V. F. Freitas

Ferroic states in La doped BiFeO3-PbTiO3 multiferroic compounds

In this paper, the relationship between ferroic states and the physicochemical mechanism which governs the (Bi/Pb/La)–O bonds in polycrystalline La doped (0.6)BiFeO3–(0.4)PbTiO3 compounds were investigated. An abrupt change in the symmetries of perovskite-structured samples, from tetragonal (P4mm) to rhombohedral (R3c), occurs with the increase of the La concentration, highlighting the diminishment of the hybridization of the (Bi/Pb)–O bonds when La is added into the solid solution, and the emergence of a characteristic morphotropic phase boundary tuned by doping. A maximum remnant polarization (10 μC/cm2) was observed for low La concentrations (where R3c and P4mm phases coexist), while the maximum remnant magnetization (∼75 × 10−3 emu/g) was observed for highly La concentrated samples. These results show that ferroelectric and weak-ferromagnetic orders are strongly correlated to the structural arrangement, which in turn are directly related to the chemical A–O and B–O bonds (long-range and short-range fo...

A phenomenological model for ferroelectric domain walls and its implications for BiFeO3–PbTiO3 multiferroic compounds

Multiferroics are an important family of materials with special properties suitable for applications in advanced technological devices. In most of the cases, the ferroelectric ordering and domain wall formation determine and control their operation and functionality. However, the physical mechanisms by which these domain walls are formed are still not yet completely clarified. Also, in the last few years, few advances are found in the mechanisms used to explain the domain walls and their influence on the materials properties. In this work, the domain walls in ferroelectric multiferroics were investigated by using high-resolution transmission electron microscopy and image simulations. The ferroelectric switching was also observed by piezoresponse force microscopy. A three-dimensional atomic-level framework of 90° ferroelectric domain walls was proposed and the structural and ferroelectric features at the domain walls, such as length, width, and angle between domains, were determined. From these studies, it was found that ferroelectric and structural features of multiferroic BiFeO3–PbTiO3 compounds, such as domain-orientation, electrical conductivity, magnetic ordering, and brittleness due to strains at the domain walls, can be controlled by particular atomic substitutions at the A site of the perovskite structure.

Overlap of the intrinsic and extrinsic magnetoelectric effects in BiFeO3-PbTiO3 compounds: Potentialities for magnetic-sensing applications

The potentialities of BiFeO3-PbTiO3 multiferroic compounds for magnetic-field sensing applications is investigated by characterizing the dynamical magnetoelectric response of La doped (0.6)BiFeO3-(0.4)PbTiO3 piezoceramics. The results are compared with those obtained in Nb doped (1 wt. %) PbZr0.53Ti0.47O3 (PZT-5A) piezoceramics and the observed non-linearity of the oscillating voltage response of (0.6)BiFeO3-(0.4)PbTiO3 piezoceramics is attributed to the overlapping of the intrinsic and extrinsic magnetoelectric effects that were successfully decoupled by analyzing the in-phase and out-of-phase voltage components relative to applied oscillating magnetic field. For an alternating cosine-like magnetic field of 0.32 Oe, at the piezoelectric resonance frequency (184.2 kHz), the alternating voltage response decreases from 7.77 mV, at −10 kOe, to 2.71 mV, at 1.8 kOe, to subsequently increases until 5.97 mV at 10 kOe. This non-linear voltage response, which can be easily converted into a linear signal by using the phase difference, can be used for static magnetic field sensing in a wide range of magnetic fields.The potentialities of BiFeO3-PbTiO3 multiferroic compounds for magnetic-field sensing applications is investigated by characterizing the dynamical magnetoelectric response of La doped (0.6)BiFeO3-(0.4)PbTiO3 piezoceramics. The results are compared with those obtained in Nb doped (1 wt. %) PbZr0.53Ti0.47O3 (PZT-5A) piezoceramics and the observed non-linearity of the oscillating voltage response of (0.6)BiFeO3-(0.4)PbTiO3 piezoceramics is attributed to the overlapping of the intrinsic and extrinsic magnetoelectric effects that were successfully decoupled by analyzing the in-phase and out-of-phase voltage components relative to applied oscillating magnetic field. For an alternating cosine-like magnetic field of 0.32 Oe, at the piezoelectric resonance frequency (184.2 kHz), the alternating voltage response decreases from 7.77 mV, at −10 kOe, to 2.71 mV, at 1.8 kOe, to subsequently increases until 5.97 mV at 10 kOe. This non-linear voltage response, which can be easily converted into a linear signal by using t...

Thermal decomposition synthesis and assessment of effects on blood cells and in vivo damages of cobalt ferrite nanoparticles

In the search to reduce the side effects, toxicity and assuring the desired effectiveness of the drugs, many efforts has been made to improve specific drugs’ delivery characteristics. Several carrier nanoparticles have been used to assist the drugs incorporation, absorption and transport through the bloodstream. However, most chemical synthesis routes are multistep and time-consuming treatments and, also, many carrier nanoparticles have toxic effects. In this work, we report a simple one-pot approach for the synthesis of CoFe2O4 nanoparticles (20 to 100 nm). The magnetic measurements revealed nanoparticles with a magnetic saturation nearly one third of that for bulk CoFe2O4. In vitro assays showed no hemolytic potential and negligible toxicity. By in vivo experiments using adult male mice we found no potential risk alterations by the nanoparticles administration. Therefore, the CoFe2O4 nanoparticles, synthesized by the current approach, can be a model drug-carrier, which makes them useful for the biomedical applications.

Multiferroic Behavior of Lead-free AlFeO3 and Mn, Nb Doped Compositions

The lead-free AlFeO3-based compounds are attractive multiferroic materials, as those present piezoelectricity and ferrimagnetism at low temperatures. In this work the synthesis and ferroic properties of AlFeO3-based ceramics were investigated. Stoichiometric proportions of the precursors - α-Fe2O3 and α-Al2O3 - were milled and sintered to obtain AlFeO3 ceramics. Also, AlFeO3 ceramics doped with 2at% of Nb2O5 or MnO2 were sintered in oxygen flow at 1700 K. The crystal structure of the samples was studied by X-ray diffraction. The diffractograms were refined by the Rietveld method. The results obtained from refinements showed an orthorhombic symmetry with space group Pna21. Dielectric, ferroelectric and magnetic investigations suggested a magnetoelectric coupling between the electric and magnetic orderings at temperatures below ∼ 160 K for AlFeO3, Al(Fe0.98Nb0.02)O3 and Al(Fe0.98Mn0.02)O3, where the compositions show the ferromagnetic and ferroelectric ordering.

Structural phase relations in perovskite-structured BiFeO 3 based multiferroic compounds

In this review, the state of the art in understanding the structural phase relations in perovskite-structured BiFeO3-based polycrystalline solid solutions is presented and discussed. Issues about the close relation between the structural phase and overall physical properties of the reviewed systems are pointed out and discussed. It is shown that, by adjusting the structural symmetric arrangement, the ferroelectric and magnetic properties of BiFeO3-based polycrystalline solid solutions can be tuned to find specific multifunctional applications. However, an intrinsic mechanism linking structural arrangement and physical properties cannot be identified, revealing that this subject still deserves further discussion and investigation.

Structural Refinement and Ferroic Properties in BiFeO3–Based Compounds

Nanostructured BiFeO3 - XTiO3 (X = Ba or Pb) multiferroic ceramics were processed by mechanosynthesis and a detailed structural study was conducted through Rietveld refinement of X-ray diffraction patterns. It was found that the structural and ferroic properties have been improved by the formation of double-phased compounds in both cases. In fact, ferroelectric investigations indicated a structurally tuned ferroic state, where the ferroelectricity is strongly affected by the coexistence of two distinct symmetries in BiFeO3 -based solid solutions.

Intrinsic features of the magnetoeletric coupling mechanism in displacive multiferroics

Structural, ferroelectric, and magnetic arrangements, and electron density in the vicinity of cations, were modeled from high-resolution X-ray and neutron powder diffraction data in La modified BiFeO3-PbTiO3 compounds. Important features for controlling the intrinsic mechanism for the magnetoelectric coupling in these materials, as prototypes for perovskite structured magnetoelectric multiferroics, are pointed out and discussed. It is shown that the magnetoelectric coupling angle is governed by covalent-like forces, which also affect the structural and ferroelectric distortions in the unit cell.

Room temperature nonlinear magnetoelectric effect in lead-free and Nb-doped AlFeO3 compositions

It is still a challenging problem to obtain technologically useful materials displaying strong magnetoelectric coupling at room temperature. In the search for new effects and materials to achieve this kind of coupling, a nonlinear magnetoelectric effect was proposed in the magnetically disordered relaxor ferroelectric materials. In this context, the aluminum iron oxide (AlFeO3), a room temperature ferroelectric relaxor and magnetic spin glass compound, emerges as an attractive lead-free magnetoelectric material along with nonlinear magnetoelectric effects. In this work, static, dynamic, and temperature dependent ferroic and magnetoelectric properties in lead-free AlFeO3 and 2 at. % Nb-doped AlFeO3 multiferroic magnetoelectric compositions are studied. Pyroelectric and magnetic measurements show changes in ferroelectric and magnetic states close to each other (∼200 K). The magnetoelectric coefficient behavior as a function of Hbias suggests a room temperature nonlinear magnetoelectric coupling in both sing...

Tuning ferroic states in La doped BiFeO3-PbTiO3 displacive multiferroic compounds

In this manuscript, X-ray and high-resolution neutron powder diffraction investigations, associated with Rietveld refinements, magnetic hysteresis curves and a modeling of electron-density distributions around the ions, are used to describe the driving forces responsible for tuning the ferroic states in La doped (0.6)BiFeO3-(0.4)PbTiO3 compositions. The intrinsic relations between the ferroic orders and the structural arrangements (angles, distances and electron-density distributions around the ions) are revealed, helping in the understanding of some aspects comprising the ferroic properties of perovskite-based displacive multiferroic compounds.