[en] The investigation focuses on the crystal structure, microstructure, local ferroelectric and magnetic properties of the Bi_0_._9Sr_0_._1Fe_1_−_xTi_xO_3_−_δ (x  =  0.05, 0.1, 0.15; δ  =  (0.1  −  x)/2) multiferroics prepared by a solid-state reaction method. All the samples have been found to be isostructural with the pure BiFeO_3 (the material crystallizes in a polar rhombohedral structure belonging to the space group R3c). It has been shown that the pattern of changes in the lattice parameters of the Bi_0_._9Sr_0_._1Fe_1_−_xTi_xO_3_−_δ samples can be interpreted as consistent with the doping-driven elimination of anion vacancies at x  ⩽  0.1 and the formation of cation vacancies at x  >  0.1. The readjustment of the defect structure associated with the mechanism of charge compensation in the aliovalent-substituted BiFeO_3 is accompanied by correlated changes in the morphology, ferroelectric/ferroelastic domain structure and magnetic properties of the materials. In particular, it has been found that the deviation from the ideal (δ  =  0) cation–anion stoichiometry in the Bi_0_._9Sr_0_._1Fe_1_−_xTi_xO_3_−_δ system leads to a significant decrease in the average size of crystal grain and ferroelectric domains and gives rise to an antiferromagnetic-weak ferromagnetic transformation. Results of this study have been compared with those obtained for equally substituted samples of the Bi_0_._9Ca_0_._1Fe_1_−_xTi_xO_3_−_δ series (Khomchenko and Paixão 2015 J. Phys.: Condens. Matter 27 436002) to demonstrate how the variation in the chemical pressure introduced by the partial replacement of Bi"3"+ with bigger (Sr"2"+) and smaller (Ca"2"+) ions can affect the multiferroic behavior of Ti-doped bismuth ferrites. (paper)

[en] Recognition of the factors that may significantly affect the multiferroic properties of BiFeO_3-based perovskites remains one of the most challenging tasks in condensed matter physics. To reveal the reasons behind the doping-driven instability of the cycloidal antiferromagnetic order in the polar phase of Bi_1_−_xCa_xFeO_3_−_x_/_2, synthesis and investigation of the crystal structure, microstructure, local ferroelectric and magnetic properties of the ceramic samples of Bi_0_._9Ca_0_._1Fe_1_−_xTi_xO_3_−_δ (x  =  0.05, 0.1, 0.15) have been carried out. The compounds possess a rhombohedral structure (space group R3c). The compositional dependence of unit cell volume in this series can be interpreted as suggesting the doping-induced elimination of anion vacancies at x  ⩽  0.1 and the formation of cation vacancies at x  >  0.1. The filling of oxygen vacancies suppresses a weak ferromagnetic contribution characteristic of the parent Bi_0_._9Ca_0_._1FeO_2_._9_5. The appearance of cation vacancies restores the weak ferromagnetic phase. The key role of lattice defects in the magnetic behavior of Ca-doped BiFeO_3 has been confirmed by the observation of a correlation between the magnetic properties and the morphology/ferroelectric domain structure of the Bi_0_._9Ca_0_._1Fe_1_−_xTi_xO_3_−_δ ceramics. (paper)

[en] Investigation of the crystal structure, microstructure, local ferroelectric, and magnetic properties of the Bi_0_._9La_0_._1Fe_1_−_yTi_yO_3 (0  ⩽  y  ⩽  0.1) perovskites has been carried out at room temperature to shed light on the conditions that favor the appearance of spontaneous magnetization in the polar phase of BiFeO_3-based multiferroics. The compounds have been shown to possess the noncentrosymmetric rhombohedral structure (space group R3c) specific to the antiferromagnetic (cycloidal) phase of the parent Bi_0_._9La_0_._1FeO_3. A gradual suppression of the rhombohedral distortions and a decrease of the ferroelectric polarization have been found in the series with increasing Ti content. The substitution dramatically affects the morphology of the ceramic samples: a drastic (from ∼10 μm for y = 0 to ∼1 μm for 0.02  ⩽  y  ⩽  0.1) decrease of the average grain size has been revealed. The decrease is accompanied by the formation of a nanodimensional ferroelectric domain structure. The origin of the morphological changes has been explained by the charge-compensating mechanism that involves the formation of lattice defects in the donor-doped materials. It has been proposed that the same mechanism can be responsible for the substitution-induced removal of the cycloidal modulation resulting in the establishment of a weak ferromagnetic state in the Bi_0_._9La_0_._1Fe_1_−_yTi_yO_3 series. (paper)

[en] Solid-state synthesis and investigation of crystal structure and magnetic properties of Bi_0.86La_0.14-xSm_xFeO₃ (0 ≤ x ≤ 0.14) ceramics were performed. It was found that a rhombohedral to orthorhombic phase transition took place in the series with decreasing average ionic radius of the substituting elements occupying the A-site of the ABO₃ perovskite. Magnetic properties of the compounds were shown to correlate with evolution of their structural state. Pure rhombohedral samples 0 ≤ x ≤ 0.06 were obtained in a mixed antiferromagnetic/weak ferromagnetic state. A small residual magnetization characteristic of the compounds was found to weakly depend on change of the chemical composition. Progressive increase of the residual magnetization was observed upon the rhombohedral-to-orthorhombic transition. Reasons for the appearance of the weak ferromagnetism in orthorhombic and rhombohedral phases of the Bi_0.86(La, Sm)_0.14FeO₃ compounds were analysed.

[en] The microstructure, crystal structure, and magnetotransport properties of microsized and nanosized Badoped manganites have been investigated. A 'two-step' reduction-reoxidation procedure has been used to obtain nanosized ceramic manganite Nd_0.70Ba_0.30MnO₃ (II). The parent microsized manganite Nd_0.70Ba_0.30MnO₃ (I) was prepared by usual ceramic technology in air. Then the sample was annealed in vacuum. The grain size of the reduced sample, determined by scanning electron microscopy, decreased from ∼5 μm down to ∼100 nm. To obtain the oxygen stoichiometry nanosized sample, the Nd_0.70Ba_0.30MnO_2.60 was again annealed in air. It is established that the (I) sample is a pseudocubic perovskite, whereas (II) is tetrahedral as a consequence of Nd³⁺ and Ba²⁺ ions as well as the ordering of oxygen vacancies. The (I) sample is a ferromagnet with T_C ∼ 140 K. It has metal-insulator transition at T_MI ∼ 135 K and a peak of magnetoresistance ∼50% in a field of 9 kOe. For the (II) sample, the critical points of phase transitions move to higher temperatures, T_C ∼ 320 K and T_MI ∼ 310 K. The magnetoresistance of the (II) sample at room temperature (T ∼ 293 K) is about 7% in a field of 9 kOe. The magnetotransport properties are interpreted in the framework of the nanosized effect

[en] The crystalline and magnetic structures and magnetic properties of La_1-xBi_xMnO_3+δ (0.4 ≤ x ≤ 0.6, 0 ≤ δ ≤ 0.06) manganites have been studied. The solid solutions having the stoichiometric oxygen content are shown to be orbitally ordered A-type antiferromagnets. An increase in the oxygen content above the stoichiometric value is found to cause Mn⁴⁺ ions in the perovskite lattice, to remove the cooperative Jahn-Teller distortions, and to form a long-range ferromagnetic order. This order becomes broken as the concentration of the tetravalent manganese ions increases further. The tendency toward breaking the ferromagnetic order increases with the bismuth content. The magnetic properties are interpreted in terms of superexchange interactions on the assumption of local lattice distortions induced by anisotropy of the 6s²(Bi³⁺)-2p⁶(O^2-) chemical bonds

[en] Crystal structure, microstructure, local ferroelectric, and magnetic properties of the Bi_0.9Nd_0.1Fe_1−xTi_xO₃ samples have been investigated at room temperature to reveal the effect of Ti⁴⁺ doping on the multiferroic behavior of the lanthanide-modified compound representing a polar (space group R3c) antiferromagnetic phase of the Bi_1−xLn_xFeO₃ perovskites. Ti doping results in a gradual suppression of the rhombohedral distortions, however, symmetry of the crystal structure remains the same in the entire concentration range allowing the single-phase perovskites to be obtained (x ≤ 0.08). The doping tends to reduce existing lone-pair cation-driven polar displacements, thus giving rise to a decrease of the ferroelectric polarization in the Bi_0.9Nd_0.1Fe_1−xTi_xO₃ (x→0.08) series. A drastic (from ∼10 μm for x = 0 to ∼1 μm for 0.02 ≤ x ≤ 0.08) decrease of the average grain size induced by the doping is accompanied by the formation of a ferroelectric domain structure with the average domain width of ∼40 nm. Finally, the doping was shown to induce an antiferromagnetic to weak ferromagnetic transformation. The maximum remanent magnetization observed in the Bi_0.9Nd_0.1Fe_1−xTi_xO₃ series at x ∼ 0.05 coincides with the locked magnetization releasing upon the magnetic field-induced suppression of the magnetic cycloid in pure BiFeO₃

[en] Investigations of crystal structure and magnetic properties of Bi_0.8(Gd_1-xBa_x)_0.2FeO₃ (x = 0, 0.5, 1) samples have been performed. The Bi_0.8Gd_0.2FeO₃ and Bi_0.8Ba_0.2FeO₃ 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 Bi_0.8(Gd_1-xBa_x)_0.2FeO₃ system: the crystal structure of the Bi_0.8Gd_0.1Ba_0.1FeO₃ sample is characterized by a coexistence of Pnma and R3c structural phases which differ in their chemical compositions. All of the Bi_0.8(Gd_1-xBa_x)_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.

[en] Bi_1-xA_xFeO₃ 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

[en] Mn substitution-driven structural and magnetic phase evolution in the predominantly antiferromagnetic polar rhombohedral (space group R3c) and weak ferromagnetic antipolar orthorhombic (space group Pnam) phases of the Bi_1-xPr_xFeO₃ perovskites was studied by x-ray diffraction, ⁵⁷Fe Mössbauer spectroscopy and magnetometry techniques at room temperature. Mn doping was found to effectively modify both the initial structures to stabilize a new phase demonstrating incommensurable modulation. Magnetic behaviour of the Mn-containing samples changes in correlation with the evolution of their structural state. Within the compositional range of the rhombohedrally distorted compounds, the manganese substitution gives rise to suppression of the dominant antiferromagnetic interaction. In the weak ferromagnetic doping-induced phase, a gradual decrease in room temperature spontaneous magnetization takes place with increasing Mn content. (paper)