transparent, at room temperature. With pump pulses at 1.55 eV, which is off-resonant to the strong inter-band charge transfer (CT) transition, we observed instantaneous transient absorption with a pencil-like temporal profile originating from the two-photon CT transition from the oxygen 2p to the iron 3d levels. In contrast, under pumping with 3.10 eV photons, the pencil-like absorption change was not observed but decay profiles showed longer time constants. Although the two-photon absorption coefficient estimated to be 1.5 cm/GW is 10 (100) times smaller than that of two (one)-dimensional cuprates, it is larger than those of common semiconductors such as ZnSe at the optical communication wavelength.