Magnetoelectric Ordering of BiFeO<Subscript>3</Subscript> from the Perspective of Crystal Chemistry

In this paper we examine the role of crystal chemistry factors in creating conditions for formation of magnetoelectric ordering in BiFeO₃. It is generally accepted that the main reason of the ferroelectric distortion in BiFeO₃ is concerned with a stereochemical activity of the Bi lone pair. However, the lone pair is stereochemically active in the paraelectric orthorhombic ß-phase as well. We demonstrate that a crucial role in emerging of phase transitions of the metal-insulator, paraelectric-ferroelectric and magnetic disorder-order types belongs to the change of the degree of the lone pair stereochemical activity—its consecutive increase with the temperature decrease. Using the structural data, we calculated the sign and strength of magnetic couplings in BiFeO₃ in the range from 945?°C down to 25?°C and found the couplings, which undergo the antiferromagnetic?→?ferromagnetic transition with the temperature decrease and give rise to the antiferromagnetic ordering and its delay in regard to temperature, as compared to the ferroelectric ordering. We discuss the reasons of emerging of the spatially modulated spin structure and its suppression by doping with La³⁺.     

Investigation on Electrical and Magnetic Properties of Gd-doped BiFeO<Subscript>3</Subscript>

Multiferroic ceramic samples of Bi_1−x Gd _x FeO₃ (x=0, 0.05, 0.1 and 0.15) have been prepared by rapid liquid-phase sintering technique. The effect of Gd substitution on ferroelectric and magnetic properties of Bi_1−x Gd _x FeO₃ ceramics has been investigated. The results of X-ray diffraction (XRD) patterns show that the single-phase BiFeO₃ sample has a rhombohedral structure and Gd³⁺ substitution for Bi³⁺ has not affected its structure. Experimental results suggest that for Bi_1−x Gd _x FeO₃ system, the ferroelectric and magnetic properties of BiFeO₃ are improved by Gd doping and the loop area increases with the Gd content. When x=0.15, saturated ferroelectric hysteresis loop is observed at room temperature with the maximal 2Pr=1.62 μC/cm², which is about 578.6% higher than that of BiFeO₃.     

Preparation and microwave absorption properties of BiFeO<Subscript>3</Subscript> and BiFeO<Subscript>3</Subscript>/PANI composites

BiFeO₃(BFO) particle was successfully synthesized by normal citric acid sol–gel method and the size of BiFeO₃ particle is about 200 nm. BiFeO₃/polyaniline (PANI) composites with the different weight ratio were synthesized by in situ emulsion polymerization. The citric acid doped PANI is fibrous and form a loose structure outside the BFO particle. With the increasing of PANI, the conductivity value of composites are increasing to 9.34?×?10^?2 S/cm. Moreover, the permittivity also enhance with the increasing of conductivity, which contribute to the improvement of dielectric loss. Microwave absorbing properties were investigated with a vector network analyzer in 1–18 GHz. The minimum reflection loss (RL) value is about ?40.2 dB at 8.3 GHz when the thickness is 3.5 mm, and the maximum bandwidth less than ?10 dB is 3.5 GHz (from 13.5 to 18 GHz) at the thickness of 2 mm. 3 mm millimeter-wave-attenuation properties were also tested, and the maximum attenuation value of BFO/PANI composites reach 15.71 dB. The composites can dissipate microwave energy into heat effectively by dielectric relaxation because of the suitable conductivity. The interface scattering and multiple reflections also play a important role because of the increasing of a loose structure. The BFO/PANI composite can be taken as a promising lightweight and multiband microwave absorber.     

Magnetic Properties and Photocatalytic Behavior of Co Co-doped BiFeO<Subscript>3</Subscript>:Er

The multiferroic materials Bi_0.95Er_0.05Fe_1?xCo_x O₃ (x = 0, 0.01, 0.03, and 0.05) are synthesized by solvothermal method. X-ray diffraction pattern is used to confirm the formation of rhombohedral crystal structure. TEM images show agglomerated nanosize particles. With the concentration of Co, a red shift is observed in the charge transfer transition of O_2p-Fe_3d band and band gap decreases. At higher concentration of Co, the saturation magnetization enhances and coercivity also varies with the concentration of Co. The photocatalytic degradation of methylene blue is evaluated under the visible light with the assistance of H₂O₂. With the increase in Co concentration, the photocatalytic activity decreases. The main species responsible for the degradation is found out using scavengers like ammonium oxalate, silver nitrate, p-benzoquinone, and tert-butyl-alcohol. The possible mechanism for the degradation of dye is explained.     

Dynamical Magnetoelectric Coupling in Multiferroic BiFeO<Subscript>3</Subscript>

In this study, the magnon excitations in multiferroic BiFeO₃ (BFO) have been discussed. The studies are based on the spin wave theory and Katsura’s model. The influence of the spin wave excitations on the terahertz absorption is discussed. The antiferromagnetic and ferroelectric interactions in multiferroic BFO were included using an effective fermion Hamiltonian. This Hamiltonian is bosonized and diagonalized, using Holstein–Primakoff and Bogoliubov transformations, respectively. An effective boson Hamiltonian is diagonalized to determine the excitation energy of the spin wave. The results obtained in this study are in qualitative agreement with the experimental data.     

Skyrmion lattices in the BiFeO<Subscript>3</Subscript> multiferroic

This work examines conditions for skyrmion lattice stability in BiFeO₃ multiferroic films possessing record high ferroelectric and antiferromagnetic transition temperatures, giant polarization, and a giant magnetoelectric effect. Using analytical and numerical calculations, we demonstrate stability of solitary spin vortices (skyrmions) and skyrmion lattices in BiFeO₃ films owing to the Dzyaloshinskii-Moriya interaction. Our results confirm that BiFeO₃ can be used as a matrix for chips with ultrahigh data density, up to 10 Tb/cm².     

Low-temperature synthesis of the multiferroic compound BiFeO<Subscript>3</Subscript>

A novel, low-temperature process is proposed for the synthesis of the multiferroic compound BiFeO₃. It enables the preparation of nanoparticulate material at temperatures as low as 200–250°C. An important role in the low-temperature synthesis of bismuth orthoferrite is played by ammonium nitrate additions and excess bismuth oxide.     

Dielectric and impedance spectroscopy of Ni doped BiFeO<Subscript>3</Subscript>-BaTiO<Subscript>3</Subscript> electronic system

A nickel modified BiFeO₃–BaTiO₃ electronic system has been fabricated by using a high-temperature solid-state reaction process. Preliminary X-ray structural analysis has confirmed the formation of a single-phase material in the orthorhombic crystal system. The dielectric and impedance characteristics of the prepared material have been studied in a wide range of frequency (1 kHz-1 MHz) at different temperatures (25–500 °C) for the better understanding of the frequency-temperature dependence of its capacitive and resistive behavior respectively. A significant effect of grains and grain boundaries of the resistive characteristics of the material is observed at high temperatures. The electrical conductivity of the material increases with increase in frequency in the low-temperature region. Preliminary study of a small amount of Ni doping in the above binary system (i.e., BiFeO₃–BaTiO₃) has provided many interesting results which may be useful for the fabrication of an electronic device.     

Toroidal spin ordering in BiFeO<Subscript>3</Subscript>, GaFeO<Subscript>3</Subscript>, and Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> in the Faddeev model with a magnetic field

The Fadeev model is used for describing the recently discovered toroidal spin ordering in piezoelectric and ferrimagnetic GaFeO₃ and piezo- and magnetoelectric Cr₂O₃ and BiFeO₃. A stable toroidal solution of the Faddeev model with the topological charge Q= 1 in an external homogeneous magnetic field was obtained using the trial function method. The energy of a toroid as a function of its radius (R) was determined at various values of the external magnetic field (H). It was shown that the energy minimum is shifted toward smaller R’s with an increase in H. At a critical field value, the torus collapses so that the local spin structure disappears. It is suggested to use magnetic field for controlling the torus size in multiferroics, promising materials of spintronics.     

Structural and Magnetic Properties of La<Subscript>2/3</Subscript>Sr<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript>/SrTiO<Subscript>3</Subscript>/CoFe<Subscript>2</Subscript> Hard-Soft Magnetic Systems

We report on the growth and magnetic properties of La_2/3Sr_1/3MnO₃/SrTiO₃/CoFe₂ hard-soft magnetic systems prepared by pulsed laser deposition on SrTiO₃(001) substrates. In situ reflection high-energy electron diffraction along the [100]SrTiO₃ substrate azimuth and atomic force microscopy measurements reveal that La_2/3Sr_1/3MnO₃ and SrTiO₃ grow both in a three dimensional mode and that the roughness of the lower and upper magnetic/non-magnetic interfaces ranges between 2 and 4 Å. Cross-section transmission electron microscopy observations show that the layers are continuous, with an homogeneous thickness, and that the interfaces are mostly sharp and correlated. The magnetization curves show a two step reversal of the magnetization, with very distinct coercive fields. A small anisotropy is observed for the CoFe₂ layer with an in plane easy magnetization axis along the [110]SrTiO₃ direction. Minor magnetization loops indicate that the coupling between the magnetic layers is negligible.     

Magnetoelectric interactions in BiFeO<Subscript>3</Subscript>, Bi<Subscript>0.95</Subscript>Nd<Subscript>0.05</Subscript>FeO<Subscript>3</Subscript>, and Bi<Subscript>0.95</Subscript>La<Subscript>0.05</Subscript>FeO<Subscript>3</Subscript> multiferroics

The technology of ceramic BiFeO₃, Bi_0.95Nd_0.05FeO₃, and Bi_0.95La_0.05FeO₃ multiferroics is described. The room-temperature magnetization, magnetoelectric (ME), and magnetodielectric (MDE) effects in these compounds have been studied. It is established that even a small fraction (x = 0.05) of rare-earth additives (La, Nd) to bismuth ferrite not only enhance its magnetic properties, but also significantly influence the ME and MDE effects. The dependence of the ME effect on the frequency of modulation of the alternating magnetic field in Bi_0.95Nd_0.05FeO₃, and Bi_0.95La_0.05FeO₃ is more pronounced than in pure BiFeO₃.     

Effects of addition of BiFeO<Subscript>3</Subscript> on phase transition and dielectric properties of BaTiO<Subscript>3</Subscript> ceramics

Samples of xBiFeO₃–(1 − x)BaTiO₃ (x = 0, 0.02, 0.04, 0.06, 0.07 and 0.08) were synthesized by solid state reaction technique and sintered in air in the temperature range 1,220–1,280 °C for 4 h. X-ray diffraction data showed that 2–8 mol% BiFeO₃ can dissolve into the lattice of BaTiO₃ and form single perovskite phase. The crystal structure changes from tetragonal to cubic phase at room temperature when 8 mol% of BiFeO₃ was added into BaTiO₃. Scanning electron microscope images indicated that the ceramics have compact and uniform microstructures, and the grain size of the ceramics decreases with the increase of BiFeO₃ content. Dielectric constants were measured as functions of temperatures (25–200 °C). With rising addition of BiFeO₃, the Curie temperature decreases. For the sample with x = 0.08, the phase transition occurred below room temperature. The boundary between tetragonal and cubic phase of the BiFeO₃–BaTiO₃ system at room temperature locates at a composition between 7 and 8 mol% of BiFeO₃. The diffusivity parameter γ for compositions x = 0.02 and x = 0.07 is 1.21 and 1.29, respectively. The relaxor-like behaviour is enhanced by the BiFeO₃ addition.     

The synergetic electromagnetic properties and enhanced microwave absorption of BiFeO<Subscript>3</Subscript>/BaFe<Subscript>7</Subscript>(MnTi)<Subscript>2.5</Subscript>O<Subscript>19</Subscript> composite

The effective microwave absorption materials could contribute to alleviating the electromagnetic wave pollution. However, conventional microwave absorption materials usually suffer from insufficient absorption intensity and the narrow effective absorption bandwidth. Herein, BiFeO₃/BaFe₇(MnTi)_2.5O₁₉ composites are proposed to address these issues through offering synergetic electromagnetic properties and proper electromagnetic properties. BiFeO₃ combined with BaFe₇(MnTi)_2.5O₁₉ exhibits dielectric multiple relaxation behaviors, strong ferromagnetic resonance and electromagnetic matching, ensuring increased multi-band microwave absorption. Accordingly, the minimum reflection loss (RL) of the composite with volume ratio of 1.5:1 reaches ??48 dB, and the bandwidth less than ??10 dB covers multi-frequencies at C, X and Ku band. These results suggest that BiFeO₃/BaFe₇(MnTi)_2.5O₁₉ composite could be a promising microwave absorption material in imaging, healthcare, information safety and military fields.     

Defect-induced asymmetry of local hysteresis loops on BiFeO<Subscript>3</Subscript> surfaces

Local piezoresponse hysteresis loops were systematically studied on the surface of ferroelectric thin films of BiFeO₃ grown on SrRuO₃ and La_0.7Sr_0.3MnO₃ electrodes and compared between ultrahigh vacuum and ambient environment. The loops on all the samples exhibited characteristic asymmetry manifested in the difference of the piezoresponse slope following local domain nucleation. Spatially resolved mapping has revealed that the asymmetry is strongly correlated with the random-field disorder inherent in the films and is not affected by the random-bond disorder component. The asymmetry thus originates from electrostatic disorder within the film, which allows using it as a unique signature of single defects or defect clusters. The electrostatic effects due to the measurement environment also contribute to the total asymmetry of the piezoresponse loop, albeit with a much smaller magnitude compared to local defects.     

Crystal chemistry and domain structure of rare-earth doped BiFeO<Subscript>3</Subscript> ceramics

Bi_(1−x)RE _x FeO₃ (BREF100x, RE = La, Nd, Sm, Gd) has been investigated with a view to establish a broad overview of their crystal chemistry and domain structure. For x ≤ 0.1, the perovskite phase in all compositions could be indexed according to the rhombohedral, R3c cell of BiFeO₃. For Nd and Sm doped compositions with 0.1 < x ≤ 0.2 and x = 0.15, respectively, a new antipolar phase was stabilised similar in structure to PbZrO₃. The orthoferrite, Pnma structure was present for x > 0.1, x > 0.15, and x > 0.2 in Gd, Sm, and Nd doped BiFeO₃, respectively. For x > 0.2, La doped compositions became pseudocubic at room temperatures but high angle XRD peaks were broad and asymmetric. These compositions have been indexed as the orthoferrite structure. It was concluded therefore that the orthoferrite phase appeared at lower values of x as the RE ferrite, end member tolerance factor decreased. However, the compositional window over which the PbZrO₃-like phase was stable increased with increasing end member tolerance factor but was not found as single phase in La doped compositions at room temperature. On heating, the PbZrO₃-like phase in BNF20 transformed to the orthoferrite, Pnma structure. T _C for all compositions decreased with decreasing A-site, average ionic polarizabilty and tolerance factor. For compositions with R3c symmetry, superstructure and orientational, and translational (antiphase) domains were observed in a manner typical of an antiphase-tilted, ferroelectric perovskite. For the new PbZrO₃-like phase orientational domains were observed.     

Hill Plot Focusing on Ce Compounds with High Magnetic Ordering Temperatures and Consequent Study of Ce<Subscript>2</Subscript>AuP<Subscript>3</Subscript>

The Hill plot is a well-known criterion of the f-electron element interatomic threshold distance separating the nonmagnetic state from the magnetic state in actinides and lanthanides. We have reinvestigated the Hill plot of Ce compounds using the commercial crystallographic database CRYSTMET, focusing on a relationship between the Ce-Ce distance and the magnetic ordering temperature because a Ce compound with no other magnetic elements rarely has a magnetic ordering temperature that is higher than 20 K. The Hill plot of approximately 730 compounds has revealed that a Ce compound, particularly a ferromagnet, showing a high magnetic ordering temperature would require a short Ce-Ce distance with a suppression of the valence instability of the Ce ion. From the study, we developed interest in Ce₂AuP₃ with a Curie temperature of 31 K. The ferromagnetic nature of this material has been examined in terms of a doping effect, which suggests a possible increase in magnetic anisotropy energy.     

Synthesis and dielectric properties of BiFeO<Subscript>3</Subscript> derived from molten salt method

BiFeO₃ powder was synthesized in NaCl media at temperature range from 700 to 800 °C, using Bi₂O₃ and Fe₂O₃ as raw materials. Effects of calcining temperature and salt ratios on the synthesis of BiFeO₃ powder had been investigated. It was found that NaCl effectively promoted the formation of BiFeO₃. Almost pure BiFeO₃ phase with a very small amount of Bi₂Fe₄O₉ phase was synthesized at 750 °C with salt weight ratios of 1:1. A large amount of BiFeO₃ phase decomposed to Bi₂Fe₄O₉ and Bi₂₅FeO₃₉ phase when the temperature was up to 800 °C. In the present method, the calcining temperature played an important role in the formation of BiFeO₃ phase. BiFeO₃ ceramics derived from molten salt method were prepared and exhibited the higher dielectric constant.     

La-modification of multiferroic BiFeO<Subscript>3</Subscript> by hydrothermal method at low temperature

This study reports the effect of La doping on structural, morphological and magnetic characteristics of BiFeO₃ synthesized under mild hydrothermal conditions at 200°C for 16 h using the KOH concentration of 4 M. The as-synthesized powders of Bi_1−x La _x FeO₃ for x = 0.05, 0.10, and 0.15 were characterized by X-ray powder diffraction scanning electron microscope and the vibrating sample magnetometer. The formation mechanism of Bi_1−x La _x FeO₃ powders can be expressed by “dissolution-nucleation-crystallization” process. The magnetism of Bi_1−x La _x FeO₃ increased with increase of La content due mainly to increscent suppressing spiral magnetic structure and magnetic contribution of La.     

Magnetic properties of Nd-Y<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript> nanoparticles

Nb³⁺-substituted garnet nanoparticles Y_3−xNd_xFe₅O₁₂ (x = 0.0, 0.5, 1.0, 1.5, and 2.0) were fabricated by a sol-gel method and their crystalline structures and magnetic properties were investigated by using X-ray diffraction (XRD), thermal analysis (DTA/TG), and vibrating sample magnetometer (VSM). The XRD patterns of Y_3−xNd_xFe₅O₁₂ have only peaks of the garnet structure and the sizes of particles range from 34 to 70 nm. From the results of VSM, it is shown that when the Nd concentration x ( 1.0, the saturation magnetization of Y_3−xNd_xFe₅O₁₂ increases as the Nd concentration (x) is increased, and gets its maximum at x = 1.0, but when x ( 1.0, the saturation magnetization decreases with increasing the Nd concentration (x), this may be due to the distortion of the microstructure of Y_3−xNd_xFe₅O₁₂, which leads to the decrease of the effective moment formed by Fe³⁺. Meanwhile, it is observed that with the enhancement of the surface spin effects, the saturation magnetization rises as the particle size is increased.     

Liquid-phase epitaxy of NdAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> and Yb-doped YAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Epitaxial layers of NaAl₃(BO₃)₄ (NAB) and YAl₃(BO₃)₄〈Yb〉 (YAB〈Yb〉) containing up to 10 at % Yb have been grown by liquid-phase epitaxy on YAB substrates. Their growth kinetics have been studied at relative supersaturations of the high-temperature solution from 2 × 10^?2 to 16 × 10^?2. The ytterbium concentration in YAB〈Yb〉 has been shown to vary little during the epitaxial process. Near the edges of the substrate, the surface morphology of the layers is complicated by vicinals, which have a spiral form in the case of YAB〈Yb〉. On \(\{ 10\overline 1 1\} \) YAB substrates, homogeneous single-crystal NAB films have been grown.