Suppressing the non-switching contribution in BiFeO3-Bi4Ti3O12 based thin film composites to produce room-temperature multiferroic behavior

Exploiting the exceptional multiferroic characteristics of BiFeO₃-based systems depends largely on controlling the high leakage currents that often constrain the ferroelectric response of this material. This limiting circumstance is even more restrictive in the film geometry, where the high area/volume ratio can add complications related to the uncontrolled loss of a particularly volatile element such as bismuth. In this work we address the suppression of such non-switching contribution by preparing BFO-BiT thin film composites and using a low-temperature processing protocol in a fully aqueous medium. With an adequate and systematic doping of both oxides, the produced composites show both magnetic and ferroelectric response at room temperature, in a process that is also related to the fine matching between the two crystal lattices involved. The results obtained further indicate the possibility of applying a simple, sustainable protocol with high scalability prospects to fabricate exploitable multiferroic systems, i.e. with no need for large energy inputs nor sophisticated equipment.     

STRUCTURE AND PROPERTIES OF EPITAXIAL THIN FILMS OF Bi2FeCrO6: A MULTIFERROIC MATERIAL POSTULATED BY AB-INITIO COMPUTATION

ABSTRACT

Experimental results on Bi₂FeCrO₆ (BFCO) epitaxial films deposited by laser ablation on SrTiO₃ substrates are presented. It has been theoretically predicted using first-principles density functional theory that BFCO is ferrimagnetic (with a magnetic moment of 2μ_B per formula unit) and ferroelectric (with a polarization of ～ 80 μ C/cm² at 0K). The crystal structure investigated using X-ray diffraction shows that the films are epitaxial with a high degree of crystallinity. Chemical analysis carried out by X-ray Microanalysis and X-ray Photoelectron Spectroscopy indicates the correct cationic stoichiometry in the BFCO layer, namely (Bi:Fe:Cr = 2:1:1). Cross-section high-resolution transmission electron microscopy images together with selected area electron diffraction confirm the crystalline quality of the epitaxial BFCO films with no identifiable foreign phase or inclusion. The multiferroic character of BFCO is proven by piezoresponse force microscopy (PFM) and magnetic measurements showing that the films exhibit ferroelectric and magnetic hysteresis at room temperature. The local piezoelectric measurements show the presence of ferroelectric domains and their switching at the sub-micron scale.     

Effect of CuO addition on magnetic and electrical properties of Sr2Bi4Ti5O18 lead-free ferroelectric ceramics

The effect of CuO addition on magnetic and electrical properties of Sr₂Bi₄Ti₅O₁₈ (SBT) lead-free bismuth layered structure ferroelectric ceramics have been studied and reported. Interestingly, the prepared samples exhibit multiferroic behavior with the coexistence of magnetic and ferroelectric phase transition temperature. Magnetic phase transition with Neel׳s temperature (T_N) of 657 K is observed at 0.75 mol% of CuO added SBT ceramics, which is higher than the well known multiferroic BiFeO₃ (643 K) and the ferroelectric phase transition with Curie temperature (T_C) of 587 K is observed at 1 mol% of CuO added SBT ceramics, which is relatively higher than the reported pure SBT ceramics (558 K). Further, the electrical properties such as dielectric, ferroelectric, piezoelectric, leakage current density characteristics and optical properties were investigated as a function of x (x=0, 0.25, 0.5, 0.75 and 1 mol%). Presence of strong magnetic super-exchange interactions in CuO and the creation of oxygen vacancies play a vital role in the enhancement of magnetic and electrical properties of CuO doped SBT ceramics. Moreover, the present results indicate that, small amount (0.25 mol%) of CuO addition in SBT ceramics enhances the electrical properties significantly and vice versa, large amount (0.75 mol%) of CuO addition enhances the magnetic properties. Thus, the presence of magneto-electric coupling effect was observed in CuO doped Sr₂Bi₄Ti₅O₁₈ ferroelectric ceramics.     

Structural, optical and electromagnetic properties of Bi1−xHoxFeO3 multiferroic materials

Bi_1−xHo_xFeO₃ (x = 0.00, 0.05, 0.10, 0.15 and 0.20) polycrystalline ceramics were synthesized by a solid-state reaction and their structural, absorption, Raman scattering, impedance and magnetic properties were investigated. The substitution of rare earth Ho for Bi was found to decrease the impurity phase in BiFeO₃ ceramics. There appears an anomalous change in the lattice constants, optical band gap as well as the impedance spectroscopy and magnetization of samples at x = 0.10, suggesting a limit of dissolubility of Ho doped ions in BiFeO₃. Additionally, the Raman measurement performed for the lattice dynamics study of Bi_1−xHo_xFeO₃ samples reveals a band centered at around 1000-1300 cm⁻¹ which is associated with the resonant enhancement of two-phonon Raman scattering in the multiferroic Bi_1−xHo_xFeO₃ samples. Ho-doped BiFeO₃ also showed a ferromagnetic-like behavior with M_r = 1070 × 10⁻⁴ and M_s = 1.60 emu/g for optimum content x = 0.10, which is similar to the solid solution system of BiFeO₃.     

Preparation and magnetic properties of Bim + 1Fem − 3Ti3O3m + 3 thin films with magnetic order above room temperature

Thin films of Bi_m + 1Fe_m − 3Ti₃O_3m + 3 (BFTO) with m ≦ 9 have been successfully grown on (100) SrTiO₃ by chemical solution deposition. These films had the c axis normal to the film plane. The conversion electron Mössbauer spectoroscopy (CEMS) showed that the spectra of BFTO thin films exhibit an asymmetric quadrupole doublet for m = 8 at 300 K, indicative of being paramagnetic, while, for m = 9, clearly show six hyperfine lines indicating presence of magnetic order at 300 K. From the intensity ratio of asymmetric peaks, the polarization axis of BFTO films with m ≥ 8 was deduced to be likely along <101> of the perovskite-like unit. On the other hand, it was found from the spectral fitting that the BFTO thin film with m = 9 has the Néel temperature around 310 K and the spin axis making an angle of about 60° to the c-axis. These indicate that the BFTO (m = 9) thin film is a promising candidate for room-temperature multiferroics.     

Sputter-epitaxy and electric properties of multiferroic Bim + 1Fem-3Ti3O3m + 3 thin films

Growth conditions suitable for sputter-epitaxy of Bi_m + 1Fe_m-3Ti₃O_3m + 3 (BFTO) thin films with layered structure have been investigated. The amount of oxygen during deposition was found to be specifically essential for obtaining a good-quality thin film of BFTO with a large m. The (001) epitaxial thin films of BFTO with m of nearly 10 which is expected to retain magnetic order up to room temperature have been successfully grown on (001) SrTiO₃ substrates under the determined optimum condition. The film exhibited leakage current as low as order of 10⁻²-10⁻¹ A/m² limited by Schottky emission at the interfaces between the electrodes and the film. In addition, the film showed a ferroelectric polarization curve with Pr = 6 μC/cm² for applied field of 35 MV/m at room temperature though the curve was unsaturated. These indicate that the BFTO (m = 10) thin films are promising as multiferroics at room temperature.     

X-ray phase,structural, dielectric and magnetic studying of BiFe1-х(M1/2Ti1/2)хO3 (M = Co,Ni, Zn, х = 0–0.11) multiferroics

Perovskite-like rhombohedral distorted solid solutions of BiFe_1-х(M_1/2Ti_1/2)_хO₃ (M = Co, Ni, Zn, x = 0–0.11) were obtained by solid-phase synthesis. An indicator of the solid solution formation is the change of unit cells parameters, that corresponds to the ionic radii of mixed cations (M1/2Ti1/2)3+ (M = Co, Ni, Zn. Solid solutions of BiFe_1-х(M_1/2Ti_1/2)_хO₃ (M = Co, Ni), in contrast to BiFe_1-x(Zn_1/2Ti_1/2)_xO₃ demonstrate ferromagnetic hysteresis pels at room temperature. The x growth in the range from 0.01 to 0.11 for the BiFe_1-х(M_1/2Ti_1/2)_хO₃ system leads to, the saturation magnetization M_S and the remanent magnetization M_R increase from ∼0.1 and ∼2.4⋅10⁻³ emu/g to ∼0.4 and ∼0.038 emu/g respectively. In the same time the coercive force H_c decreases from ∼120 to ∼80 Oe. For the BiFe_1-х(Co_1/2Ti_1/2)_хO₃ system, a noticeably higher magnetic properties with a more complex dependence on x are observed. The maximum parameter values are observed at x = 0.04–0.05: M_S = 0.83 and M_R = 0.24 emu/g, H_c = 1.8 kOe. It is suggested that the detected anomalies of Co-containing solid solutions behavior are related to the one-ionic magnetocrystalline anisotropy of Co²⁺ cations. The BiFe_1-х(M_1/2Ti_1/2)_хO₃ (M = Co, Ni) samples demonstrate piezoelectric constant d₃₃ up to 7 pC/N. Due to the set of properties the materials obtained can be classified as high-temperature multiferroics.     

Electroelastic study of the relaxor–normal phase transition in PFW–PT solid solution

Relaxor–normal transition in ferroelectric materials have been extensively studied through their optical and dielectric properties. However, only few reports concerning simultaneously elastic and dielectric characterization were presented. In this work, multiferroic ceramic solid solutions between the ferroelectric relaxor Pb(Fe_2/3W_1/3)O₃ (PFW) and “normal” ferroelectric PbTiO₃ (PT) [PFW–PT] have been synthesized by a modified B-site precursor method and investigated by the ultrasonic pulse echo technique and dielectric measurements, as a function of the frequency and temperature. The nature of the phase transition was characterized through the diffusivity exponent obtained from both measurements. Additionally, an interplay between ferroelectric and ferromagnetic properties of the samples, as well as the influence of the proximity of the temperatures of the establishment of both orderings with electroelastic coupling coefficient was represented.     

Structure,synthesis and multiferroic nature of BiFeO<Subscript>3</Subscript> and 0·9BiFeO<Subscript>3</Subscript>-0·1BaTiO<Subscript>3</Subscript>: An overview

A brief review of the crystal structure and multiferroic nature of pure BiFeO₃ and 0·9BiFeO₃-0·1BaTiO₃ (BF-0·1BT) is presented. An atomic level evidence for magnetoelectric coupling of intrinsic multiferroic origin in BF-0·1BT is presented.     

Preparation and characterization of self-assembled percolative BaTiO3–CoFe2O4 nanocomposites via magnetron co-sputtering

BaTiO₃–CoFe₂O₄ composite films were prepared on (100) SrTiO₃ substrates by using a radio-frequency magnetron co-sputtering method at 750 °C. These films contained highly (001)-oriented crystalline phases of perovskite BaTiO₃ and spinel CoFe₂O₄, which can form a self-assembled nanostructure with BaTiO₃ well-dispersed into CoFe₂O₄ under optimized sputtering conditions. A prominent dielectric percolation behavior was observed in the self-assembled nanocomposite. Compared with pure BaTiO₃ films sputtered under similar conditions, the nanocomposite film showed higher dielectric constants and lower dielectric losses together with a dramatically suppressed frequency dispersion. This dielectric percolation phenomenon can be explained by the ‘micro-capacitor’ model, which was supported by measurement results of the electric polarization and leakage current.