Effects of BaTiO<Subscript>3</Subscript> and SrTiO<Subscript>3</Subscript> as the buffer layers of epitaxial BiFeO<Subscript>3</Subscript> thin films

BiFeO₃ (BFO) thin films with BaTiO₃ (BTO) or SrTiO₃ (STO) as buffer layer were epitaxially grown on SrRuO₃-covered SrTiO₃ substrates. X-ray diffraction measurements show that the BTO buffer causes tensile strain in the BFO films, whereas the STO buffer causes compressive strain. Different ferroelectric domain structures caused by these two strain statuses are revealed by piezoelectric force microscopy. Electrical and magnetical measurements show that the tensile-strained BFO/BTO samples have reduced leakage current and large ferroelectric polarization and magnetization, compared with compressively strained BFO/STO. These results demonstrate that the electrical and magnetical properties of BFO thin films can be artificially modified by using a buffer layer.     

EPR Monitoring of BaTiO<Subscript>3</Subscript> Formation

BaCO₃ and anatase-type TiO₂ were adopted as initial materials to prepare BaTiO₃ powder by the solid-state reaction method at a heating rate of 350°C/h. The electron paramagnetic resonance (EPR) technique was employed to monitor the formation of BaTiO₃. TiO₂ showed a series of complicated EPR signals associated primarily with Fe impurities. The formation of BaTiO₃ can be monitored in terms of the evolution of EPR signals associated with Fe impurities with calcination and measurement temperatures. The activation of the g = 2.004 signal above the Curie point of BaTiO₃ and the disappearance of the other EPR signals in the BaCO₃/TiO₂ mixture at room temperature are characteristic of the formation of BaTiO₃.     

Femtosecond pulsed-laser deposition of BaTiO<Subscript>3</Subscript>

The elemental composition and the surface morphology of thin films grown by laser ablation of barium titanate with femtosecond pulses at 620 nm laser wavelength have been systematically studied according to the experimental pulsed-laser deposition parameters : laser energy density, oxygen pressure, substrate temperature, target–substrate distance and substrate position (in- and off-axis geometry). Firstly, even at high temperature (700 °C), the deposits consist of coalesced particles up to 1-μm in size, mixed in a poorly crystallised tetragonal BaTiO₃ thin film. The particles formed in femtosecond pulsed-laser deposition induce a high surface roughness, which is observed whatever the experimental growth conditions and does not correspond to the droplets often observed during laser ablation in the nanosecond regime. As shown by plasma expansion dynamics, these particles propagate toward the substrate in the plasma plume with a low velocity, and are assumed to be produced by gas-phase reactions. Moreover, the cationic concentration evaluated through the Ba/Ti ratio strongly depends on the oxygen pressure in the ablation chamber and the angular position of the substrate along the normal to the target at laser impact. Indeed, the films appear to be enriched in the heavy element (Ba) when the substrate is located at high angular deviation. This fact is correlated to an increase in the lighter species (i.e. Ti) in the central part of the plasma plume. Received: 30 April 2002 / Accepted: 26 August 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +33-1/4354-2878, E-mail: millon@gps.jussieu.fr RID="**" ID="**"Also at: LSMCL, Université de Metz, 57078 Metz Cedex 3, France     

Weak ferromagnetism in BiFeO<Subscript>3</Subscript>-based multiferroics

The magnetic properties of the Bi_{1 ? x} Ln _x FeO₃ (Ln is a rare-earth ion), Bi_{1 ? x} A _x FeO_{3 ? x/2} (A is an alkali earth ion), and BiFe_{1 ? x} Ti _x O_{3 + δ} solid solutions in magnetic fields up to 14 T have been studied. The concentration ranges of the existence of the ferroelectric phase described by the space group R3c have been determined. It is shown that the substitution of the rare-earth ions for the Bi³⁺ ions leads to a sharp decrease in the critical fields inducing the metamagnetic transition from a modulated antiferromagnetic state to a weakly ferromagnetic one; however, the modulated structure in the concentration range of the R3c phase is mainly retained. The substitution of the alkali earth ions (x ～ 0.1) for the bismuth ions leads to the total destruction of the modulated structure and to the implementation of the weakly ferromagnetic state within the R3c phase. A homogeneous weakly ferromagnetic state has been revealed when the Ti⁴⁺ ions (x = 0.1) are substituted for the Fe³⁺ ions in the ferroelectric R3c phase.     

Lattice dynamics of BiFeO<Subscript>3</Subscript> under hydrostatic pressure

The vibrational frequencies of the BiFeO₃ crystal lattice in the cubic phase (Pm3m) and the rhombohedral paraelectric phase (R3c) are calculated in terms of the ab initio model of an ionic crystal with the inclusion of the dipole and quadrupole polarizabilities. In the ferroelectric phase with the symmetry R3c, the calculated spontaneous polarization of 136 μC cm^?2 agrees well with the experimental data. The dependences of the unit cell volume, the elastic modulus, and the vibrational frequencies on the pressure are calculated. It is found that the frequency of an unstable ferroelectric mode in both the cubic (Pm3m) and rhombohedral (R3c) phases are almost independent of the applied pressure, in contrast to classical ferroelectrics with a perovskite structure, where the ferroelectric instability is very sensitive to a variation in the pressure.     

Specific features of structural states in the BiFeO<Subscript>3</Subscript>-YMnO<Subscript>3</Subscript> solid solutions

The (1 − x)BiFeO_3−x YMnO₃ solid solutions have been found to undergo the following sequence of phase transformations with increasing x: R3c → Pbnm → C2 → Pnma → P6₃ cm. It has been established that the Pbnm and Pnma phases have different orientations of atomic displacements and can exhibit antiferroelectric properties.     

<Superscript>57</Superscript>Fe NMR study of multiferroic BiFeO<Subscript>3</Subscript>

The effects of the ⁵⁷Fe isotope content and high-frequency magnetic field amplitude h ₁ on the shape of the NMR spectrum of multiferroic BiFeO₃ at T = 4.2 K are studied by pulsed nuclear magnetic resonance. The NMR spectrum shape and transverse relaxation time T ₂ are found to depend strongly on the ⁵⁷Fe isotope content and h ₁ in multiferroic BiFeO₃ in the presence of a spatial spin-modulated structure of a cycloid type. In a sample with a high ⁵⁷Fe isotope content, the Suhl-Nakamura interaction contributes substantially to T ₂. When these dynamic effects are taken into account for analysis of the NMR spectrum shape, an undisturbed (without an anharmonicity effect) spatial spin-modulated structure of a cycloid type is shown to exist in BiFeO₃.     

Structural and magnetic phase transformations in the BiFeO<Subscript>3</Subscript>-CaFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> system

The crystal structure and magnetic properties of the Bi_{1 ? x} Ca _x Fe_{1 ? x/2}Nb _x/2O₃ system were studied. It is shown that, at x ≤ 0.15, the unit-cell symmetry of solid solutions is rhombohedral (space group R3c). Solid solutions with x ≥ 0.3 have an orthorhombic unit cell (space group Pbnm). The rhombohedral compositions are antiferromagnetic, while the orthorhombic compositions exhibit a small spontaneous magnetization due to Dzyaloshinski?-Moriya interaction. In CaFe_0.5Nb_0.5O₃, the Fe³⁺ and Nb⁵⁺ ions are partially ordered and the unit cell is monoclinic (space group P2₁/n). In the concentration range 0.15 < x < 0.30, a two-phase state (R3c + Pbnm) is revealed.     

Rapid optical measurement of surface roughness of BiFeO<Subscript>3</Subscript> films for nonvolatile memory application

An optical inspection system for rapid surface roughness measurement of BiFeO₃ (BFO) thin films is developed. It is found that y = −121.45 x + 212.81 is a trend equation for characterizing the surface roughness of BFO thin films. The incident angle of 60◦ is a good candidate for measuring the surface roughness of BFO thin films. The maximum measurement error rate of the optical inspection system developed is less than 2.6%. The savings in inspection time of the surface roughness of BiFeO₃ thin films is up to 90%.     

Effect of KNbO<Subscript>3</Subscript> modification on structural,electrical and magnetic properties of BiFeO<Subscript>3</Subscript>

The polycrystalline samples of (Bi_1?x K _x ) (Fe_1?x Nb _x ) O₃ (BKFN) for x = 0.0, 0.1, 0.2 and 0.3 were synthesized by a solid-state reaction method. The X-ray diffraction patterns of BKFN exhibit that the addition of KNbO₃ in BiFeO₃ gradually changes its structure from rhombohedral to pseudocubic. The analysis of scanning electron micrograph clearly showed that the sintered samples have well-defined and uniformly distributed grains. Addition of KNbO₃ to BiFeO₃ enhances the dielectric, ferroelectric and ferromagnetic properties of BiFeO₃. Detailed studies of impedance and related parameters of BKFN using the complex impedance spectroscopic technique exhibit the significant contributions of grain and grain boundaries in the resistive and transport properties of the materials. Some oxygen vacancies created in the ceramic samples during high-temperature processing play an important role in the conduction mechanism. The leakage current or tangent loss of BiFeO₃ is greatly reduced on addition of KNbO₃ to the parent compound BiFeO₃. Preliminary studies of ferroelectric and magnetic characteristics of the samples reveal the existence of ferroelectric, and weak ferromagnetic ordered ceramics.     

Structural and electrical properties of BiFeO<Subscript>3</Subscript>-Pb(ZrTi)O<Subscript>3</Subscript> composites

The polycrystalline (Bi_1-xPb_x)(Fe_1-xZr_0.6xTi_0.4x)O₃ (x=0.15, 0.25, 0.40, 0.50) (BPFZT) nanoceramic composites were synthesized using mechanical activation and solid-state reaction techniques. The formation of single-phase compounds with 100% solubility of BiFeO₃ and Pb(Zr_0.6Ti_0.4)O₃ was confirmed by an X-ray diffraction (XRD) technique. Detailed structural analysis of the fabricated BPFZT composites suggests the formation of tetragonal structure (i.e., distorted perovskite) for all composition. The dielectric constant and loss-tangent of the BPFZT composites decrease on increasing frequency and temperature. It has also been observed that the leakage current and loss-tangent are reduced by increasing the contents of PZT in the BPFZT composites, and hence they may be considered useful for some applications. The values of activation energies and the nature of variation of conductivity with temperature and frequencies suggest that the space charge and oxygen ion vacancies play a significant role in the conduction process. PACS 61.10.Nz; 77.22.Ch; 77.84.Lf; 81.20.Ev     

A planar complex phase interface in BaTiO<Subscript>3</Subscript>

A model is proposed for a complex phase interface separating the cubic and tetragonal phases in barium titanate, which consists of an intermediate layer of the induced phase with small monoclinic distortions near the interface. It is demonstrated that the induced phase can have two different structures, which are formed during the direct and reverse phase transitions, respectively. The parameters of these structures, equations of phase boundaries, relative orientations of the phases upon the phase transition, and the number of possible orientation states are determined.     

Synthesis of transparent BaTiO<Subscript>3</Subscript> nanoparticle/polymer hybrid

Transparent BaTiO₃ nanoparticle/polymer hybrid was synthesized by polymerization and hydrolysis of barium titanium alkoxide modified with 2-vinyloxyethoxy ligand. Barium alkoxide, titanium alkoxide and 2-vinyloxyethanol were reacted affording a BaTiO₃ precursor, which was then hydrolyzed and polymerized to form BaTiO₃ particle/polymer hybrids below 100°C. BaTiO₃ particles increased in crystallinity with increasing water amount for hydrolysis. The absorption edge of the hybrid film on silica plates shifted to shorter wavelength with decreasing crystallite size. Nano-sized BaTiO₃ particle/polymer hybrid polymerized with methyl methacrylate (MMA) was shaped into a transparent and self-standing film with a refractive index of 1.595 at 589 nm.     

Phase states in solid solutions based on BiFeO<Subscript>3</Subscript>

Concentration and temperature regions of the existence of different phases (R3c, Pbnm, P4mm, Amm2, Pbcm, and Pm3m) in solid solutions of BiFeO₃-BiMnO₃, BiFeO₃-KNbO₃, and BiFeO₃-NaNbO₃ are determined.     

Magnetic Properties of CaMnO<Subscript>3</Subscript> Layers on the (100) Surface of BaTiO<Subscript>3</Subscript>

“Ab-initio” calculations of the electronic structure of the heterojunction (001) between cubic CaMnO₃ and BaTiO₃ perovskites are performed on the basis of density functional theory for different variants of magnetic ordering in calcium manganite. The paper considers some cases of ferromagnetic and antiferromagnetic A-type ordering. Comparison of the total energies of these structures shows that antiferromagnetic ordering in CaMnO3 is the most favorable. All the studied structures in a ferromagnetic state are half-metallic ferromagnets.     

Nanoparticulate BaTiO<Subscript>3</Subscript> film produced by aerosol-type nanoparticle deposition

Aerosol-type nanoparticle deposition (NPD) is a magical method to form a dense electroceramic film with a fine, nanoscale structure on a substrate surface by depositing ceramic particles through a nozzle at room temperature. This film has the potential to be applied to various electronic, environmental, and energy devices. However, the deposition mechanism and the nanostructure in the film are not understood sufficiently. This study aimed at investigating the crystal structure of an NPD as-deposited film, and compared the crystal structures of the NPD as-deposited film, annealed film, and the raw powders consisting of particles with diameters of 200 and 50 nm, respectively, using barium titanium oxide (BaTiO₃). We found that the crystal in BaTiO₃ with a disordered phase due to the Ba displacement within the BaTiO₃ was responsible for the adhesion between the BaTiO₃ crystalline particles having a diameter of approximately 10 nm, as well as with the substrate.     

Dielectric properties of (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)(Nb<Subscript>0.93</Subscript>Sb<Subscript>0.07</Subscript>)O<Subscript>3</Subscript> ferroelectric ceramics modified with BaTiO<Subscript>3</Subscript>

Processes of the polarization and repolarization of ferroelectric ceramics based on potassium sodium niobate in the region of infralow frequencies are discussed. The effect aging and subsequent annealing in a strong alternating electric field have on the nonlinearity of the dielectric response of a sample at different temperatures is determined.     

Modulation of ultrafast laser-induced magnetization precession in BiFeO<Subscript>3</Subscript>-coated La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> thin films

The ultrafast laser-excited magnetization dynamics of ferromagnetic (FM) La_0.67Sr_0.33MnO₃ (LSMO) thin films with BiFeO₃ (BFO) coating layers grown by laser molecular beam epitaxy are investigated using the optical pump-probe technique. Uniform magnetization precessions are observed in the films under an applied external magnetic field by measuring the time-resolved magneto-optical Kerr effect. The magnetization precession frequencies of the LSMO thin films with the BFO coating layers are lower than those of uncoated LSMO films, which is attributed to the suppression of the anisotropy field induced by the exchange interaction at the interface between the antiferromagnetic order of BFO and the FM order of LSMO.     

Dielectric and polar order behaviors of BaTiO<Subscript>3</Subscript>-Bi(Mg<Subscript>1/2</Subscript>Ti<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> ceramics

New perovskite solid solution ceramics of (1−x)BaTiO₃-xBi(Mg_1/2Ti_1/2)O₃ ((1−x)BT-xBMT, x≤0.09) were synthesized by the solid-state reaction technique. X-ray diffraction studies have revealed a stable single perovskite structure for all samples. Dielectric measurements were carried out at different frequencies and temperatures. The polarization evolutions with temperatures were measured to investigate the ferroelectric properties. All the compositions show features of ferroelectrics with diffuse phase transition, though the temperature T _m of their dielectric constant maximum ε _m is frequency dependent. The dielectric constant peak ε(T) of (1−x)BT-xBMT ceramics become broad with increasing BMT content. During the temperature range of ε(T) peak summit, (1−x)BT-xBMT ceramics present quasi-linear dielectric phenomenon under high electric field with very high dielectric constant.     

Thin ferroelectric Nd-doped BiFeO<Subscript>3</Subscript> films with orthorhombic structure

X-ray diffraction and Raman spectroscopy of epitaxial Nd-doped bismuth ferrite films on MgO substrates reveal their orthorhombic symmetry Fmm2 (a = 7.914 Å, b = 7.913 Å, and c = 7.937 Å).