Effect of Nd and Nb co-doping on the structural,magnetic and optical properties of multiferroic BiFeO3 nanoparticles prepared by sol-gel method

BiFeO₃, BiFe_0.95Nb_0.05O₃ and Bi_1−xNd_xFe_0.95Nb_0.05O₃ (x=0.0, 0.05, 0.10, 0.15 and 0.20) nanoparticles are successfully synthesized via a tartaric acid-assisted sol-gel technique for the first time. The effect of Nd and Nb co-doping on the structure, morphology, and magnetic and optical properties is investigated. X-ray diffraction (XRD) and Raman measurements demonstrate that Nd³⁺ ions and Nb⁵⁺ ions co-doping at A and B-sites of BiFeO₃ can result in a structural transformation (from single rhombohedral phase (R3c) to two coexisting phases (R3c and Pbam)). The morphology of the nanoparticles seems to be approximately cubic, and by the co-doping, the size of the nanoparticles decreases from ~132 to ~35 nm. Notable improvement in the remanent magnetization of the sample with x=0.15 is observed with a value of 0.285 emu/g, being 20 times higher than that of the undoped sample (BiFeO₃). A decrease in the optical band gap is also observed in the Nd and Nb co-doped nanoparticles, indicating their favorable potential in photocatalytic applications.     

High quality TbMnO3 films deposited on YAlO3

High quality thin films of TbMnO₃ were grown by pulsed laser deposition on orthorhombicYAlO₃ (1 0 0). The interface and surface roughness of a 55 nm thick film were probed by X-ray reflectometry and atomic force microscopy, yielding a roughness of 1 nm. X-ray diffraction revealed untwinned films and a small mosaic spread of 0.04° and 0.2° for out-of-plane and in-plane reflections, respectively. This high degree of epitaxy was also confirmed by Rutherford backscattering spectrometry. Using polarized neutron diffraction we could identify a magnetic structure with the propagation vector (0 0.27 0), identical to the bulk magnetic structure of TbMnO₃.     

Crystal structure transformation and improved dielectric and magnetic properties of La-substituted BiFeO3 multiferroics

Samples of Bi_1−xLa_xFeO₃ with x = 0.1, 0.3, 0.5, and 0.7 have been synthesized by two stage solid state reaction method. Structural characterization was performed using powder x-ray diffraction at room temperature. The crystal structure of perovskite phases are further characterized via Rietveld analysis which revealed a structural transition from R3c symmetry of the parent phase of BiFeO₃ to orthorhombic Pnma symmetry of LaFeO₃. However the intermediate samples with x = 0.3 and 0.5 are bi – phasic (i.e. a combination of rhombohedral R3c and orthorhombic Pbam phases co-exist). Rietveld Refinement presents a good agreement between measured and simulated patterns. The transition from a rhombohedral to orthorhombic unit cell is suggested to be driven by the dilution of the stereochemistry of the lone pair of Bi³⁺ at A- site. M-H hysteresis loops are recorded at room temperature up to a field of 15 kOe. The G-type antiferromagnetic spin structure and the magnetic moment are very sensitive to increasing La concentration at A-site. La substitution transformed antiferromagnetic BiFeO₃ into ferromagnetic which is closely related to the structural phase transition and modification of antiparallel spin structure. Dielectric constant (ε′) and dissipation factor (tan δ) measured in frequency range 1 kHz to 5 MHz showed dispersion behaviour at low frequencies.     

Hydrothermal epitaxy and resultant properties of EuTiO3 films on SrTiO3(001) substrate

We report a novel epitaxial growth of EuTiO₃ films on SrTiO₃(001) substrate by hydrothermal method. The morphological, structural, chemical, and magnetic properties of these epitaxial EuTiO₃ films were examined by scanning electron microscopy, transmission electron microscopy, high-resolution X-ray diffractometry, X-ray photoelectron spectroscopy, and superconducting quantum interference device magnetometry, respectively. As-grown EuTiO₃ films with a perovskite structure were found to show an out-of-plane lattice shrinkage and room-temperature ferromagnetism, possibly resulting from an existence of Eu³⁺. Postannealing at 1,000°C could reduce the amount of Eu³⁺, relax the out-of-plane lattice shrinkage, and impact the magnetic properties of the films.

PACS

81.10.Aj; 81.15.-z; 61.05.-a     

Structural phase evolution in Bi7/8Ln1/8FeO3 (Ln = La-Dy) series

Regularities of the structural phase evolution associated with changing ionic radius of the A-site dopants in Bi_7/8Ln_1/8FeO₃ (Ln = La-Dy) series were investigated by X-ray diffraction technique at room temperature. A strong tendency to the structural phase separation was found. Initial polar rhombohedral R3c structure characteristic of pure bismuth ferrite is retained only for La-doped compound. For most other samples, intrinsic polar-antipolar R3c + Pnam (Ln = Pr, Nd, Sm), antipolar-nonpolar Pnam + Pnma (Ln = Gd), or polar-nonpolar R3c + Pnma (Ln = Tb, Dy) phase coexistence was revealed. Investigation of temperature evolution of crystal structure of La-doped compound performed with a neutron powder diffraction technique found that its high-temperature paraelectric phase had the orthorhombic Pnma symmetry.     

Structural,strong magnetic exchange,and room temperature multiferroic properties of single-phase 0.75BiFeO3–BaFe1/2Nb1/2O3 bulk ceramics

The ceramic 0.75BiFeO₃–BaFe_1/2Nb_1/2O₃ (0.75BF–BaFN) exhibits enhanced magnetisation and polar order regions at room temperature. This study investigated the microscopic structure and the magnetic, ferroelectric, magneto-ferroelectric, and optical properties of 0.75BF–BaFN by conducting experiments and performing density functional theory (DFT) calculations. Evidence of weak amplitude R-type octahedral rotations was observed using transmission electron microscopy (TEM). X-ray diffraction (XRD) refinement revealed that the P1 space group more appropriately described the structure of 0.75BF–BaFN. The weak influence of the magnetic field H on the polarisation of 0.7BF–BaFN and 0.75BF–BaFN ceramics was investigated. A notable magnetic enhancement was observed in 0.75BF–BaFN ceramics with a saturation magnetisation of 0.19 μB per Fe. DFT calculations predicted that the stable magnetic configuration of 0.75BF–BaFN is a G-type antiferromagnetic (AFM) structure. This study found stronger magnetic interactions between first-neighbour Fe ions (Ja = 12 meV) compared to pure BiFeO₃. With a significant remanent moment, strong exchange coupling, and magnetic field control of polarisation, 0.75BF–BaFN may facilitate the development of room temperature single-phase magnetoelectric components.     

Effect of mechanical milling on structural,dielectric and magnetic properties of BaTiO3–Ni0.5Co0.5Fe2O4 multiferroic nanocomposites

The coexistence of ferroelectricity and ferromagnetism has triggered great interest in multiferroic materials. Multiferroic with strong room temperature magnetoelectric (ME) coupling can provide a platform for future technologies. In this paper, we have investigated the effect of mechanical milling on the properties of multiferroic nanocomposites synthesized by mixing barium titanate (BaTiO₃) (BT) and nickel cobalt ferrite (Ni_0.5Co_0.5Fe₂O₄) (NCF). This process has resulted into reliable disposal of a given quantity of NCF nanoparticles in BT grid and composite samples of different particle sizes (<500 nm) have been obtained by varying the duration of ball-milling for 12, 24, and 48 h. The presence of NCF within BT powder has been confirmed by X-ray Diffraction (XRD) and magnetization measurements (MH). Structural analysis was performed by using Reitveld refinement method that shows that the tetragonality of BaTiO₃ structure get reduced in submicron range. Variations in ferroelectric and dielectric properties with reduction in particle size/milling duration have been studied by P-E loop tracer and Impedance analyzer. The dielectric constant value of 400 has been observed for BT-NCF0 that increases to 9.7 K for composite sample ball mill at 48 h whereas remnant polarization increases to 4.2 μC/cm². These composites with high dielectric constant that changes with temperature and particles size find application in energy storage devices, sensor and memory devices.     

On some properties of PZT-NZF composite films manufactured by hybrid synthesis route

Composite magnetoelectric films using ferroelectric lead zirconate titanate (PZT) and ferromagnetic nickel zinc ferrite (NZF) were prepared using the combination of sol-gel and hydrothermal process on Pt/Ti/SiO₂/Si substrates. The thickness was estimated ∼2 μm using cross-sectional SEM. Structure, morphology and electro-magnetic characterization were assessed using XRD, XPS, SEM, dielectric, leakage current, ferroelectric, and magnetic property analyze. The composite films exhibit coexistence of ferroelectric and ferromagnetic ordering at room temperature with a remnant polarization (P_r), and coercive field (E_c) of 1.2 μC/cm² and 7.8 kV/cm, respectively, and saturation magnetization (M_s) ∼20 emu/cm³. Polarization improved ∼5.2% upon poling the composite film using a magnetic field of 1 T.     

Room temperature ferromagnetism and magnetoelectric coupling in (K0.5Na0.5)NbO3 PLD nanocrystalline films

(K_0.5Na_0.5)NbO₃ nanocrystalline films have been successfully prepared on conductive Si substrate under different conditions by pulsed laser deposition. Room temperature ferromagnetism has been achieved through the introduction of cation (K, Na) vacancies during deposition at the expense of the deterioration of ferroelectricity. In addition to positive magnetocapacitance effect and electrical manipulation of magnetization, the new phenomenon that the orthometric application of an electric field and magnetic field on the surface of film leads to an enormous enhancement of in-plane saturation magnetization has been first observed, indicative of a strong magnetoelectric coupling which has a memory effect on the saturation magnetization at room temperature.