V. A. Khomchenko

Effect of diamagnetic Ca, Sr, Pb, and Ba substitution on the crystal structure and multiferroic properties of the BiFeO3 perovskite

In this work, we studied the effect of heterovalent Ca, Sr, Pb, and Ba substitution on the crystal structure, dielectric, local ferroelectric, and magnetic properties of the BiFeO3 multiferroic perovskite. Ceramic solid solutions with the general formula Bi0.7A0.3FeO3 (A is a doping element) were prepared and characterized by x-ray diffraction, dielectric, piezoresponse force microscopy (PFM), and magnetic measurements. It is shown that the crystal structure of the compounds is described within the space group R3c, permitting the spontaneous polarization, whose existence was confirmed by the PFM data. Magnetic properties of the solid solutions are determined by the ionic radius of the substituting element. Experimental results suggest that the increase in the radius of the A-site ion leads to the effective suppression of the spiral spin structure of BiFeO3, resulting in the appearance of net magnetization.

Synthesis and multiferroic properties of Bi0.8A0.2FeO3 (A=Ca,Sr,Pb) ceramics

Bi1−xAxFeO3 ceramics (A=Ca,Sr,Pb) were sintered by conventional mixed oxide route. The crystallographic structure of all samples is characterized by the rhombohedral symmetry (space group R3c). The existence of switchable ferroelectric polarization is verified by piezoresponse force microscopy that is proven to be a useful technique in semi-insulating ferroelectrics. Magnetic properties of Ca and Sr-doped ceramics are found to reproduce the antiferromagnetic behavior of undoped BiFeO3 without any enhancement of the magnetization. On the contrary, Pb-doped compound demonstrates appearance of a weak ferromagnetism. It is thus shown that Pb doping of BiFeO3 is a promising way for preparing multiferroic materials.

Intrinsic nature of the magnetization enhancement in heterovalently doped Bi1−xAxFeO3 (A = Ca, Sr, Pb, Ba) multiferroics

The mechanism of the formation of heterovalent-substitution-induced defects as well as their influence on the magnetic properties of BiFeO3-based multiferroics has been studied. It has been shown that heterovalent A2+ substitution results in the formation of oxygen vacancies in the host lattices of both antiferromagnetic and weak ferromagnetic Bi1−xAxFeO3 (A = Ca, Sr, Pb, Ba; x = 0.2, 0.3) compounds, thus indicating the intrinsic (i.e. not related to defects themselves) mechanism of doping-induced enhancement of magnetization. A correlation between the ionic radius of the substituting element and the value of the spontaneous magnetization of the corresponding solid solution has been found. The experimental results suggest that A-site substitution with the biggest ionic radius ions effectively suppresses the spiral spin configuration of antiferromagnetic BiFeO3.

Crystal structure and magnetic properties of Bi0.8(Gd1?xBax)0.2FeO3 (x = 0, 0.5, 1) multiferroics

Investigations of crystal structure and magnetic properties of Bi0.8(Gd1−xBax)0.2FeO3 (x = 0, 0.5, 1) samples have been performed. The Bi0.8Gd0.2FeO3 and Bi0.8Ba0.2FeO3 compounds have been shown to crystallize in the polar space groups Pn21a and R3c, respectively. It has been found that no continuous series of solid solutions is formed in the Bi0.8(Gd1−xBax)0.2FeO3 system: the crystal structure of the Bi0.8Gd0.1Ba0.1FeO3 sample is characterized by a coexistence of Pnma and R3c structural phases which differ in their chemical compositions. All of the Bi0.8(Gd1−xBax)0.2FeO3 (x = 0, 0.5, 1) compounds have been found to possess a spontaneous magnetization at room temperature. For Gd-containing samples, a significant enhancement of the magnetization takes place with decreasing temperature.

Coexistence of spontaneous ferroelectricity and weak ferromagnetism in Bi0.8Pb0.2FeO2.9 perovskite

Polycrystalline samples with the nominal composition Bi0.8Pb0.2FeO3 have been studied via x-ray diffraction, M?ssbauer spectroscopy, dielectric, magnetic, and local ferroelectric measurements. It has been found that the heterovalent Pb2+ substitution in Bi0.8Pb0.2FeOy is realized through the formation of oxygen vacancies. The crystal structure of the compound has been shown to be described by the non-centrosymmetric space group R3c. Investigations of local ferroelectric and magnetic properties have confirmed that spontaneous polarization and weak ferromagnetism coexist in this material at room temperature. The nature of the weak ferromagnetic moment in this compound is discussed in terms of a doping-induced change in the magnetic anisotropy.

Correlation between Ionic Radius of Substituting Element and Magnetic Properties of Bi1-xAxFeO3-x/2 (A= Ca, Sr, Pb, Ba) Multiferroics

Investigation of crystal structure and magnetic properties of the diamagnetically- substituted Bi1-xAxFeO3-x/2 (A= Ca, Sr, Pb, Ba; x= 0.2, 0.3) polycrystalline samples has been carried out. It has been shown that the heterovalent A2+ substitution result in the formation of oxygen vacancies in the host lattice. The solid solutions have been found to possess a rhombohedrally distorted perovskite structure described by the space group R3c. Magnetization measurements have shown that the magnetic state of these compounds is determined by the ionic radius of the substituting elements. A-site substitution with the biggest ionic radius ions has been found to suppress the spiral spin structure of BiFeO3 giving rise to the appearance of weak ferromagnetism.

Heterovalent A-site doping of multiferroic BiFeO3

Multiferroics, where magnetic order and ferroelectric polarization might be combined in a single phase, have been an object of the growing interest. However, most magnetic ferroelectrics tend to have low magnetic ordering temperature and are often antiferromagnets, in which the magnetoelectric effect is intrinsically small. BiFeO3 perovskite seems to be the most suitable object for multiferroic research in view of their high magnetic and ferroelectric ordering temperatures. On the other hand, spatially modulated G-type antiferromagnetic spin structure of BiFeO3 prevents any net magnetic signal and inhibits the observation of linear magnetoelectric effect. A-site doping of BiFeO3 has been found to be one of the way to suppress the spin modulation, however main regularities of changes of the magnetic and ferroelectric properties of the BiFeO3-based multiferroics under the A-site doping are still unclear. To reveal them, we have carried out complex investigation of crystal structure, magnetic and ferroelectric behavior of A= Ca, Sr, Pb and Ba- substituted multiferroic perovskites. The Bi1-xAxFeOy (x= 0.2, 0.3) solid solutions have been prepared and characterized by x-ray diffraction, magnetic, dielectric, Mossbauer spectroscopy and PFM measurements. It has been found that the heterovalent A2+ substitution in Bi1-xAxFeOy is realized through the formation of the oxygen vacancies. It has been shown that the crystal structure of the compounds is described within the space group R3c, permitting the spontaneous polarization, whose existence is confirmed by the PFM data. Magnetic properties of the solids have been found to be determined by the ionic radius value of the substituting element. Experimental results suggest that the A-site substitution with the biggest ionic radius ions effectively suppress the spiral spin structure of BiFeO3 and result in the net magnetization appearance.

Effect of Diamagnetic A 2+ Substitution on the Magnetic and Ferroelectric Properties of the Bi 1−x A x FeO 3 Multiferroics

Investigation of crystal structure, magnetic and local ferroelectric properties of the diamagnetically-doped Bi 1−x A x FeO 3 (A= Ca, Sr, Pb, Ba; x= 0.2, 0.3) ceramic samples has been carried out. It has been shown that the solid solutions have a rhombohedrally distorted perovskite structure described by the space group R 3 c . Piezoresponse force microscopy data have revealed the existence of the spontaneous ferroelectric polarization in the samples at room temperature. Magnetization measurements have shown that the magnetic state of these compounds is determined by the ionic radius of the substituting elements. A-site substitution with the biggest ionic radius ions has been found to suppress the spiral spin structure of BiFeO 3 and to result in the appearance of weak ferromagnetism. The magnetic properties have been discussed in terms of doping- induced changes in the magnetic anisotropy.