Study on ferroelectric polarization induced resistive switching characteristics of neodymium-doped bismuth ferrite thin films for random access memory applications

The present work reports on resistive switching (RS) characteristics of Neodymium (Nd)-doped bismuth ferrite (BFO) layers. The Nd (2–10 at%) doped BFO thin film layers were deposited using a spray pyrolysis method. The structural analysis reveals that a higher Nd doping concentration in BFO leads to significant distortion of the prepared Nd:BFO thin films from rhombohedral to tetragonal characteristics. The morphological analysis shows that all the deposited Nd:BFO thin films have regularly arranged grains. The X-ray photoelectron spectroscopy (XPS) analysis reveals that the prepared Nd:BFO thin films have a higher Fe ³⁺/Fe ²⁺ratio and less oxygen vacancy (V_O) defects which enriches the ferroelectric characteristics in Nd:BFO layers. The polarization-electric field (P-E) and RS characteristics of the fabricated Nd:BFO-based RS device were examined. It was observed that the Nd (7 at%) doped BFO RS device shows large remnant polarization (P _r) of 0.21 μC/cm² and stable RS characteristics.     

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.     

Rare earth doped cobalt ferrite thin films deposited by PLD

CoFe₂O₄ thin films with preferential texture structure, small grain size, and perpendicular magnetic anisotropy can be obtained by the pulsed laser deposition (PLD) technique. In this work, we studied the influence of the Fe³⁺ ions substitution by three elements from lanthanide group (Dy, La, and Gd) on the structural properties of the thin films. The samples were deposited by Nd:YAG laser (λ=532 nm, 10 ns) ablation of CoFe_1.8RE_0.2O₄, (RE=Dy, La, Gd) targets at various substrate temperatures ranging from room temperature to 600 °C. The microstructure and chemical composition of the thin films were investigated by Raman spectroscopy, XRD, SEM-EDS, and ToF-SIMS. The XRD patterns and Raman spectra of the thin films indicated the formation of a single spinel structure. Thus, the desired substitution of the iron ions in the spinel lattice with the RE elements was achieved in the thin films, although in the bulk material, their presence determined the formation of a residual phase with a perovskite-type structure.     

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.     

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.     

Identification of Mn‐related Raman modes in Mn‐doped ZnO thin films

This article aims to investigate the Raman modes present in Mn‐doped ZnO thin films that are deposited using the magnetron co‐sputtering method. A broad band ranging from 500 to 590 cm⁻¹ is present in the Raman spectra of heavily Mn‐doped ZnO films. The multi‐peak‐fitting results show that this broad band may be composed of six peaks, and the peak at 528 cm⁻¹ could be a characteristic mode of Mn₂O₃. The results of this study suggest that the origin of the Raman peaks in Mn‐doped ZnO films may be due to three major types: structural disorder and morphological changes caused by the Mn dopant, Mn‐related oxides and intrinsic host‐lattice defects. Copyright © 2010 John Wiley & Sons, Ltd.     

Enhancement in multiferroic properties of Bi0.7−xLaxDy0.3FeO3 system with removal of La

La doping at Bi Site in multiferroic BiFeO₃ (BFO) is known to stabilize the ferroelectric perovskite phase and reduce the leakage current. It has been observed that in Dy modified BFO sample, La is not required for the said reason. In fact, the removal of La from Dy modified BFO system leads to the reduction in leakage current. Moreover, the magnetic properties are enhanced by order of magnitude in the absence of La in the system. Remarkably, a similar trend in properties has been also observed in thin films grown on Pt/TiO₂/SiO₂/Si substrate by using the pulsed laser deposition technique. This significant variation in the properties after removal of La from the system could be attributed to the change in lattice parameters, bond lengths and Fe–O–Fe bond angle as determined by powder x-ray and neutron diffraction study.     

Off-stoichiometry effects on BiFeO3 thin films

The effects of Bi and Fe-excess on the structure, ferroelectric, leakage current and magnetic properties of BiFeO₃ (BFO) thin films are reported. BFO with 5% excess exhibits no change in the structure with an improvement in leakage current properties in comparison to stoichiometric BFO. Raman spectroscopy of 10% Bi excess suggests a structural change from monoclinic to rhombohedral accompanied with an improvement of resistivity and ferroelectric polarization switching. A higher Fe-excess leads to the formation of pyrochlore Bi₂Fe₄O₉ and gamma-Fe₂O₃ that cause an increase in conductivity at the macroscopic scale. The results are discussed in terms of Fe and Bi-excess effects on the defect structure of BFO.     

Effect of bilayer structure and a SrRuO3 buffer layer on ferroelectric properties of BiFeO3 thin films

Effects of the BiFe_0.95Mn_0.05O₃ thickness and a SrRuO₃ (SRO) buffer layer on the microstructure and electrical properties of BiFeO₃/BiFe_0.95Mn_0.05O₃ (BFO/BFMO) bilayered thin films were investigated, where BFO/BFMO bilayered thin films were fabricated on the SRO/Pt/Ti/SiO₂/Si(100) substrate by a radio frequency sputtering. All thin films are of a pure perovskite structure with a mixture of (110) and (111) orientations regardless of the BFMO layer thickness. Dense microstructure is demonstrated in all thin films because of the introduction of BFMO layers. The SRO buffer layer can also further improve the ferroelectric properties of BFO/BFMO bilayered thin films as compared with those of these thin films without a SRO buffer layer. The BFO/BFMO bilayered thin film with a thickness ratio of 220/120 has an enhanced ferroelectric behavior of 2P _r??165.23???C/cm² and 2E _c??518.56?kV/cm, together with a good fatigue endurance. Therefore, it is an effective way to enhance the ferroelectric and fatigue properties of bismuth ferrite thin films by constructing such a bilayered structure and using a SRO buffer layer.     

Magnetic field annealing effects on self-oriented BiFeO3 thin films prepared by chemical solution deposition

Self-oriented BiFeO₃ (BFO) thin films are prepared via chemical solution deposition method with magnetic field in-situ annealing process. The effects of magnetic annealing on the microstructure, magnetic and dielectric properties as well as magnetoelectric coupling effect of the BFO thin films are investigated. With increasing the annealing magnetic field, the crystallization quality, texture, grain boundary connectivity and densification of the films are enhanced, which is attributed to the improvement of connection and diffusion of components. The magnetization of the field-annealing films and dielectric constant as well as remanent polarization increases with increasing the strength of annealing magnetic field. In addition, it is observed that magnetocapacitance value of the magnetic-field-annealing BFO thin film is higher than the non-field-annealing one. Moreover the BFO thin films annealed at 3 kOe magnetic field show the magnetoelectric effect with 4% under 2 kOe at room temperature.     

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.     

Improvement of fatigue resistance on La modified BiFeO3 thin films

The effect of lanthanum (La) addition in BiFeO₃ (BFO) thin films deposited on Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates prepared by soft chemical method was explained. Increasing La concentration promotes changes on structure, microstructure and dielectric/ferroelectric response of films. X-ray diffraction reveals that the films are free of preferred orientations and structural distortion. La addition promotes an increase in dielectric permittivity. The polarization switching and the fatigue behavior of the BFO films were significantly enhanced by the La concentration.     

Structural, optical and magnetic properties of Nd-doped BiFeO3 thin films prepared by pulsed laser deposition

Nd-doped BiFeO₃ thin films were grown by pulsed laser deposition on quartz substrate and their structural, optical and magnetic properties have been studied. X-ray diffraction analysis revealed that Nd addition caused structural distortion even with 5% of Nd concentration, additional secondary phase appeared in all samples but its intensity was greatly reduced with Nd addition. Doping-induced variations in texture and structure modifying both magnetic and optical properties of BiFeO₃ thin films. The energy band gap decreases while the refractive index increases with addition of Nd³⁺ in BiFeO₃ for Bi³⁺. These variations in energy band gap and refractive index have been explained on the basis of density of states and increase in disorders in the system. All the samples were found to exhibit ferromagnetism at room temperature and the saturation magnetization increases with the increase in structural distortion with addition of Nd. Finally, Nd-doping modifies the physical properties of BiFeO₃ in comparison to undoped BiFeO₃ thin films.     

Ferroelectric domain structures and polarization switching characteristics of polycrystalline BiFeO3 thin films on glass substrates

We investigated ferroelectric characteristics of BiFeO₃ (BFO) thin films on SrRuO₃ (SRO)/yttria-stabilized zirconia (YSZ)/glass substrates grown by pulsed laser deposition. YSZ buffer layers were employed to grow highly crystallized BFO thin films as well as SRO bottom electrodes on glass substrates. The BFO thin films exhibited good ferroelectric properties with a remanent polarization of 2P_r = 59.6 μC/cm² and fast switching behavior within about 125 ns. Piezoelectric force microscopy (PFM) study revealed that the BFO thin films have much smaller mosaic ferroelectric domain patterns than epitaxial BFO thin films on Nb:SrTiO₃ substrates. Presumably these small domain widths which originated from smaller domain energy give rise to the faster electrical switching behavior in comparison with the epitaxial BFO thin films on Nb:SrTiO₃ substrates.     

Effect of non-magnetic doping on leakage and magnetic properties of BiFeO3 thin films

In this paper, we show that the leakage current properties of BiFeO₃ (BFO) thin films have been greatly improved by Zr-doping. In contrast, the magnetic properties of Zr-doped BFO films are affected as a weak ferromagnetism. Beyond the double-exchange interactions arising from the creation of Fe²⁺, we propose another simple model considering the replacement of the magnetically active Fe³⁺, time to time, by a non-active Zr⁴⁺, which is expected to induce a local ferromagnetic coupling rather than an antiferromagnetic one.     

Tunable magnetic anisotropy of CoFe2O4 nanopillar arrays released from BiFeO3 matrix

We deposited epitaxial BiFeO₃–CoFe₂O₄ (BFO–CFO) self‐assembled thin films on (001) SrTiO₃ (STO) substrates. We find that a combined annealing and etching process could remove the BFO matrix, thereby resulting in free‐standing CFO nanopillar arrays. Scanning electron and atomic force microscopies showed well separated CFO nanopillars, which were very similar to the original CFO ones in the self‐assembled structure. Finally, comparison of the magnetic hysteresis loops before and after removal of the BFO matrix showed a significant decrease of the coercive field and a dramatic decrease in the strain dominated magnetic anisotropy. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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)     

Effect of lanthanum substitution on the Raman spectra of barium titanate thin films

Thin films of Ba_1−xLa_xTiO₃ on platinum substrates were synthesized using the sol–gel method for x values of 0.0, 0.03, 0.05, and 0.10, and the effect of trivalent La³⁺ substitution on the structural and dielectric properties was studied. Using X‐ray diffraction, structural analysis of these compositions revealed a slight increase in the tetragonal distortion of the unit cell with increase in La content. Accordingly, an increase in the tetragonal to cubic transition temperature T_T/C was detected by temperature‐dependent Raman spectroscopy in the range of 70–500 K. Unlike the results from Raman scattering for the La‐doped BaTiO₃ films, the dielectric measurements showed broad and diffused dielectric maxima, making the estimation of the transition temperature merely qualitative. Copyright © 2006 John Wiley & Sons, Ltd.     

Magnetoelectric properties of Mn-substituted BiFeO3 thin films with a TiO2 barrier

Multiferroic thin films with the general formula TiO₂/BiFe_1−xMn_xO₃ (x=0.00, 0.05, 0.10 and 0.15) (TiO₂/BFMO) were synthesized on Au/Ti/SiO₂/Si substrates using a chemical solution deposition (CSD) method assisted with magnetron sputtering. X-ray diffraction analysis shows the thin films contained perovskite structures with random orientations. Compared with BFMO films, the leakage current density of the TiO₂/BFMO thin films was found to be lower by nearly two orders of magnitude, and the remnant polarizations were increased by nearly ten times. The enhanced ferroelectric properties may be attributed to the lower leakage current caused by the introduction of the TiO₂ layer. The J-E characteristics indicated that the main conduction mechanism for the TiO₂/BFMO thin film was trap-free Ohmic conduction over a wide range of electric fields (0-500 kV/cm). In addition, ferromagnetism was observed in the Mn doped BFO thin films at room temperature. The origin of ferromagnetism is related to the competition between distortion of structure and decrease of grain size and decreasing net magnetic moment in films due to Mn doping.     

Influence of the ion energy on the structure of Bi and Fe2O3 thin films

Compounds containing bismuth, iron and oxygen (BFO) can result in materials with important magnetic and electrical properties for high-technology applications. We plan to prepare such compounds using the simultaneous ablation of bismuth and iron oxide targets. For that reason in the first part of this work we study the plasmas and the materials produced by ablation of bismuth or Fe₂O₃ targets, and then the two plasmas are combined in order to deposit the BFO compounds. The individual plasmas were characterized using a Langmuir probe, in order to measure the mean kinetic ion energy (E _p) and plasma density (N _p). Bismuth and magnetite-Fe₃O₄ thin films were obtained in high vacuum (2.7×10^?4 Pa). Meanwhile for the deposition of α-Fe₂O₃ (hematite) or amorphous bismuth oxide thin films a reactive atmosphere (Ar/O₂=80/20) was used. All depositions were made at room temperature. The bismuth thin films crystallized in the rhombohedral metallic system with preferential orientations that depended on the Bi-ion energy used. Bismuth oxide phases were only obtained after annealing of the Bi thin films at different temperatures. Iron oxide thin films reproducing the target stoichiometry were obtained at a certain value of iron-ion energy. Preliminary structural results of the BFO thin films obtained by the combination of the individual plasmas are presented.