Simultaneous reduction in leakage current and enhancement in magnetic moment in BiFeO3 nanofibers via optimized Sn doping

Sn⁴⁺‐doped BiFeO₃ (BFO) nanofibers have been fabricated by sol–gel based electrospinning method with nanofiber diameter in the range of 30–100 nm in the annealed state. The leakage current density dropped by two orders of magnitude in 3% Sn‐doped BFO nanofibers compared to undoped BFO samples. Our density functional theory (DFT) simulation results indicate that Sn⁴⁺ prefers to occupy the Fe³⁺ site in BFO with a formation energy of 1.89 eV. The impurity Sn acts as a shallow donor with a low transition energy level of 41 meV. Furthermore, an enhancement in the saturation magnetization was simultaneously observed for 3% Sn‐doped nanofibers, which correlated well with our theoretical calculations. In other words, by carefully tailoring the degree of Sn doping in BFO nanofibers, we can reduce the leakage current and concurrently enhance the magnetic moment. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Correlation of structure and spin–phonon coupling in (La,Nd) doped BiFeO3 films

We studied the effect of structural change on the spin–phonon coupling in doped BiFeO₃ (BFO) films (Bi_0.8La_0.1Nd_0.1FeO₃) grown on SrTiO₃ (001) substrate. The temperature‐dependent Raman studies show phonon anomalies in the vicinity of magnetic ordering temperature T_N owing to the spin–phonon coupling. Doped films exhibit strong anomalies in the line widths of Raman bands around T_N revealing the presence of strong spin–lattice coupling. The modification in structure as a result of A‐site doping in BFO films plays an important role in controlling the nature of spin–phonon coupling. Copyright © 2014 John Wiley & Sons, Ltd.     

Modification in structure of La and Nd co‐doped epitaxial BiFeO3 thin films probed by micro Raman spectroscopy

The influence of La and Nd co‐substitution on structure, electric and magnetic properties of epitaxial thin films of BiFeO₃ (BFO) was examined. We demonstrate structural phase transition in co‐doped La and Nd BFO thin films using Raman spectroscopy. Based on group theoretical analysis of the number and symmetry of Raman lines, we provide strong experimental evidence that the structure has been changed from rhombohedral to monoclinic due to co‐doping in BFO. The change in structure was also reflected in morphology of these films. Room temperature magnetic hysteresis curves showed that doped films exhibit enhanced ferromagnetic properties with remnant magnetization of ~10 emu/cm³ and coercive field of 1.2 kOe. The enhanced magnetic properties highlight the potential applications of doped BLNFO thin film for smart devices. Copyright © 2015 John Wiley & Sons, Ltd.     

Magnetoresistance of epitaxial SrRuO3 thin films on a flexible CoFe2O4-buffered mica substrate

We have investigated the magnetoresistance of epitaxial SrRuO₃ (SRO) thin films on a flexible CoFe₂O₄ (CFO)-buffered mica substrate. High-resolution X-ray diffraction and transmission electron microscopy revealed that the SRO film could be epitaxially grown on a mica substrate with a 22-nm-thick CFO buffer layer. The epitaxial relationships were SRO [1–10] || CFO [1–10] || mica [010] and SRO [111] || CFO [111] || mica [001]. Epitaxial SRO thin films exhibited two magnetoresistance (MR) peaks; one peak occurred at a Curie temperature of 160 K (HT-MR) and the other at a low temperature of 40 K (LT-MR). The LT-MR increased more rapidly with an increase of the buffer layer thickness than the HT-MR. The LT-MR was similar for the two orthogonal current directions with respect to the magnetic field. We explained the HT-MR and LT-MR in terms of the suppression of spin fluctuations and the magnetic rotation of crystallographic domains, respectively.     

Preparation and characterization of multiferroic CoFe2O4/Bi0.97Ce0.03FeO3 coaxial nanotubes

Multiferroic CoFe₂O₄ (CFO)/Bi_0.97Ce_0.03FeO₃ (BCFO) coaxial nanotubes were prepared by a sol-gel template method. Transmission electron microscopy revealed that the coaxial nanotubes featured with inner CFO and outer BCFO nanotubes. Selected area electron diffraction confirmed the coexistence of spinel CFO and perovskite BCFO phases in the coaxial nanotubes. Vibrating sample magnetometer measurements showed that the saturated magnetization of the coaxial nanotubes was 3.3?emu/g, smaller than that of CFO nanotubes. The P–E hysteresis loop of the coaxial nanotubes was of poor shape due to possible high conductivity in the inner CFO nanotubes. Dielectric measurements exhibited that the dielectric constant of the coaxial nanotubes decreased while the dielectric loss increased due probably to the small dielectric constant and high conductivity in the inner CFO nanotube. Ferroelectric and magnetic properties were simultaneously demonstrated in the CFO/BCFO coaxial nanotubes.     

Structural,optical, and multiferroic properties of single phased BiFeO3

A soft chemical coprecipitation method has been proposed for synthesis of nano-sized multiferroic BiFeO₃ (BFO) powders. The X-ray diffraction pattern confirms the perovskite structure of BFO and Rietveld refinement reveals the existence of rhombohedral R3c symmetry. Crystallite size and strain value are studied from Williamson–Hall (W–H) analysis. The transmission electron microscope (TEM) image shows that the particle size of BFO powders lies between 50–100 nm. 4A₁ and 7E Raman modes have been observed in the range 100–650 cm^?1 and a prominent band centered around 1150–1450 cm^?1 have also been observed corresponding to the two-phonon scattering. Differential Thermal Analysis (DTA) shows the existence of two prominent peaks at 330 ^°C and 837 ^°C corresponding to the magnetic and ferroelectric ordering, respectively. From the temperature dependent dielectric studies, an anomaly in the dielectric constant is observed at the vicinity of Neel temperature (T _N ) indicating a magnetic ordering. Also, BFO shows antiferromagnetic behavior measured from the magnetic studies.     

Multiferroic behavior and magnetoelectric effect in CoFe2O4/Pb(Zr0.53Ti0.47)O3 thick films

A CoFe₂O₄/Pb(Zr_0.53Ti_0.47)O₃ (CFO/PZT) multiferroic composite thick film assisted by polyvinylpyrrolidone (PVP) was prepared by a hybrid sol–gel processing and spin coating technique. Scanning electronic microscopy indicated a porous microstructure with a thickness of 6.2 μm. Pure PZT perovskite phase observed from x-ray diffraction suggested that the low ratio of CFO particles was deeply buried in the PZT matrix. Ferroelectric and ferromagnetic properties were observed simultaneously at room temperature as well as a lower leakage current compared with a CFO/PZT thin film. The dynamic and static magnetoelectric effects were strongly dependent on the applied DC/AC magnetic field but their values were very low. The results demonstrated the prediction that ferroelectric and ferromagnetic properties can induce a strong magnetoelectric coupling only if a dense microstructure was achieved.     

Probing the ferroelectric phase transition in sol–gel‐derived polycrystalline bismuth ferrite thin films

Polycrystalline BiFeO₃ (BFO) thin films were successfully grown on Pt/Ti/SiO₂/Si(100) and SrTiO₃ (STO) (100) substrates using the chemical solution deposition (CSD) technique. X‐ray diffraction (XRD) patterns indicate the polycrystalline nature of the films with rhombohedrally distorted perovskite crystal structure. Differential thermal analysis (DTA) was performed on the sol–gel‐derived powder to countercheck the crystal structure, ferroelectric (FE) to paraelectric (PE) phase transition, and melting point of bismuth ferrite. We observed a significant exothermic peak at 840 °C in DTA graphs, which corresponds to an FE–PE phase transition. Raman spectroscopy studies were carried out on BFO thin films prepared on both the substrates over a wide range of temperature. The room‐temperature unpolarized Raman spectra of BFO thin films indicate the presence of 13 Raman active modes, of which five strong modes were in the low‐wavenumber region and eight weak Raman active modes above 250 cm⁻¹. We observed slight shifts in the lower wavenumbers towards lower values with increase in temperature. The temperature‐dependent Raman spectra indicate a complete disappearance of all Raman active modes at 840 °C corresponding to the FE–PE phase transitions. There is no evidence of soft mode phonons. Copyright © 2008 John Wiley & Sons, Ltd.     

Role of crystal orientation on the magnetic properties of CoFe2O4 thin films grown on Si (1 0 0) and Al2O3 (0 0 0 1) substrates using pulsed laser deposition

We report the influence of crystal orientation on the magnetic properties of CoFe₂O₄ (CFO) thin films grown on single crystal Si (1 0 0) and c-cut sapphire (Al₂O₃) (0 0 0 1) substrates using pulsed laser deposition technique. The thickness was varied from 200 to 50 nm for CFO films grown on Si substrates, while it was fixed at 200 nm for CFO films grown on Al₂O₃ substrates. We observed that the 200 and 100 nm thick CFO-Si films grew in both (1 1 1) and (3 1 1) directions and displayed out-of-plane anisotropy, whereas the 50 nm thick CFO-Si film showed only an (1 1 1) orientation and an in-plane anisotropy. The 200 nm thick CFO film grown on an Al₂O₃ substrate was also found to show a complete (1 1 1) orientation and a strong in-plane anisotropy. These observations pointed to a definite relation between the crystalline orientation and the observed magnetic anisotropy in the CFO thin films.     

Microscopic evidence of a metallic state in the one‐pot organic conductor ammonium tetrathiapentalene carboxylate

¹H NMR (nuclear magnetic resonance) and high‐field ESR (electron spin resonance) measurements were carried out for self‐doped organic conductors in the ammonium tetrathiapentalene carboxylate (TTPCOO)₂[(NH₄¹⁺)_1–x(NH₃⁰)_x ] system. While the pristine TTPCOOH molecule is closed‐shell, self‐doped carriers are generated by substitution of the carboxyl proton by (NH₃⁰) and (NH₄¹⁺), which can be regarded as a charge reservoir. The π‐extended system TTPCOO has a uniaxial g ‐tensor, indicating a 2D isotropic structure such as a herring‐bone‐like or parallel cross donor arrangement. The NMR‐relaxation rate indicated the Korringa relation in the temperature dependence, and the ESR linewidth followed the Elliot mechanism. Both of these observations provide supporting evidence for a stable metallic state. In this paper, we introduce self‐doped organic conductors as a branch of materials design, and emphasize that advanced magnetic resonance measurements are powerful tools for developing functional materials. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Temperature‐dependent Raman scattering studies of polycrystalline BiFeO3 bulk ceramics

This work reports the temperature‐dependent Raman scattering study of mutiferroic BiFeO₃ (BFO) bulk ceramics in a wide temperature range of 93–843 K. The polycrystalline samples are sintered at four different temperatures and characterized by X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), vibrating sample magnetometry, differential scanning calorimetry (DSC), and optical microscopy. The microstructure shows remarkable changes in terms of grain size and domain pattern as the sintering temperature increases. The DSC curves show prominent exothermic peaks at 645 K, the antiferromagnetic–paramagnetic phase transition temperature. The Raman spectra of all the four specimens reveal strong anomalies in the vicinity of the Neel temperature, which can be attributed to the multiferroic nature of BFO. The Raman scattering studies also reveal considerable spectral changes at a temperature range of 140–200 K in all the specimens, which can be inferred to a further spin–reorientation transition exhibited in BFO at a cryogenic temperature. Copyright © 2008 John Wiley & Sons, Ltd.     

Enhanced multiferroic properties in La and Ce co-doped BiFeO3 nanoparticles

Room temperature multiferroic properties of BiFeO₃ (BFO), Bi_0.9La_0.1FeO₃ ((La)BFO) and Bi_0.9La_0.075Ce_0.025FeO₃ ((La,Ce)BFO) nanoparticles have been reported in this paper. XRD (X-ray diffraction) analyses of the nanoparticles show a decrease in the lattice constants and cell volume with the substitution of La and Ce. It is evident from the SEM (scanning electron microscope) micrographs that the (La,Ce) co-doped sample possesses dense microstructure made of smaller particles. Raman study accounts for the weakening of the strong hybridization between Bi-O by the substitution of La and Ce ions. This is also accompanied by an increase in the remanent magnetization, dielectric constant, and ferroelectric polarization. BFO nanoparticles show exchange bias effect under an applied magnetic field while the (La)BFO and (La,Ce)BFO samples show no trace of such effect. Ac-conductivity of (La,Ce) co-doped sample is observed to be several orders lesser in magnitude than bulk BFO ceramics. These results are interpreted by means of the subtle change in the structure, suppression of the spin cycloid and reduction of oxygen vacancies in the doped samples.     

Thickness Dependence of Photoconductance in Strained BiFeO3 Thin Films With Planar Device Geometry

The recent discovery of efficient charge separation in tetragonal–rhombohedral (T‐R) polymorphic phase boundaries (PPBs) in strained BiFeO₃ (BFO) films is of great interest, and also raised a question of whether the PPBs could enhance the performance of BFO‐based planar photodetectors. To address it, we prepare BFO films with thickness ranging from 8 to 90 nm on the LaAlO₃ substrates, in which the BFO evolves from a pure T phase (without PPBs) to a T‐R mixed phase (with PPBs) due to the strain relaxation. Then, we comparatively investigate the photoconductive properties of these BFO films with the planar device geometry. It is found that the photoconductance first increases and then decreases with increasing film thickness. Particularly, the 50‐nm film containing the pure T phase without any detectable PPBs exhibits the highest photoconductance. This unexpected observation can be understood by analyzing the effects of increasing film thickness and associated phase evolution on the photoconduction‐related parameters.     

Oxygen vacancy dominant strong visible photoluminescence from BiFeO3 nanotubes

In this Letter, we report oxygen vacancy dominant strong visible photoluminescence (PL) from multiferroic BiFeO₃(BFO) nanotubes (NTs) prepared by sol–gel template method. Abundant oxygen vacancies present in BFO NTs provide alternate paths for the photo‐induced carrier generation and recombination thus affecting the PL and photoabsorption characteristics. This study not only assists in understanding the optoelectronic characteristics of BFO NTs at nanoscale but also suggests BFO nanostructures as potential candidates for future photonic and sensing applications. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

In situ synthesis of cobalt ferrite nanoparticle/polymer hybrid from a mixed Fe–Co methacrylate for magnetic hyperthermia

Hyperthermic CoFe₂O₄ nanoparticle (CFO NP)/polymer hybrids were synthesized by hydrolysis–condensation from a complex of Co and Fe possessing methacrylate ligands. Single-crystal analysis revealed that the complex consisted of two Co and four Fe metal atoms coordinated by methacrylate and 2-methoxyethoxy groups. The complex was copolymerized with 2-hydroxyethyl methacrylate (HEMA) and the resulting copolymer was then hydrolyzed to form a CFO NP/copolymer of poly(methacrylate) and poly(2-hydroxyethyl methacrylate) hybrid. Copolymerization with HEMA enhanced the stability of the hybrid in water. The size and magnetic properties of CFO in the hybrid were controlled by adjusting the hydrolysis conditions. Moreover, the hybrid generated heat under an alternating current magnetic field; its exothermal properties depended on the magnetic properties of the hybrid, the strength of the applied field, and the CFO NP content in the agar phantom matrix.     

Magnetic separation and SERS observation of analyte molecules on bifunctional silver/iron oxide composite nanostructures

We report the preparation of bifunctional silver–iron oxide composite nanostructures (Ag@Fe₂O₃) and demonstrate their magnetic separation with an analyte molecule from silver nanoparticles in a mixed solution. Magnetic and non‐magnetic plasmonic nanostructures and their separation are monitored by the surface‐enhanced Raman scattering (SERS) spectra of two different analytes attached to each kind of particles. In general, such separation experiments can provide insight into basic phenomena of adsorption and exchange of adsorbed molecules which are of strong interest in SERS. The formation of stable Ag@Fe₂O₃ nanoparticle–molecule complexes suggests small magnetic SERS labels without additional protective layers for application in analytical assays. The magnetic plasmonic nanostructures have great promise for targeted imaging and sensing in biological structures by directing nanosensors to places of interest using magnetic fields. The option of magnetic separation and collection of plasmonic particles improves the analytical capabilities of SERS. Copyright © 2012 John Wiley & Sons, Ltd.     

Thin film bilayers of multiferroic bismuth ferrite on Pt–Si substrate

A multiferroics/multiferroics BiFeO₃/Bi_0.90La_0.10Fe_0.90Zn_0.10O₃ (BFO/BLFZO) bilayer was deposited on Pt/TiO₂/SiO₂/Si substrates by radio frequency sputtering. The BLFZO layer strongly affects the phase purity, orientation growth, and leakage current of BFO layer. The bilayered capacitor exhibits a high dielectric permittivity of ～162 and an improved magnetic behavior of 2M_s ～ 34.6 emu/cm³, together with an excellent fatigue endurance. A remanent polarization of 2P_r ～ 116.2 μC/cm² for the bilayered capacitor is better than those of reported BFO bilayers. The impedance study indicates that lower freely mobile charges are responsible for the improved electrical behavior of the BFO/BLFZO bilayer. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thickness dependent magnetic and ferroelectric properties of LaNiO3 buffered BiFeO3 thin films

BiFeO₃ (BFO) thin films with thickness increasing from 40 to 480 nm were successfully grown on LaNiO₃ (LNO) buffered Pt/Ti/SiO₂/Si(100) substrate and the effects of thickness evolution on magnetic and ferroelectric properties are investigated. The LNO buffer layer promotes the growth and crystallization of BFO thin films. Highly (100) orientation is induced for all BFO films regardless of the film thickness together with the dense microstructure. All BFO films exhibited weak ferromagnetic response at room temperature and saturation magnetization is found to decrease with increase in film thickness. Well saturated ferroelectric hysteresis loops were obtained for thicker films; however, the leakage current dominated the ferroelectric properties in thinner films. The leakage current density decreased by three orders of magnitude for 335 nm film compared to 40 nm film, giving rise to enhanced ferroelectric properties for thicker films. The mechanisms for the evolution of ferromagnetic and ferroelectric characteristics are discussed.     

Enhanced electrical and ferroelectric properties in a multiferroic (BiFeO3/Bi0.5Na0.5TiO3)3/LaNiO3 superlattices structure

Artificial multiferroic superlattices (SL), consisting of BiFeO₃ (16 nm)/Bi_0.5Na_0.5TiO₃ (5 nm) (BFO/BNT SL), were grown on (001) SrTiO₃ single crystal by pulsed laser deposition method. The cross-sectional, surface morphology, and crystallographic structure of BFO/BNT SL and BFO single layer were investigated. It was found that the electrical, ferroelectric, and magnetic properties of BFO/BNT SL exhibit a remarkably enhancement compared with BFO single layer. The influence of BNT buffering layer, lattice strain, and interfaces interplay of the SL structure are supposed to benefit their ferroelectric and ferromagnetic properties. Our works suggested the BFO/BNT SL with an improved multiferroic characteristics have a promising application for the future informational storage devices.     

复合薄膜NiFe2 O4-BiFeO3 中的磁电耦合

采用化学溶液沉积法(CSD)在Au/Ti/SiO₂/Si衬底上通过自组装生长制备了BiFeO₃-NiFe₂O₄ (BFO-NFO)多铁性复合薄膜.X射线衍射图谱(XRD)显示形成了分离的钙钛矿结构的铁电相BFO和尖晶石结构的铁磁相NFO. NFO的引入导致复合薄膜的泄漏电流减小,剩余极化强度增加.相比于纯BFO薄膜,0.25NFO-0.75BFO样品泄漏电流下降了约两个数量级,剩余极化强度( M _{关键词： 多铁性复合薄膜 磁电耦合 铁电性 铁磁性}