Effect of Non-Stoichiometry Oxygen Content on the Magnetization of La<Subscript>1.5</Subscript>Sr<Subscript>0.5</Subscript>NiO<Subscript>4+<Emphasis Type="Italic">δ</Emphasis></Subscript>

We report on magnetization measurements of La_1.5Sr_0.5NiO_4+δ for δ=0.01 and δ=−0.01. Zero field-cooled and field-cooled bulk magnetization measurements were performed parallel to the Ni–O planes of single crystals, and on a polycrystalline δ=0.01 sample. Striking differences in the magnetization curves are observed between the two doping levels that are identifiable in polycrystalline δ=0.01. The bulk magnetization differences indicate a crossover in the magnetic properties of La_2−x Sr _x NiO_4+δ at half doping level, which can be used as a simple non-destructive way to determine whether La_1.5Sr_0.5NiO_4+δ is under or over half doping.     

An analysis of the error in measuring the field of the maximum magnetic permeability of steels

The error δ in measuring the parameters of the field Hμ _m of the maximum magnetic permeability μ _m of steels is estimated. It is established that δ is 5–10 times greater than the error of ±5% in measuring μ _m . It is shown that not all the reference data on μ _m and Hμ _m of steels are reliable. The inadvisability of using Hμ_m in magnetic structural analysis is demonstrated.     

Inter- and Intra-granular Interactions of REBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript>, RE: Eu,Gd, Ho and Er

Here, the superconducting and AC/DC magnetic properties of REBa₂Cu₃O_7−δ (RE: Eu, Gd, Ho and Er) are investigated in order to understand the inter-granular and intra-granular behavior of these compounds. These compounds were prepared by standard solid-state reaction route. ErBa₂Cu₃O_7−δ (Er-123) and HoBa₂Cu₃O_7−δ (Ho-123) (consisting of heavier rare earth) show higher orthorhombic splitting in their XRD patterns than those observed for EuBa₂Cu₃O_7−δ (Eu-123) and GdBa₂Cu₃O_7−δ (Gd-123) (consisting of lighter rare earth). Even though Eu, Gd, Ho and Er are magnetic elements, substitution of these at Y site in Y-123 does not change the superconducting transition temperature (T _c). Er-123 and Ho-123 exhibit very sharp superconducting transition while Eu-123 and Gd-123 exhibit relatively broad transition with little hump at the lower part of transition close to the long-range superconducting state with zero resistance. In addition to that, Eu-123 and Gd-123 show relatively higher normal state resistivity than those of Er-123 and Ho-123. The AC susceptibility measurements reveal that these compounds possess different strength of inter-granular couplings. The hump in the superconducting transition of Eu and Gd is not due to the lack of inter-grain couplings in these samples because of having better grain connectivity in the samples as compared to other rare earth elements in REBa₂Cu₃O_7−δ system. Though intra-grain peak is insensitive to the applied magnetic field, inter-grain peak is sensitive to the applied magnetic field.     

Magnetic and dielectric properties of the M-type barium strontium hexaferrite (Ba x Sr1−x Fe12O19) in the RF and microwave (MW) frequency range

This paper presents results for the dielectric permittivity (ε′), dielectric loss (tg δ _E) dielectric permeability (μ′) and magnetic loss (tg δ _M) in the radio-frequency and microwave frequency range of Ba _x Sr_1−x Fe₁₂O₁₉ hexaferrite (0 ≤ x ≤ 1). The samples were prepared by a new route of the ceramic method. The magnetic permeability (μ′) and magnetic loss (tg δ _M) measurements in the range 100 MHz to 1.5 GHz, reveals that 1.32 ≤ μ′ ≤ 1.68 for the permeability of BFO100 (BaFe₁₂O₁₉) and 1.16 < μ′ ≤ 1.88 for SFO100 (SrFe₁₂O₁₉) in the range of studied frequencies. The BFO100 sample presented lower loss (tg δ _M = 4.10⁻³ at 1.5 GHz). The permittivity of BFO100 and SFO100 in 1.5 GHz are, respectively 8.18 and 8.19, and in the 1 MHz are, respectively 52.04 and 19.09. The samples presented coercive field in the range of 3–5 kOe and remanence magnetization in the range of 33–36 emu/g. The subjects of paper were study the dielectric and magnetic properties of the barium and strontium hexaferrite, in view of applications as a material for permanent magnets, high density magnetic recording and microwave devices.     

Effect of magnetic field on the temperature gradient in Y1Ba2Cu3O7 ? δ + Eu1Ba2Cu3O7 ? δ rods

A method for growing Y₁Ba₂Cu₃O_{7 − δ} + Eu₁Ba₂Cu₃O_{7 − δ} (YBCO-EBCO) high-temperature superconductor rods is described. The temperature dependences of the resistance of rods in a magnetic field of B = 0.6 T and in the absence of the field (B = 0 T) are presented. The current-voltage characteristics of YBCO-EBCO rods have been measured at liquid nitrogen temperature have been measured in the absence and presence of a magnetic field. It is shown that the magnetic field significantly decreases the temperature gradient arising when the rod leaves the superconducting state.     

Resistive and magnetic properties of Y1−x Zr x Ba2Cu3O7−δ and Y1−2x Zr x Eu x Ba2Cu3O7−δ superconductive compounds

The results of X-rays, electrical resistivities and magnetic studies performed on Y_−x Zr _x Ba₂Cu₃O_t7−δ and Y_1−2x Zr _x Eu _x Ba₂Cu₃O_7−δ superconducting compounds, in the temperature range 4.2–500 K and external fields up to 50 kOe are reported. The presence of zirconium decreases the superconducting transition temperatures,T _c. BelowT _c, the hysteresis loops narrow when the zirconium content increases. From the magnetization curves, the critical current densities were determined. A time dependence of the magnetizations of logarithmic form was found. The flux-pinning energy was estimated to beU ≃0.03 eV. Above the transition temperatures, Y_−x Zr _x Ba₂Cu₃O_t7−δ samples show a Pauli-type paramagnetism, while Y_1−2x Zr _x Eu _x Ba₂Cu₃O_7−δ compounds have, in addition, temperature-dependent contributions to the magnetic susceptibilities. The effective europium moments are ∼3.40 μ_B suggesting that the ion is in (+3) valence state.     

Ordered oxygen deficient ‘112’ perovskites,LnBaCo<Subscript>2</Subscript>O<Subscript>5·50+<Emphasis Type="Italic">δ</Emphasis></Subscript>: complex magnetism and transport properties

The ordered oxygen deficient ‘112’ perovskites, LnBaCo₂O_5·50+δ (Ln = lanthanide or Y), exhibit a very flexible structure which can either uptake (δ > 0) or release oxygen (δ < 0) depending on the experimental conditions of synthesis and on the size of the lanthanide. These compounds exhibit remarkably complex magnetic transitions, metal-insulator transition and exceptionally high magnetoresistance. We show herein that their physics is mainly dominated by three different ferromagnetic states, depending on the cobalt valency: FM1 for δ = 0 (Co³⁺), FM2 for δ < 0 (Co²⁺/Co³⁺) and FM3 for δ > 0 (Co³⁺/Co⁴⁺). The competition between ferromagnetism and antiferromagnetism in these phases and the various transitions are discussed taking into consideration the spin state of cobalt, the issue of phase separation and the effect of cobalt coordination and disproportionation. In honour of Prof. C N R Rao on his 75th birthday     

Synthesis and electrical and magnetic properties of LaSr2Mn2 ? yNiyO7 ? δ solid solutions

LaSr₂Mn_{2 − y} Ni _y O_{7 − δ} layered manganite solid solutions with 0 < y ≤ 0.6 have been prepared by solid-state reactions and sol-gel processing. The synthesis procedure is shown to have a significant effect on the electrical and magnetic properties of LaSr₂Mn_{2 − y} Ni _y O_{7 − δ} ceramics. The highest conductivity is offered by the y = 0.2 material, which is due to partial destruction of Mn³⁺/Mn⁴⁺ charge order. Increasing the degree of nickel substitution on the manganese site influences the ferromagnetic properties of LaSr₂Mn_{2 − y} Ni _y O_{7 − δ} and shifts the magnetoresistance peak to lower temperatures.     

Partial thermodynamic functions of aluminum in sodium aluminum vanadium Β-bronzes

The partial thermodynamic functions of Al in the Β-bronzes Na_2-yAl_yV₁₂O₃₀ (0 <y ≤ 0.7), Na₂Al_yV₁₂O₃₀ (0 <y ≤1.4), and Na_1.3Al_yV₁₂O₃₀ (0 <y≤ 2.1) were determined as a function of composition in the temperature range 294–348 K. The δG (Al), δ-H (Al), and δS (Al) of the bronzes were found to vary in a complex manner with Al content, passing through extrema neary = 1 as a result of the structural ordering of Al³⁺ ions. The temperature dependences of δG (Al) show changes in slope between 303 and 318 K, attributable to redistribution of the Al³⁺ ions over the possible crystallographic sites in the Β-bronze structure.     

Effect of non-stoichiometry and Ce-doping on the tetragonal superconducting phase CaBaLaCu3O7−δ

We have synthesized CaBaLaCu₃O_7−δ and related compounds Ca_0.9BaLaCu₃O_7−δ , Ca₁₁ BaLaCu₃O_7−δ and Ca_0.9Ce_0.1 BaLaCu₃O_7−δ in order to investigate the effect of doping and non-stoichiometry at Ca-sites on the superconducting behaviour of tetragonal superconductor CaBaLaCu₃O_7−δ . The superconducting transition temperature (T _c) of Ca_0.9 BaLaCu₃O_7−δ is the same as that of pure CaBaLaCu₃O_7−δ (67 ± 1 K). Calcium excess compound Ca_1.1 BaLaCu₃O_7−δ showed a drop inT _c to 72 K while the cerium-substituted compound Ca_0.9 Ce_0.1 BaLaCu₃ O_7−δ showed a drop inT _c to 43 K. The results are explained on the basis of difference in valency between the host and the dopant ion.     

On δ-record observations: asymptotic rates for the counting process and elements of maximum likelihood estimation

Given a sequence of random variables {X _n ,n≥1} and δ∈ℝ, an observation X _n is a δ-record if X _n >max {X ₁,…,X _n−1}+δ. We obtain, for δ≤0, weak and strong laws of large numbers for the counting process of δ-records among the first n observations from a sequence of independent identically distributed random variables, with common distribution F, possibly discontinuous. We provide examples of our results in the context of common probability distributions. Finally, we show how δ-records can be used for maximum likelihood estimation.     

Effect of A-site vacancies on the magnetoresistive Effect in La1 ? x ? yCaxNayMnO3 ± δ

We have studied the effect of composition and heat-treatment conditions on the structural and transport properties of La_{1 − x − y} Ca _x Na _y MnO_{3 ± δ} solid solutions. The materials obtained offer a magnetoresistance of about 48% in a magnetic field of 1.2 MA/m.     

Properties of Mg(Fe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>O<Subscript>4 + δ</Subscript> solid solutions in stable and metastable states

Using pyrohydrolytic synthesis, we have obtained a continuous series of Mg(Fe_{1 − x} Ga _x )₂O_{4 + δ} solid solutions, as checked by X-ray diffraction. The magnetization and coercivity of the samples have been determined from field dependences in applied magnetic fields of ±5 T, and their conductivity has been assessed using current-voltage measurements at 300 K in electric fields from −200 to +200 V. The effective band gap of the solid solutions has been evaluated from their absorption spectra obtained by diffuse reflectance measurements. The optimal composition for spintronic applications is Mg(Fe_0.8Ga_0.2)₂O_{4 + δ}.     

Thermodynamic principles of the synthesis of CdTe,ZnTe, and CdZnTe solid solutions

Results of thermodynamic investigations of the P-T-X phase equilibria in Cd-Te, Zn-Te and Cd-Zn-Te systems carried out by measuring total vapor pressure in the 1–760 Torr (0.13–101 kPa) range at temperatures up to ∼1370 K are systematized. Concentration boundaries of the existence of intermediate phases with the compositions CdTe_1±δ, ZnTe_1±δ, and Cd_{1 − x} Zn _x Te_1±δ (x = 0.05, 0.10, 0.15) are established. Vapor compositions in crystal-vapor equilibria are determined and isopleths of partial pressures for the CdTe_1±δ and Cd_{1 − x} ZnxTe_1±δ (x = 0.05, 0.10, 0.15) systems are calculated. Being presented in an analytical form, the thermodynamic data provide a basis for targeted synthesis of the indicated compounds.     

Investigation of high temperature superconductivity through microwave absorption method

The field dependence and near zero magnetic field microwave absorption as a function of rf power in YBa₂Cu₃O_7−δ and Bi-Ca-Sr-Cu-O has been studied using a varian ESR spectrometer. A model of microwave absorption built on diamagnetic tensor susceptibility has been proposed which explains the observed results satisfactorily.     

Effect of substitution at the barium site in YBa2Cu3O7-δ

The oxygen content in YBa₂Cu₃O_7-δ has been varied by substitution at Ba site with Sr or K and its influence on the structural and superconducting properties has been studied. These properties are compared with that of unsubstituted YBa₂Cu₃O_7-δ prepared by quenching. The increase inδ value of YBa₂Cu₃O_7-δ decreases theT _c drastically, whereas in YBa_2-χK_χCu₃O_7-δ system increase in theδ value (as high as 0·92) does not affect the superconducting transition temperature, which remains above 77 K. In the YBa_2-χSr_χCu₃O_7-δ system, as Sr content increases there is marginal decrease in the oxygen content as well as in the superconducting transition temperature.     

Gap-to-T c ratio as a function of the Fermi level shift

Within the framework of the BCS theory, the gap-to-T_C ratioR=2‡ ₀/kT_c is evaluated numerically (‡₀ is the energy gap atT = 0 andT> _c is the critical temperature) for a superconductor with a van Hove singularity (vHs) in the density of states as a function of the shifts (δ) of the Fermi level with respect to the vHs. It is found thatR varies asymmetrically with δ and that the variations are strong near δ = 0. Our numerical calculation shows that the largest R’s occur at certain values of δ⊋0.     

Studies on electrical properties of ( x ) BaTiO3?+?(1? x ) Ni0.92Co0.03Mn0.05Fe2O4 ME composites

The (1−x) Ni_0.92Co_0.03Mn_0.05Fe₂O₄ + (x) BaTiO₃ magnetoelectric (ME) composite have been prepared using conventional double sintering ceramic process where x varies as 1.00, 0.85, 0.70, 0.55 and 0.00. The presence of both phases has been confirmed by X-ray diffraction and the microstructure study will be carried out by SEM technique. The dc resistivity and thermo-emf of the samples have been studied with variation in temperature. The variation of dielectric constant (έ) and loss tangent (tan δ) will be measured in the frequency range 100 Hz–1 MHz. The ac conductivity has been derived from dielectric constant (έ) and loss tangent (tan δ). The static value of magnetoelectric conversion factor dc (ME)_H has been studied as a function of intensity of magnetic field.     

Oxygen deficiency dependence of transition temperature in (Sm, Er) Ba2Cu3O7−δ superconductors

We have investigated the effects of oxygen deficiency (δ) on the transition temperature (T _c) of (Sm, Er)Ba₂Cu₃O₇₋ δ superconductors by incorporating the effects of the two dimensional (2D) acoustic phonons and plasmons in the framework of strong coupling theory. The proposed approach for yttrium cuprates properly takes care of the double CuO₂ plane in a unit cell and has been found earlier to be successful in describing the pairing mechanism as well as the variation ofT _c withδ in Y Ba₂Cu₃O₇₋ δ system. The coupling strength (λ), the screening parameter (μ*) and the two dimensional acoustic phonon (plasmon) energyħω ₋(ω₊) as a function of oxygen deficiency is worked out. Finally, the transition temperature is evaluated and is found to be consistent with the earlier experimental data on yttrium cuprates. Thus, coupled phonon-plasmon mechanism is adequate to understand the nature of pairing mechanism and oxygen deficiency dependence of transition temperature in 90 K (Sm, Er)Ba₂Cu₃O₇₋ δ superconductors.