Magneto‐transport properties of polycrystalline YBa2(Cu1‐xMx)3O7‐δ (M = B and Mn)

The magnetic and transport properties of polycrystalline YBa₂ (Cu_1‐xM_x)₃ O_7‐δ (M = B and Mn) superconductor was investigated. Samples of YBa₂(Cu_1‐xB_x)₃O_7‐δ doped with several concentrations of boron B(x = 0.05 and 0.1) were investigated using magnetization measurements. A YBa₂(Cu_1‐xMn_x)₃O_7‐δ sample doped with Mn with concentration of x = 0.02 was investigated using current‐voltage (I‐V) measurements. Our results on the YBa₂(Cu_1‐xB_x)₃O_7‐δ samples reveal a considerable increase in the hysterisis width of the magnetization, M versus the applied magnetic field H with increasing boron concentration. The lower critical field was also found to be enhanced by boron doping. The critical current density, J_c was found to be significantly enhanced in the Mn‐doped sample. The enhancement of J_c was found to be more significant at the lower temperatures for all applied magnetic fields used (0 Oe, 300 Oe, and 500 Oe). Thus, chemical doping is suggested to enhance the vortex pinning forces in the YBCO samples. From the resistivity (R‐T) measurements, chemical doping of the samples was found to have no significant effect on the critical temperature, T_c. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Co nanoclusters as origin of ferromagnetism in sol‐gel synthesized Zn1‐xCoxO (x = 0.05, 0.10 and 0.15) samples

We report on the structural and magnetic properties of the polycrystalline samples of Zn_1‐xCo_xO (x = 0.05, 0.10 and 0.15) synthesized via sol‐gel route. The air sintered samples of all compositions exhibit paramagnetic behaviour at room temperature, on the other hand the same samples on annealing in Ar/H₂ atmosphere show room temperature ferromagnetism (RTFM) with enhanced magnetization. The value of magnetization increases with the Co concentration (x) in both the air sintered and Ar/H₂ annealed samples. The observed ferromagnetism in the Ar/H₂ annealed Zn_1‐xCo_xO (x = 0.05, 0.10 and 0.15) samples is attributed to the presence of Co nanoclusters as detected by XRD and FESEM‐EDAX. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and Characterisation of (CuInTe2)1‐x(2 ZnTe)x Solid Solution Single Crystals

The crystal structure as well as the optical properties in the band gap region of (CuInTe₂)_1‐x(2 ZnTe)_x solid solution single crystals grown by directional freezing have been studied. The lattice constants exhibit a linear dependence on crystal composition. The chalcopyrite‐sphalerite phase transition was observed between x = 0.3 and x = 0.4°. The variation of the band gap with respect to crystal composition can be described by a quadratic expression.     

Growth and characterization of superconductive Nd2‐xCexCuO4‐y single crystals

Nd_2‐xCe_xCuO_4‐y single crystals with different x values have been successfully grown by Traveling Solvent Floating Zone method (TSFZ). Electronic properties of Nd_2‐xCe_xCuO_4‐y single crystals with various x have been studied in detail. The results show that superconductivity can be found in crystals with x > 0.1 (0.13‐0.18) directly grown at oxygen‐reduced atmosphere without post‐annealed, while no superconductivity appears in crystals with x < 0.1 at the same atmosphere. It is also found that, the segregation coefficient of Ce is determined to be 0.946 and transition temperature Tc (onset) reaches maximum value of 23.5 K at nominal composition x = 0.165. With further increase of Ce concentration, transition temperature of single crystals declines due to the precipitation of secondary Phases. In addition, the variation of lattice constants of Nd_2‐xCe_xCuO_4‐y single crystals with different x is also given. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study on the ferroelectric phase transition in potassium lithium niobate (KLN)

The ferroelectric phase of potassium lithium niobate K₃Li_2–xNb_5+xO_15+2x (KLN) in the range of 0.15 < x < 0.5 is a very promising material for the second harmonic generation (SHG) in the blue visible region (∼410 nm). The ferroelectric phase transition was shown to occur between 400 and 500°C depending on the composition of the KLN phase. In this study several analysis techniques were used to investigate the phase transition on ferroelectric (x = 0.3) KLN samples. The temperature‐dependent measurements of the relative dielectric constant ε₃₃ provided a phase transition temperature of about 470°C. In our DTA experiments, a small but reproducible thermal effect at the phase transition in KLN was indicated. The temperature‐dependent birefringence measurement technique, applied the first time on KLN, shows a second order behaviour at a temperature of 467 °C. However, this phase transition is accompanied by a small thermal effect. The DSC analysis for the other KLN composition (x = 0.5) provided a phase transition temperature of 514 °C. The appearance of a phase transition in the paraelectric KLN phase (Nb content higher than 55 mol%) was also studied. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of Ba ions substitution on multiferroic properties of BiFeO3 perovskite

Dielectric, magnetic and ferroelectric properties of Bi_1‐xBa_xFeO₃(x = 0.1, 0.2 and 0.3) synthesized by solid state reaction method are reported. X‐ray diffraction pattern showed that Bi_1‐xBa_xFeO₃was single phase. Ba substitution has led to a decrease in grain size and hence an increase in the electrical resistivity. The variation of dielectric constant and loss has been studied over a frequency range of 1 kHz – 2 MHz at room temperature. Ba doping leads to effective suppression of the spiral spin structure of BiFeO₃, resulting in the appearance of net magnetization. Improved multiferroic properties of Bi_0.8Ba_0.2FeO₃ ceramic with remnant magnetization and polarization 0.093 emu/g and 0.808 µC/cm², respectively, were obtained. The evidence of ferromagnetism and ferroelectric hysteresis loops in Bi_1‐xBa_xFeO₃system at room temperature makes it a good candidate for potential applications. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystallization temperature of amorphous Fe80B20 under pressure

The resistance of rapidly quenched Fe₈₀B₂₀ was recorded versus T at a rate of 7 K/min and various pressures up to 2.5 GPa. The onset of crystallization, T_x, was defined by extrapolation at the sharp change in resistance associated with crystal growth. The resulting slope of T_x is 15 K/GPa. Separate measurements at atmospheric pressure yielded T_x versus rate of temperature increase, R = dT/dt. crystallization times from the literature were recalculated to yield T_x, and the results are compared with our data. Nominally amorphous materials of the same composition show markedly different properties. It is also shown that the variation of T_x with R is mostly due to the change in viscosity.     

Synthesis and characterization of compounds LixMn1+xFe2–2xO4 with spinel structure in the quasiternary system “LiO0,5 – MnOx – FeOx“

The thermal decomposition of freeze‐dried Li‐Mn(II)‐Fe(III)‐formate precursors was investigated by means of DTA, TG and mass spectroscopy. By the thermal treatment of the prefired precursors between 400 and 1000°C, single phase solid solutions Li_xMn_1+xFe_2–2xO₄ (0 ≤ x ≤ 1) with cubic spinel structure were obtained. To get single phase spinels, special conditions concerning the temperature T and the oxygen partial pressure p(O₂) during the synthesis are required. Because of the high reactivity of the freeze‐dried precursors, in comparison with the conventional solid state reaction, the reaction temperature can be lowered by 200°C. The cation distribution and the properties of the Li‐Mn‐ferrites were studied by chemical analysis, X‐ray powder diffraction and magnetization measurements. It was found that for high substitution rates, almost all lithium occupies the tetrahedral coordinated A‐sites of the spinel lattice AB₂O₄, while at small x‐values, lithium ions are distributed over the tetrahedral and octahedral sites. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Neutron powder diffraction studies in CaMn1‐xCuxO3 (x = 0, 0.2)

Neutron powder diffraction patterns were recorded on CaMn_1‐xCu_xO₃ (x = 0 and 0.20) compounds at different temperatures down to 11K. All the patterns were analyzed by employing Rietveld refinement technique and using the Fullprof program. The observed crystallographic peaks could be refined by using Pbnm space group and no structural transition has been observed down to 11K. An additional peak at 2θ = 16.7° has been observed with decrease in temperature below T_N and its intensity was found to increase with decrease in temperature. It could be indexed to magnetic (101) plane. The magnetic ordering is found to be G‐type antiferromagnetic behaviour. The magnetic moment at 11K for the samples x = 0.0 and 0.20 are found to be 2.69 and 2.42μ_B. The doped Cu ions are found to be in Cu²⁺ state and take part antiferromagnetic interactions with Mn ions. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fabrication and magnetic properties of Cu50(Fe69Si10B16C5)50 thin microwires

In this work, we study the granular samples of mixed Cu₅₀(Fe₆₉Si₁₀B₁₆C₅)₅₀ composition where half of alloy composition is commonly used amorphous soft magnetic material and the other half-Cu. Glass covered Cu₅₀(Fe₆₉Si₁₀B₁₆C₅)₅₀ microwires were produced and their magnetic properties were studied. The evolution of the structure after the annealing was observed using X-ray diffraction with Cu K_α radiation. The as-prepared Cu₅₀(Fe₆₉Si₁₀B₁₆C₅)₅₀ microwires present a relatively low coercivity of about 5 Oe and exhibit non-regular hysteresis loop typical behavior for two-phases systems. Annealing resulted in magnetic hardening of the samples with coercivity of about 50 Oe. The variation of the coercivity and remanent magnetization with the temperature at 5–300 K were obtained from those curves. Temperature dependence of magnetization at 5–300 K exhibits significant difference between field-cooled and zero-field cooled behaviour. Observed dependences interpreted in terms of two-phase structure of as-prepared samples and evolution of the structure under annealing.     

Growth of big single crystals of a new magnetic superconducting double perovskite Ba2PrRu1–xCuxO6

Single crystals of Ba₂PrRu_1–xCu_xO₆ with x = 0 to 0.2, have been grown from high temperature solutions of a mixture of PbO‐PbF₂ in the temperature range 1100–1200 °C. Thin crystals with mostly a hexagonal and triangular plate like habit measuring up to 1–2 mm across and 0.1–0.2 mm thick were obtained. The size, quality and morphology of the crystals were improved by varying the solution volume as well as additives like B₂O₃. Large crystals measuring up to 3 mm across and 0.3 to 0.5 mm thick were obtained with 5–7 wt% solute concentration and 0.51 wt% of B₂O₃. The ZFC curves exhibit a spin glass like behavior with x = 0 and a superconducting transition at 8 to 11 K depending on x = 0.05 to 0.1. The transition was also influenced by the growth temperature and post growth annealing. Powder x‐ray diffraction, EDS and Raman spectroscopic measurements confirm the presence of Cu in the crystals. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrical and magnetotransport properties of La0.7Sr0.3MnO3/TiO2 composites

The composite samples with nominal compositions of (1‐x) La_0.7Sr_0.3MnO₃ + x TiO₂ (x = 0, 0.05, 0.10, 0.15 and 0.20) were synthesized via solid state reaction process. The X‐ray diffraction and scanning electronic microscopy observations reveal no reaction between La_0.7Sr_0.3MnO₃ (LSMO) and TiO₂ phases. Temperature dependence resistivity measurements show that TiO₂ phase shifts the metal‐insulator transition temperature (T_p) towards lower temperature and increases the resistivity. Moreover, the magnetization of the composite samples decreases with TiO₂ content. An enhancement in magnetoresistance is observed in the composite samples with x = 0.05 and x = 0.10 at low magnetic fields, which is encouraging for potential application of magnetoresistive materials at low field. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and characterization of hexagonal ferrites BaFe12‐2xZnxTixO19 (0 ≤ x ≤ 2) by thermal decomposition of freeze‐dried precursors

Substituted barium hexaferrites, BaFe_12‐2xZn_xTi_xO₁₉ (0 ≤ x ≤ 2), have been synthesized by thermal decomposition of freeze‐dried acetate precursors. Decomposition and phase formation were investigated by means of thermal analysis, XRD and IR spectroscopy. The initially amorphous decomposed precursor reacts to the substituted hexaferrite via a spinel‐like maghemite (γ‐Fe₂O₃) and Zn/Ti containing spinel ferrites. The synthesis method allows a decrease of the reaction temperature and time, necessary for producing a single phase hexaferrite. At relative low reaction temperatures, the substitution rate x shows remarkable differences at different iron sublattices. For x ≤ 0,8 this selective substitution results in an increase of magnetization as x grows. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure,microstructure and magnetic properties of mixed rare earth oxide (Dy1‐xErx)2O3

The mixed rare earth oxide (Dy_1‐xEr_x)₂O₃ (0.0 ≤ x ≤ 1.0) were synthesized by a sol–gel process. X‐ray and neutron diffraction data were collected and crystal structure and microstructure analyses were performed using Rietveld refinement method. All samples were found to have the same crystal structure and formed solid solutions over the whole range of x. Preferential cationic distribution is found for all samples but with different extent with Dy³⁺ preferring the 8b among the two non‐equivalent sites 8b and 24d of the space group Ia3. The lattice parameter is found to vary linearly with the composition x and a systematic variation is found in the r.m.s microstrain . Magnetization measurements were done in the temperature range 5‐300 K and a behavior in accordance with Curie‐Weiss law was found. Anomalous concentration dependence is found in magnetic susceptibility which is ascribed to the concentration dependence of effective crystal field combined with the contribution of ⁴I_15/2 and ⁶H_15/2 manifold at elevated temperature. The effective magnetic moments μ_eff is found to decrease linearly with composition parameter x, except for sample x=0.5 where the magnetization is enhanced. The Curie‐Weiss paramagnetic temperatures indicated antiferromagnetic interaction. These magnetic results are discussed in view of the cationic distribution. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Heat‐treatment influence on the microstructure and magnetic properties of rare‐earth substituted SrFe12O19

Rare‐earth substituted strontium ferrite nanopowders SrFe_12‐xR_xO₁₉ (R = La, Gd and Er; x = 0.2, 0.5 and 1) were prepared by sol‐gel‐autocombustion method and subsequent heat treatments. Structural and magnetic properties of SrFe_12‐xR_xO₁₉ powders heat treated at 800, 900 and 1000⁰C, for various times, were characterized with an X‐ray diffractometer, a vibrating sample magnetometer and a scanning electron microscope. The results of X‐ray diffraction measurements showed the M‐type hexagonal structure formation by heat treatments. Magnetic properties, such as specific saturation magnetization σ_s, specific remanent magnetization σ_r and coercivity H_c, as well as microstructure depend on the heat treatment conditions (temperature and time). The coercivity H_c exhibits a great increase after a critical heat treatment time. When the heat treatment time increases, one obtains an increase in H_c after a shorter heat treatment time. This jump of H_c was explained by a transition from the superparamagnetic state to normal state of the single domain nanoparticles. The occurrence of an agglomerated structure composed of magnetically interacting ultrafine crystallites also contributes to the increase of H_c.The heat treatment determines a reduced grain growth due to the internal stress generated by R ions. With increasing R content the σ_s and σ_r decrease due to the dissolution of R ions into the hexaferrite lattice. We believe that by selecting the time and temperature of the heat treatment, microstructure and magnetic properties suitable for magnetic recording media application can be obtained. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Casting vacuum effects on the precipitates and magnetic properties of Fe-based glassy alloys

The crystalline precipitates and magnetic properties of the Fe₆₁Co_9 − xZr₈Mo₅W_xB₁₇ (x = 0 and 2) cylinders prepared under various vacuum conditions were investigated in this paper. The fraction of the crystalline precipitates and liquidus temperature of the samples with tungsten content c_W = 0 decease with decreasing vacuum, while they increase in the samples with c_W = 2 at.%. For both tungsten contents, with decreasing vacuum, the magnetization at room temperature M_start and Curie temperature T^h_c of the samples decrease gradually, but the increment of magnetization ΔM_800 K at 800 K after heating to 973 K increases and the Curie temperature T^c_c of the residual amorphous matrix shows a decreasing tendency simultaneously. Meanwhile, with decreasing vacuum, Zr-B compounds tend to precipitate easily out from the melt or to remain undissolved during casting, and eutectic Fe-B compounds tend to be formed easily after heating to 973 K. The existence of Zr-B and Fe-B compounds changes the boron content in the residual melt/matrix and consequently affects the magnetic properties of the samples in the as-quenched state and after heating to 973 K.     

Review: Different types of ordering

Homogeneous structures A_xB_y with identical A–A or B–B self‐coordination numbers T₁, T₂, T₃, of nearest, next‐nearest and third neighbors are selected. The maximum T_i values are 2 2 2 (1D row), 6 6 6 (2D hexagonal net) and 12 6 24 (3D cubic close‐packing). Reduced T₁ or T₂ values for A_xB_y can be related with attractive or repulsive A–A (or B–B) interactions. A single set of T₁ T₂ T₃; y/x values was obtained for y/x = 1–4 and 6. The y/x = 2 structures of three equivalent positions A, B and C are unfavourable for A = ⊕ and B = ⊖ charge or direction of magnetic moments. The spatial distribution of positive and negative potential near A⁺ and B⁻ positions gives rise to neutral C positions. Frustrated magnetic moments at C positions are disordered. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and magnetic properties of nanometric Zn0.5Co0.5Fe2O4 prepared by hydrothermal method

ABSTRACT

The study of the structural, morphology and magnetic properties of Zn_0.5Co_0.5Fe₂O₄ ferrite is the objective of this work. The sample was prepared by hydrothermal method and was characterized by X-ray diffraction (XRD), (SEM) and (TEM) micrographs and magnetization measurements.

The magnetic hysteresis loops, field cooling (FC) and zero field cooling (ZFC) curves, in temperature range (0-400K), were measured using XL-SQUID magnetometer and the values of blocking temperatures (T_B) were determined. The results indicated that Zn_0.5Co_0.5Fe₂O₄ sample were formed in a single spinel phase and gives the value for the lattice parameter (8.3952 Å) and nanosizes of particles (13.8 nm) were compared with these obtained from ZnFe₂O₄ sample prepared also by synthesis method (8.4261 Å and 14 nm). Although, the superparamagnetic behaviour for Co-Zn ferrite has observed at 350K with a blocking temperature (T_B = 300K), that is maximum at the value obtained in the case of Zn-ferrite (T_B = 12K).     

Optical and magnetic properties of Nd‐doped ZnO nanoparticles

Nd‐doped ZnO nanoparticles with different concentration were synthesized by sol‐gel method. The structures, magnetic and optical properties of as‐synthesized nanorods were investigated. X‐ray diffraction (XRD) and x‐ray photoelectron spectroscopy (XPS) results demonstrated that Nd ions were incorporated into ZnO lattice; but Zn_1‐xNd_xO nanoparticles with Nd concentration of x = 0.05 showed Nd₂O₃ phase, so the saturation concentration of Nd in Zn_1‐xNd_xO is less than 5 at%. Vibrating sample magnetometer (VSM) measurements indicated that Nd doped ZnO possessed dilute ferromagnetis behaviour at room temperature. Photoluminescence spectroscopy (PL) showed that Nd ions doping induced a red slight shift and decrease in UV emission with increase of Nd concentration.     

Microstructural,thermal, and electrical properties of Bi1.7V0.3Sr2Ca2Cu3Ox glass‐ceramic superconductor

A glass‐ceramic Bi_1.7V_0.3Sr₂Ca₂Cu₃O_x superconductor was prepared by the melt‐quenching method. The compound was characterized by scanning electron microscopy, x‐ray diffraction, differential thermal analysis, current‐voltage characteristics, transport resistance measurements, and Hall effect measurements. Two main phases (BSCCO 2212 and 2223) were observed in the x‐ray data and the values of the lattice parameters quite agree with the known values for 2212 and 2223 phases. The glass transition temperature was found to be 426 °C while the activation energy for crystallization of glass has been found to be E_a = 370.5 kJ / mol. This result indicates that the substitution of vanadium increased the activation energy for the BSCCO system. An offset T_c of 80 K was measured and the onset T_c was 100 K. The Hall resistivity ρ_H was found to be almost field‐independent at the normal state. A negative Hall coefficient was observed and no sign reversal of ρ_H or R_H could be noticed. The mobility and carrier density at different temperatures in the range 140‐300 K under different applied magnetic fields up to 1.4 T were also measured and the results are discussed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)