Structural phase transitions in the Bi1−x La x FeO3 system

The crystal structure and phase composition of the Bi_1?x La _x FeO₃ system have been studied by the methods of neutron and x-ray diffractions and electron microscopy. It has been revealed that a unit cell of the solid solutions at room temperature is described by the R3c, Imma, and Pnma space groups in the concentration ranges 0 < x < 0.19, 0.4 < x < 0.5, and x > 0.5, respectively. For 0.2 < x < 0.4, the solid solutions are not formed.     

Local states of iron ions in multiferroics Bi1−x La x FeO3

Mössbauer studies of perovskites Bi_1?x La _x FeO₃ (x = 0, 0.10, 0.20, 0.61, 0.90, 1.00) were conducted at 295 and 87 K. The spatial spin-modulated structure (SSMS) observed in perovskites BiFeO₃ and Bi_0.9La_0.1FeO₃ leads to a specific distribution of hyperfine fields P(B) with two peaks. Substitution of La for Bi (x = 0.2) destructs the SSMS. The concentration dependences of the hyperfine field (B), isomer shift (?) and quadrupole shift (δ) were measured. The iron ions are in the trivalent state. The local magnetic moments μ(Fe) of the Fe³⁺ ions are determined.     

Enhancement in multiferroic properties of Bi0.7−xLaxDy0.3FeO3 system with removal of La

La doping at Bi Site in multiferroic BiFeO₃ (BFO) is known to stabilize the ferroelectric perovskite phase and reduce the leakage current. It has been observed that in Dy modified BFO sample, La is not required for the said reason. In fact, the removal of La from Dy modified BFO system leads to the reduction in leakage current. Moreover, the magnetic properties are enhanced by order of magnitude in the absence of La in the system. Remarkably, a similar trend in properties has been also observed in thin films grown on Pt/TiO₂/SiO₂/Si substrate by using the pulsed laser deposition technique. This significant variation in the properties after removal of La from the system could be attributed to the change in lattice parameters, bond lengths and Fe–O–Fe bond angle as determined by powder x-ray and neutron diffraction study.     

Dielectric and piezoelectric properties of (Bi1−x−yNdxNa1−y)0.5BayTiO3 lead-free ceramics

New lead-free (Bi_1−x−yNd_xNa_1−y)_0.5Ba_yTiO₃ ceramics were prepared by a conventional ceramic technique and their dielectric and piezoelectric properties were studied. X-ray diffraction studies reveal that Nd³⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) of rhombohedral and tetragonal phases is formed at 0.04 < y < 0.10. The partial substitutions of Nd³⁺ and Ba²⁺ decrease effectively the coercive field E_c and increase significantly the remanent polarization P_r. Because of lower E_c, larger P_r and the formation of the MPB, the piezoelectric properties of the ceramics are significantly enhanced at x/y = 0.02/0.06: d₃₃ = 150 pC/N and k_p = 30.5%. The ceramics exhibit relaxor characteristic, which is probably resulted from the cation disordering in the 12-fold coordination sites. The depolarization temperature T_d shows a strong compositional dependence and reaches a minimum value at the MPB. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both the polar and non-polar regions near the depolarization temperature T_d, which cause the polarization hysteresis loop become deformed near/above T_d.     

Magnetic and transport properties of electron-doped Ca3−x Bi x Mn2O7

Ca_3?x Bi _x Mn₂O₇ with the nominal composition x=0.05, 0.1, 0.2 and 0.3 is synthesized by solid-state reaction. The refined X-ray diffraction pattern of Ca_2.807Bi_0.193Mn₂O₇ with the nominal Bi³⁺ content x=0.2 indicates that about 71 % of the Bi³⁺ ion enters into the Ca²⁺ (2a) site and the remaining 29 % is in the Ca²⁺ (4e) site. The doped Bi³⁺ ion produces a ferromagnetic component in the antiferromagnetic matrix. Below the transition temperature, at about 110 K, the ferromagnetic and antiferromagnetic interactions coexist. The alignment of the magnetic moment is canted at 5 K. The electric transport shows insulating behavior. Around the magnetic transition, at about 110 K, the resistance sharply drops like a well. A model proposed by Glazman and Matveev (GM model) is applied to the thermal variation of the resistance from 40 K to 138 K. Above this temperature, it is due to thermally activated hopping of small polarons with the activation energy of 50 meV. A negative magnetoresistance, 17 %, is observed with the doping content as low as 0.05. The magnetoresistance is due to the spin-polarized inelastic tunneling through nonmagnetic localized states embedded in an insulating barrier.     

Magnetic and electrical properties of weakly doped manganese-deficient La1 − x Ca x Mn1 − z O3 manganites

Samples of La_{1 ? x} Ca _x Mn_{1 ? z} O_{3 + δ} (x = 0.05?0.15) with deficient manganese and excess oxygen δ do not pass into a metallic state and have low spin ordering temperatures T _C at acceptor Mn⁴⁺ concentrations near the percolation threshold. These results are explained by carrier localization in clusters near cation vacancies. A break in the carrier transport chain Mn-O-Mn in the form of absent manganese favors cluster formation and decreases the double exchange energy and T _C of the samples. Closeness to the percolation threshold results in strong (more than four orders of magnitude) changes in the electrical resistivity in a magnetic field. The changes in the cluster sizes with the temperature and the magnetic field that are determined from the magnetotransport properties are satisfactorily described in the model of phase separation into small-radius metallic droplets in a dielectric paramagnetic and an antiferromagnetic matrices.     

Temperature evolution of the crystal structure of Bi1 − x Pr x FeO3 solid solutions

The crystal structure of solid solutions in the Bi_{1 ? x} Pr _x FeO₃ system near the structural transition between the rhombohedral and orthorhombic phases (0.125 ≤ x ≤ 0.15) has been studied. The structural phase transitions induced by changes in the concentration of praseodymium ions and in the temperature have been investigated using X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. It has been established that the sequence of phase transformations in the crystal structure of Bi_{1 ? x} Pr _x FeO₃ solid solutions with variations in the temperature differs significantly from the evolution of the crystal structure of the BiFeO₃ compounds with the substitution of other rare-earth elements for bismuth ions. The regions of the existence of the single-phase structural state and regions of the coexistence of the structural phases have been determined in the investigation of the crystal structure of the Bi_{1 ? x} Pr _x FeO₃ solid solutions. A three-phase structural state has been revealed for the solid solution with x = 0.125 at temperatures near 400°C. The specific features of the structural phase transitions of the compounds in the vicinity of the morphotropic phase boundary have been determined by analyzing the obtained results. It has been found that the solid solutions based on bismuth ferrite demonstrate a significant improvement in their physical properties.     

The crystal and grain structure and physical properties of Bi1 − x A x FeO3 (A = La, Nd) solid solutions

High-quality ceramics were synthesized on the basis of Bi_{1 ? x} A _x FeO₃ (A = La, Nd; 0.00 ≤ x ≤ 0.20) solid solutions. Their crystal and grain structure, Mössbauer spectra, and other dielectric and magnetic characteristics were studied. It was shown that an increase in the content of A elements in the studied samples considerably enhanced their magnetic susceptibility and magnetoelectric effect.     

Evolution of the optical properties of single-crystal La1−x SrxMnO3

An ellipsometric method is used to study the dispersion of the real ɛ ₁(ω) and imaginary ɛ ₂(ω) parts of the complex dielectric permittivity of single-crystal La_1−x Sr_xMnO₃ (x=0.1, 0.2, and 0.3) for energies from 100 meV to 5 eV at room temperature. It is found that, when lanthanum is replaced by strontium, the optical spectrum changes fundamentally. A shift in the main features of the spectrum of initial LaMnO₃ at 1.9 and 4.7 eV to lower energies takes place, as well as a partial redistribution of the optical-conductivity spectral weight in the band gap region E<1.7 eV. For compositions with x=0.2 and 0.3, a fine structure of the interband absorption is observed against a background of non-Drude optical conductivity at low energies. Fiz. Tverd. Tela (St. Petersburg) 41, 1445–1449 (August 1999)     

Heat capacity and dielectric properties of multiferroics Bi1 − x Gd x FeO3 (x = 0–0.20)

The heat capacity and the permittivity of multiferroics Bi_{1 ? x} Gd _x FeO₃ (x = 0, 0.05, 0.10, 0.15, 0.20) have been studied in the temperature range 130–800 K. It has been found that insignificant substitution of gadolinium for bismuth markedly shifts the temperature of antiferromagnetic phase transition and increases the heat capacity over a wide temperature range. It has been shown that the temperature dependence of the excess heat capacity is due to the manifestation of three-level states. Additional anomalies characteristic of the phase transitions have been revealed in the temperature dependences of the heat capacity for the compositions with x = 0.1 and 0.15 at T ≈ 680 K and T ≈ 430 K, respectively. The results of studies of the heat capacity have been discussed simultaneously with the data of structural studies.     

Specific heat of EuxSr1−xO near the ferromagnetic phase transition

High-resolution measurements of the specific heat,C, near the ferromagnetic phase transition of the diluted ferromagnetic system Eu_xSr_1–xO are reported. Samples with four different concentrations (x=1,x=0.9,x=0.7 andx=0.5) have been studied and the expected phase diagram, i.e. the linear decrease ofT _c with decreasingx, is confirmed. Our specific-heat data of the pure EuO samples yield a critical exponent =–0.12±0.02 in contrast to literature results, but in agreement with the value expected theoretically for this three-dimensional Heisenberg system. The origin of the discrepancies is traced down to differences in data analysis.     

Magnetic properties of Fe−Mn−Al alloy system in the FCC disordered phase

In this work we report the experimental studies of Fe?Mn?Al alloys in the FCC disordered phase at room temperature by Mössbauer spectroscopy and X-ray diffraction. In this phase the alloys are antiferromagnetic with a constant mean hyperfine field ( \(\bar H\) ) near 26 kOe in the composition range from 0 to 7.5 at.% Al and 50 to 65 at.% Fe. When the Al or Fe concentration increases, the \(\bar H\) value gradually decreases to zero and the alloy becomes paramagnetic. In the same way when the Al concentration increases the lattice parameter increases linearly but when the Fe concentration increases the lattice parameter remains nearly constant for alloys with 5 at.% Al and decreases for alloys with 10 at.% Al.     

Magnetic and electric properties of Yb x Mn1 − x S alloys

Results from investigating the structural, magnetic, and electrical properties of Yb _x Mn_{1 ? x} S alloys (0 ≤ x ≤ 0.2) synthesized on the basis of manganese monosulfide are presented. Substituting manganese for ytterbium increases the concentration of charge carriers and lowers the activation energy. The observed anomalies in the temperature dependence of resistivity are explained by an impurity semiconductor model with donor 4f levels.     

Magnetic phase transition in ɛ-In x Fe2 − x O3 nanowires

This paper reports on a study of magnetic properties of ordered arrays of ?-In _x Fe_{2 ? x} O₃ (x = 0.24) nanowires possessing a high room-temperature coercive force of 6 kOe. Lowering the temperature below 190 K brings about a sharp decrease of the coercive force and magnetization of nanowires driven by the magnetic phase transition from the ferrimagnetic into antiferromagnetic phase. The transition is accompanied by a decrease of the magnetic anisotropy constant, which accounts for the anomalous frequency dependence of the position of the maximum in the temperature dependence of dynamic magnetic susceptibility. In the low-temperature phase, a spin-flop transition in the magnetic field of 28 kOe has been observed at T = 2 K. Lines related to the high-temperature hard-magnetic and low-temperature phases have been identified in electron spin resonance spectra of the nanowires. A line lying near zero magnetic field and evolving from the nonresonant signal related to the microwave magnetoresistance of the sample has also been detected.     

A Magnetic resonance study of La1−x SrxMnO3 manganites

Single crystals of La_0.9Sr_0.1MnO₃ and La_0.8Sr_0.2MnO₃ manganites are examined using magnetic resonance in the temperature range 80–370 K. It is found that magnetostatic oscillations arise near the Curie temperature. The possible reasons for the appearance of additional lines in the ferromagnetic resonance (FMR) spectrum are considered, and the anisotropy field and the type of crystalline magnetic anisotropy in the La_0.8Sr_0.2MnO₃ compound are determined. It is shown that the crystalline magnetic anisotropy in the La_0.9Sr_0.1MnO₃ compound exhibits specific features associated with its type of crystal structure.