Structural,magnetic, and electrical properties of Gd-doped BiFeO<Subscript>3</Subscript> nanoparticles with reduced particle size

Pure and Gd-doped BiFeO₃ nanoparticles have been synthesized by sol–gel method. The significant effects of size and Gd-doping on structural, electrical, and magnetic properties have been investigated. X-ray diffraction study reveals that the pure BiFeO₃ nanoparticles possess rhombohedral structure, but with 10% Gd-doping complete structural transformation from rhombohedral to orthorhombic has been observed. The particle size of pure and Gd-doped BiFeO₃ nanoparticles, calculated using Transmission electron microscopy, has been found to be in the range 25–15 nm. Pure and Gd-doped BiFeO₃ nanoparticles show ferromagnetic character, and the magnetization increases with decrease in particle size and increase in doping concentration. Scanning electron microscopy study reveals that grain size decreases with increase in Gd concentration. Well-saturated polarization versus electric field loop is observed for the doped samples. Leakage current density decreases by four orders by doping Gd in BiFeO₃. The incorporation of Gd in BiFeO₃ enhances spin as well as electric polarization at room temperature. The possible origin of enhancement in these properties has been explained on the basis of dopant and its concentration, phase purity, small particle, and grain size.     

氧含量对BiFeOδ多晶陶瓷介电特性的影响

采用固相反应法制备了不同含氧量的BiFeO_δ多晶陶瓷样品，利用HP4294A阻抗分析仪测量了样品的介电特性随频率和氧含量的变化，用正电子湮没寿命谱学的方法研究了样品中因氧含量的变化所引起的结构缺陷. 实验结果表明：引入氧空位和氧填隙离子缺陷都会使介电常数减小，而介电损耗则随氧含量的增加而增加，二者的变化范围均在10%—35%之间；对不同氧含量的BiFeO_δ样品，介电常数和介电损耗随测量频率的增加而减小. 氧空位的引入使得局域电子密度变小，正电子平均寿命τ_m增加. 在氧含量δ=2.99时电子密度最大(n_e=3.90×10²³/cm³)，继续增加氧含量对正电子寿命与局域电子密度的影响不大. BiFeO_δ样品的介电常数和介电损耗随氧含量的变化可以在空间电荷限制电导的框架下来理解.     

Nucleation and stability of defect clusters: New energetic model in the case of substituted wustites

Abstract

For wüstite Fe_1?z O (z < 0.08) an energetic model accounts for the stability of cubic defect clusters (m/n) which are partly ordered in the crystal. The Gibbs energy G_T (N) associated with clusters, including their distorted envelope, is expressed as a sum of a volume term in N ³ and of surface terms in N ⁴; N is the number of bonds characteristic of the cluster size. In the case of a (10/4) type cluster, this energy is negative and minimum for N_m ranging between 4 and 5, when the volume and surface energies range between specific values. Using simple assumptions, a volume energy ?0.80 eV per vacancy is found in accordance with the value of stabilization energy calculated by theorists for the (10/4) cluster. The substitution of Fe²⁺ by Ca²⁺ should lead to a decrease of cluster size; this has been recently suggested by neutron diffraction studies.     

Raman study of BiFeO3 with different excitation wavelengths

Raman spectra of bismuth ferrite (BiFeO₃) over the frequency range of 100-1500 cm⁻¹ have been systematically investigated with different excitation wavelengths. The intensities of the two-phonon modes are enhanced obviously under the excitation of 532 nm wavelength. This is attributed to the resonant behavior when incident laser energy closes to the intrinsic bandgap of BiFeO₃. The Raman spectra of BiFeO₃ excited at 532 nm were measured over the temperature range from 77 to 678 K. Besides the abnormal changes of the peak position and the linewidth of the A₁ mode at 139 cm⁻¹, the prominent frequency shift, the line broadening and the decrease of the intensity for the two-phonon mode at 1250 cm⁻¹ were observed as the temperature increased to Néel temperature (T_N). All these results indicate the existence of strong spin-phonon coupling in BiFeO₃.     

Enhanced ferromagnetic transition temperature in nanocrystalline lanthanum calcium manganese oxide (La0.67Ca0.33MnO3)

We report a novel, low temperature (450-600 °C) route for the synthesis of highly crystalline and homogeneous nanoparticles of lanthanum calcium manganese oxide La_0.67Ca_0.33MnO₃ (LCMO). The nanocrystallites, with average particle size of 30 nm, possess a ferromagnetic-paramagnetic transition temperature (T_c) of 300 K, which is about 50 K higher than that of a bulk single crystal. The transition temperature was found to be inversely proportional to the particle size. The Rietveld analysis of the powder X-ray diffraction data of the phase-pure nanopowders reveals that the particle size reduction leads to a significant contraction of the unit cell volume and a reduction of the unit cell anisotropy. We propose that these two lattice effects are responsible for the observed enhancement in T_c.     

<Superscript>57</Superscript>Fe NMR study of multiferroic BiFeO<Subscript>3</Subscript>

The effects of the ⁵⁷Fe isotope content and high-frequency magnetic field amplitude h ₁ on the shape of the NMR spectrum of multiferroic BiFeO₃ at T = 4.2 K are studied by pulsed nuclear magnetic resonance. The NMR spectrum shape and transverse relaxation time T ₂ are found to depend strongly on the ⁵⁷Fe isotope content and h ₁ in multiferroic BiFeO₃ in the presence of a spatial spin-modulated structure of a cycloid type. In a sample with a high ⁵⁷Fe isotope content, the Suhl-Nakamura interaction contributes substantially to T ₂. When these dynamic effects are taken into account for analysis of the NMR spectrum shape, an undisturbed (without an anharmonicity effect) spatial spin-modulated structure of a cycloid type is shown to exist in BiFeO₃.     

Structural,optical, and multiferroic properties of single phased BiFeO3

A soft chemical coprecipitation method has been proposed for synthesis of nano-sized multiferroic BiFeO₃ (BFO) powders. The X-ray diffraction pattern confirms the perovskite structure of BFO and Rietveld refinement reveals the existence of rhombohedral R3c symmetry. Crystallite size and strain value are studied from Williamson–Hall (W–H) analysis. The transmission electron microscope (TEM) image shows that the particle size of BFO powders lies between 50–100 nm. 4A₁ and 7E Raman modes have been observed in the range 100–650 cm^?1 and a prominent band centered around 1150–1450 cm^?1 have also been observed corresponding to the two-phonon scattering. Differential Thermal Analysis (DTA) shows the existence of two prominent peaks at 330 ^°C and 837 ^°C corresponding to the magnetic and ferroelectric ordering, respectively. From the temperature dependent dielectric studies, an anomaly in the dielectric constant is observed at the vicinity of Neel temperature (T _N ) indicating a magnetic ordering. Also, BFO shows antiferromagnetic behavior measured from the magnetic studies.     

Studying local structural, valence, and magnetic states of iron ions in Bi0.9Ca0.1FeO3 perovskite

Mössbauer method was used to study a perovskite compound Bi_0.9Ca_0.1FeO₃ at T = 295 K and at temperature above T _N . It has been established that Bi_0.9Ca_0.1FeO₃ has a rhombohedral crystal structure similar to that of BiFeO₃. The substitution of Ca²⁺ for Bi³⁺ ions leads to the formation of three states of Fe³⁺ ions with an octahedral surroundings and one state with a tetrahedral oxygen surroundings with substantially different hyperfine magnetic fields. All Fe ions are in a trivalent state; the compensation of the charge deficit occurs via the formation of oxygen vacancies. Above T _N , two structurally nonequivalent states of Fe³⁺ ions exist in the Bi_0.9Ca_0.1FeO₃ sample, which correspond to the Fe³⁺ ions with an octahedral and tetrahedral oxygen coordination.     

Pressure dependence of the superconducting state parameters of metallic glass superconductor Mg70Zn30

Explicit expressions have been derived for the volume dependence of electron-phonon coupling strength (λ) and the Coulomb pseudopotential (μ^*) considering the variation of Fermi momentum (κ _F) and Debye temperature (θ _D) with volume. Ashcroft’s model pseudopotential and RPA form of dielectric screening have been used for obtaining pressure dependence of transition temperature (T _C) and the logarithmic volume derivative (Φ) of the effective interaction strength (N ₀ V) for metallic glass superconductor Mg₇₀Zn₃₀. It has been observed that T _C of the metallic glass Mg₇₀Zn₃₀ decreases rapidly with increase of pressure and the superconducting phase disappears at about 30% decrease of volume, for which the μ^* curve shows a minimum and an elbow is formed in the Φ graph.     

Magnetic,ferroelectric and magnetoelectric properties of Ba-doped BiFeO3

Bi_1–xBa_xFeO₃ (0.0≤x≤0.25) ceramics are prepared by chemical synthesis route. At room temperature, antiferromagnetic BiFeO₃ is converted to ferromagnetic on doping Ba. A large change in the magnetization is observed around 370 °C which is close to the Neel temperature (T_N) of parent compound. Another magnetic transition is also observed near 600 °C. Spin canting or impurity phase could be a probable reason for the origin of ferromagnetism in both cases. Ferroelectric and magnetic transitions of the compounds shift towards higher temperature with Ba-doping concentration. Anomaly in the dielectric constant is also observed near the T_N of BiFeO₃. The composition x=0.15 shows the maximum magnetic moment at room temperature while better fatigue resistance and maximum magnetoelectric coupling are observed for x=0.20 composition.     

Anomalies of the phonon properties in multiferroic BiFeO3 thin films near the magnetic phase transition temperature

Size, substrate, doping and magnetic field effects on the phonon properties in multiferroic BiFeO₃ thin films are studied based on a microscopic model. We obtain an anomaly near the magnetic phase transition temperature TN which can be attributed to the magnetoelectric nature of BiFeO₃ and strong anharmonic spin-phonon interaction. It is shown that due to crystal lattice distortion for dopants with ionic radius smaller than that of the host ions the phonon energy decreases (for example Tb or Ti), whereas for the opposite case (larger radius of the doping ions, for example Co or Ni) it increases. The phonon damping is always enhanced compared to the undoped thin film.     

Molecular dynamics investigation of the size effect upon the β → α transformation in Zr nanocrystals

The stability of the β phase in cubic zirconium nanoparticles has been calculated as a function of the size r (r varies in the range from 2.5 to 11.5 nm) by the molecular dynamics method with the many-body interatomic interaction potential obtained within the embedded-atom model. It has been demonstrated that the temperature T _k at which the cubic cluster of body-centered cubic zirconium becomes structurally unstable depends nonlinearly on the particle size. The curve T _k (r) exhibits a pronounced maximum in the range r ≈ 4.3−4.7 nm. It has been established that the mechanism of the structural transition from the body-centered cubic phase to the hexagonal close-packed phase depends substantially on the particle size. For particles with sizes in the range from 2.5 to 5.0 nm, there exists a temperature range in which the transition from the body-centered cubic phase to the hexagonal close-packed phase remains incomplete for a long time. In this case, two phases coexist and the initial particle undergoes a strong deformation along the habit plane.     

Optical study of the antiferromagnetic-paramagnetic phase transition in chromium oxide Cr2O3

Abstract

Optical effects of the first and second order with respect to the order parameter (1 is antiferromagnetic vector) have been studied in Cr₂O₃ around its phase transition at T_N = 306 K from the antiferromagnetic to paramagnetic state. The magnetic linear birefringence is characterized by a rather large magnitude Δn _sp ? 10^?3 and by a large contribution of fluctuations of the order parameter to the birefringence. The study of the nonreciprocal optical rotation induced by an electric field has shown that the phase transition has a well defined first-order character. This result is also supported by the observation of a nonlinear (quadratic in the electric field) nonreciprocal rotation in a narrow temperature region ΔT = 0.15 K around T_N . The temperature variation of the order parameter l(T) below T_N is well described by a power low l where τ^β = (T_N - T)/T_N and β = 0.355. We also observed a very reproducible effect of the rotation of the optical indicatrix in opposite directions for two types of antiferromagnetic domains. The possible explanation of this effect could be related to the so-called gyrotropic birefringence, an effect related to k_il_i terms in the dielectric permeability.     

Error Estimates in the Sampling From Particle Size Distributions

It is often necessary to estimate the properties of particle size distributions from limited samples taken from large populations. When the distributions are broad, and higher order moments required, as in the case of volume based particle size distributions, the inferred parameters d_3,50 (volume median diameter) and GSD (geometric standard deviation) can have high intrinsic errors not immediately obvious to the measuring scientist. We show that there is a critical number of particles, N_crit, which must be counted or else the error may blow up catastrophically. N_crit is very sensitive to the width of the distribution, and is approximately proportional to GSD¹¹ We develop formulae to estimate the random sampling error inherent in measured values of the d_3,50 and GSD for the log-normal distribution; compare the predictions to a typical experimental particle size measurement; and then generalize to the median of any arbitrary moment, d_{r, 50}.     

Enhanced temperature-independent magnetoresistance below the metal-insulator transition temperature of epitaxial La0.2Nd0.4Ca0.4MnO3 thin films

Epitaxial La_0.2Nd_0.4Ca_0.4MnO₃ thin films have been deposited at 800°C on LaAlO₃ substrate using pulsed laser deposition technique. The structural and magnetotransport properties of the films have been studied. The sharp peak in the temperature dependence of the resistance corresponding to metal-to-insulator transition (T _p) has been observed at a temperature of T _p=82 K, 97 K and 110 K for 0 Oe, 20 kOe and 40 kOe magnetic fields, respectively. The film exhibits a large nearly temperature-independent magnetoresistance around 99% in the temperature regime below T _p. The zero field-cooled (ZFC) and field-cooled (FC) magnetization data at 50 Oe shows irreversibility between the ZFC and FC close to the ferromagnetic transition temperature T _c=250 K. The ZFC temperature data of the film displays ferromagnetic behavior for higher temperature regime T _c=250 K>T>T _p=82 K, and a decrease in magnetization with decreasing temperature up to 5 K below 82 K exhibiting a sort of antiferromagnetic behavior in the low temperature regime (T<82 K=T _p=T _N).     

A pseudopotential approach to the superconducting state parameters of (Ni33Zr67)1? x M x (M=Ti, V, Co, Cu) ternary metallic glasses

Superconductivity in ternary metallic glasses has been investigated using the model pseudopotential approach, which has been found quite successful in explaining superconductivity in metals, binary alloys and binary glasses. It is observed that this simple methodology successfully explains superconducting behaviour of ternary glasses without requiring the solution of Dirac equation for a many body problem or estimation of various interactions as required in ab-initio pseudopotential theory. In the present work superconducting state parameters of fourteen metallic glasses of (Ni-Zr)-M system (M=Ti, V, Co, Cu) have been determined in the BCS-Eliashberg-McMillan framework. It is observed that addition of V, Co, and Cu as the third element (M) to a binary metallic glass (Ni₃₃ Zr₆₇) causes the parameters λ,T _c, α, andN ₀ V to decrease, and Coulomb pseudopotential (μ*) to increase with concentration of M, showing that the presence of third element (M) causes suppression of superconducting behaviour of the alloy. The decrease inT _c with increasing concentration of third element (M) may be attributed to the modifications in density of states at the Fermi levelN(E _F), and probable changes in the band structure of the alloy due to addition of the third element (M). Slight difference is noticed when Ti is added to the Ni₃₃ Zr₆₇ alloy. In this caseT _c rises initially and then decreases with concentration of M, showing a peak at aboutx=0.05. This indicates that on addition of Ti, 3d states grow near the Fermi level and hence contribute substantially toN(E _F), favouring superconducting behaviour in this case. The present results forT _c show an excellent agreement with the experimental data. QuadraticT _c equations have been proposed, which provide successfully theT _c values of ternary metallic glasses under consideration. Paper presented at National Conference on Current Trends in Condensed Matter Research, Warangal, India, September 20–22, 2004.     

DC Townsend Discharge in Nitrogen: Temperature‐Dependent Phenomena

Low‐current Townsend discharge in nitrogen has been studied in the temperature range of T = 100–300 K in a semiconductor‐gas‐discharge structure. It was found that the sustaining voltage U_S increases with time when a current is passed through the structure at low T. This effect was not observed at room temperature. A hypothesis is put forward that a film of a neutral phase of nitrogen is formed on the electrodes under cryogenic discharge conditions. The presence of the condensed thin‐film phase leads to a decrease in the secondary electron emission from the electrode and to a corresponding increase in U_S. A possible mechanism of the phenomenon is associated with the formation of large neutral aggregates in the form of [N⁺₂(N₂)_n]^– in the gas discharge volume. The condensation of these aggregates seems to yield a phase that is comparatively stable at cryogenic temperatures (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of ball-milling on T c values of bi-based bulk superconductors

Superconducting powders were synthesised by solid-state reaction. They were ball-milled, and pressed into pellets. It was found that particle size prior to sintering has a substantial influence on the properties of Bi-based bulk superconductors, such as reactivity, phase homogeneity and T _c values. Powders of fine particle size were found to show low T _c values.     

Structure and phase composition of BiFeO3 : La films synthesized by chemical deposition from solutions

The transmission electron microscopy, energy-dispersive, and X-ray powder diffraction analyses have been used to study the changes in the structure and phase composition of lanthanum-doped BiFeO₃ and undoped BiFeO₃ thin films synthesized by chemical deposition from solutions and annealed in the temperature range 500?C700°C. It has been found that the temperature of the onset of crystallization with formation of the rhombohedral phase of BiFeO₃ is 500°C. As the annealing temperature increases, the phase composition of the film is changed: the Bi_3.43Fe_0.57O₆ and Bi₂Fe₄O₉ phases are formed. The lanthanum doping increases the crystallization temperature to 550°C; in this case, the film remains single-phase to T = 700°C.