Finite size effect and influence of temperature on electrical properties of nanocrystalline Ni–Cd ferrites

Electrical conductivity and dielectric measurements have been investigated for four different average grain sizes ranging from 3 to 7 nm of nanocrystalline Ni_0.2Cd_0.3Fe_2.5−xAl_xO₄ (0.0 ⩽ x ⩽ 0.5) ferrites. The impedance spectroscopy technique has been used to study the effect of grain and grain boundary on the electrical properties of the Al doped Ni–Cd ferrites. The analysis of data shows only one semi-circle corresponding to the grain boundary volume suggesting that the conduction mechanism takes place predominantly through grain boundary volume in the studied samples. The variation of impedance properties with temperature and composition has been studied in the frequency range of 120 Hz–5 MHz between the temperatures 300–473 K. The hopping of electrons between Fe³⁺ and Fe²⁺ as well as hole hopping between Ni³⁺ and Ni²⁺ ions at octahedral sites are found to be responsible for conduction mechanism. The dielectric constant and loss tangent (tan δ) are found to decrease with increasing frequency, whereas they increase with increasing temperature. The dielectric constant shows an anomalous behavior at selected frequencies, while the temperature increases, which is expected due to the generation of more electrons and holes as the temperature increases. The behavior has been explained in the light of Rezlescu model.     

Characterization of CdS1−xSex thin films by chemical bath deposition technique

The cadmium sulfo selenide CdS_1?xSe_x thin films were chemical bath deposited in aqueous media onto coated glass substrates. As-deposited CdS_1?xSe_x thin films were annealed at 350 °C in air for 30 min. The structural, morphological, compositional and optical properties of deposited CdS_1?xSe_x thin films were studied using X-ray diffractometer (XRD), scanning electron microscopy (SEM), Energy dispersive analysis by X-ray (EDAX), and UV-Vis-NIR spectrophotometer respectively. X-ray diffraction patterns of CdS_1?xSe_x thin films reveal the polycrystalline nature and hexagonal structure. The microstructural parameters such as crystallite size (D), micro strain (?), and dislocation density were calculated and found to depend on compositions. The surface morphology and grain size are found to be influenced with the annealing temperature. The presence of Cd, S and Se of the CdS_1?xSe_x thin films and the composition of CdS_1?xSe_x thin film are estimated by EDAX analysis. The optical transmittance and absorption spectra were recorded in the range 400–2500 nm. The band gap of the CdS_1?xSe_x thin films is found to decrease from 2.5 eV to 1.75 eV.     

Optical properties of PMN–PT thin films prepared using pulsed laser deposition

(1 ? x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ (PMN–PT) thin films have been deposited on quartz substrates using pulsed laser deposition (PLD). Crystalline microstructure of the deposited PMN–PT thin films has been investigated with X-ray diffraction (XRD). Optical transmission spectroscopy and Raman spectroscopy are used to characterize optical properties of the deposited PMN–PT thin films. The results show that the PMN–PT thin films of perovskite structure have been formed, and the crystalline and optical properties of the PMN–PT thin films can be improved as increasing the annealing temperature to 750 °C, but further increasing the annealing temperature to 950 °C may lead to a degradation of the crystallinity and the optical properties of the PMN–PT thin films. In addition, a weak second harmonic intensity (SHG) has been observed for the PMN–PT thin film formed at the optimum annealing temperature of 750 °C according to Maker fringe method. All these suggest that the annealing temperature has significant effect on the structural and optical properties of the PMN–PT thin films.     

Structural and magnetic properties of HCP-CoCrPt–SiO2 granular media

Granular HCP-(CoCrPt)_100−x(SiO₂)_x thin films with Cr underlayers have been fabricated by sputtering multilayers followed by post-deposition annealing. Magnetic and structural properties of the films for potential applications in magnetic recording media have been investigated in detail. In as-deposited films coercivities exceeding 2.5 kOe have been obtained with SiO₂ varying from 8 to 16 vol%; high coercivity of 5.6 kOe and anisotropy of 4.6×10⁶ erg/cm³ have been achieved at low M_rt value (about 0.4 memu/cm²) in the post-annealed films. VSM measurements showed that the magnetic moment lies well in the film plane under proper preparation conditions. Grain isolation in the magnetic layer was improved by segregating SiO₂ into grain boundaries and further enhanced by post-deposition annealing. The rapid increase of the coercivity upon annealing is most likely due to the significant decrease in intergranular exchange coupling, as shown by the δM measurement in which the peak value of δM curves changed from a positive value to a negative value upon annealing. Magnetic reversal properties of the films have also been systematically studied. These results show that the HCP-CoCrPt–SiO₂ granular film is a promising candidate for ultra-high-density recording media up to 100 Gbit/in² or beyond because of its low Pt content and desirable properties.     

Effect of nanocrystallization on the electronic conductivity of vanadate–phosphate glasses

Electronically conducting glasses of the composition xV₂O₅·(100 − x)P₂O₅ for 60 < x < 90 were prepared. The glasses of the composition corresponding to x = 90 exhibited the highest electrical conductivity and they were studied in more detail. The effects of the annealing of the samples on their electrical conductivity, structure and other characteristics were studied by impedance spectroscopy, X-ray diffractometry, DSC and SEM microscopy. It was shown that, at temperatures close to the crystallization temperature T_c (determined from DSC), these glasses turned into nanomaterials consisting of crystalline grains of V₂O₅ (average size 25–35 nm) embedded in the glassy matrix. Their electrical conductivity was higher and the temperature stability was better than those of the starting glasses. It is postulated that the major role in this conductivity enhancement is played by the interfacial regions between crystalline and amorphous phases. The annealing at temperatures exceeding T_c led to massive crystallization and to a conductivity drop. The XRD and SEM observations have shown that the material under study undergoes structural changes: from amorphous at the beginning, to partly crystalline after the annealing at 340 °C and to polycrystalline after the annealing at 530 °C.The obtained results are in agreement with those of our earlier studies on mixed electronic–ionic conducting glasses of the ternary Li₂O–V₂O₅–P₂O₅ system.     

Modified Lanthanum Zirconium Oxide buffer layers for low-cost,high performance YBCO coated conductors

The pyrochlore Lanthanum Zirconium Oxide, La₂Zr₂O₇ (LZO), has been developed as a potential replacement barrier layer in the standard RABiTS three-layer architecture of physical vapor deposited CeO₂ cap/YSZ barrier/Y₂O₃ seed on Ni–5%W metal tape. The main focus of this research is to ascertain whether: (i) we can further improve the barrier properties of LZO; (ii) we can modify the LZO cation ratio and still achieve a high level of performance; and (iii) it is possible to reduce the number of buffer layers. We report a systematic investigation of the LZO film growth with varying compositions of La:Zr ratio in the La₂O₃–ZrO₂ system. Using a metal–organic deposition (MOD) process, we have grown smooth, crack-free, epitaxial thin films of La_xZr_1?xO_y (x = 0.2–0.6) on standard Y₂O₃ buffered Ni–5W substrates in short lengths. Detailed XRD studies indicate that a single epitaxial LZO phase with only (0 0 1) texture can be achieved in a broad compositional range of x = 0.2–0.6 in La_xZr_1?xO_y. Both CeO₂ cap layers and MOD–YBCO films were grown epitaxially on these modified LZO barriers. High critical currents per unit width, I_c of 274–292 A/cm at 77 K and self-field were achieved for MOD–YBCO films grown on La_xZr_1?xO_y (x = 0.4–0.6) films. These results indicate that LZO films can be grown with a broad compositional range and still support high performance YBCO coated conductors. In addition, epitaxial MOD La_xZr_1?xO_y (x = 0.25) films were grown directly on biaxially textured Ni–3W substrates. About 3 μm thick YBCO films grown on a single MOD–LZO buffered Ni–3W substrates using pulsed laser deposition show a critical current density, J_c, of 0.55 MA/cm² (I_c of 169 A/cm) at 77 K and 0.01 T. This work holds promise for a route for producing simplified buffer architecture for RABiTS based YBCO coated conductors.     

Growth mode of Pt1−xNix films biased dc-sputter-deposited on MgO(0 0 1)

A study is made by TEM, XRD and by measuring electrical/magnetic properties, of growth mode and properties of Pt_1−xNi_x alloy films deposited on MgO(0 0 1) at 250°C by dc-sputtering at 2.5–2.7 kV in Ar. A bias voltage V_s≤−160 V was applied to the substrate during deposition. It was confirmed that the Pt film was polycrystalline with the texture of Pt(1 1 1)/MgO(0 0 1) while the films of Pt_0.14Ni_0.86 and Pt_0.19Ni_0.81 were epitaxially grown with Pt–Ni(0 0 1)[1 0 0]/MgO(0 0 1)[1 0 0] similarly to the case of Ni/MgO(0 0 1). Thus the growth mode transformation between Pt–Ni(1 1 1)/MgO(0 0 1) and Pt–Ni(0 0 1)/MgO(0 0 1) may be induced at x less than 0.81 for Pt_1−xNi_x alloy films. The temperature coefficient of resistance TCR from 100 to 300 K of Pt_0.14Ni_0.86 films was estimated to be 0.0044–0.0053 K⁻¹ and saturation magnetization at 300 K to be 1.7–3.2 kG, respectively, while TCR of Pt films was estimated to be 0.0035–0.0048 K⁻¹.     

Structure and magnetic properties of 300-nm-thick FePt films with Hf underlayer

Structure, microstructure, magnetic properties of 300-nm-thick FePt films with 10-nm-thick Hf underlayer have been studied. The experimental results showed that the very thin Hf underlayer could promote the ordering at reduced temperatures by facilitating the nucleation of the order phase, leading to refined grain size and magnetic domain size. Therefore, the permanent magnetic properties of FePt films were enhanced. (BH)_max and H_c of FePt films were greatly enhanced from 5.0–21.0 MGOe and 1.4–11.0 kOe for single layer to 10.2–23.6 MGOe and 4.5–13.2 kOe for Hf-underlayered films annealed in T_a region of 400–600 °C, respectively. Nevertheless, the severe interdiffusion between the Hf and FePt layers at T_a=800 °C resulted in the decreased S, coarsened surface morphology, grain and magnetic domain sizes, and therefore the slightly decreased (BH)_max to 18.0 MGOe.     

Influence of growth morphology on the Néel temperature of CrRu thin films and heterostructures

Dimensionality effects on epitaxial and polycrystalline Cr_1?xRu_x alloy thin films and in Cr/Cr–Ru heterostructures are reported. X-ray analysis on Cr_0.9965Ru_0.0035 epitaxial films indicates an increase in the coherence length in growth directions (1 0 0) and (1 1 0) with increasing thickness (d), in the range 20≤d≤300 nm. Atomic force microscopy studies on these films shows pronounced vertical growth for d>50 nm, resulting in the formation of columnar structures. The Néel temperatures (T_N) of the Cr_0.9965Ru_0.0035 films show anomalous behaviour as a function of d at thickness d≈50 nm. It is interesting to note that this thickness corresponds to that for which a change in film morphology occurs. Experiments on epitaxial Cr_1?xRu_x thin films, with 0≤x≤0.013 and d=50 nm, give T_N–x curves that correspond well with that of bulk Cr_1?xRu_x alloys. Studies on Cr/Cr_0.9965Ru_0.0035 superlattices prepared on MgO(1 0 0), with the Cr layer thickness varied between 10 and 50 nm, keeping the Cr_0.9965Ru_0.0035 thickness constant at 10 nm, indicate a sharp decrease in T_N as the Cr separation layers reaches a thickness of 30 nm; ascribed to spin density wave pinning in the Cr layers for d<30 nm by the adjacent CrRu layers.     

Influence of substrate temperature on physical properties of sprayed Zn0.85Mn0.15O films

Zn_1−xMn_xO thin films have been synthesized by chemical spray pyrolysis at different substrate temperatures in the range, 250–450 °C for a manganese composition, x = 15%, on corning 7059 glass substrates. The as-grown layers were characterized to evaluate their chemical and physical behaviour with substrate temperature. The change of dopant level in ZnO films with substrate temperature was analysed using X-ray photoelectron spectroscope measurements. The X-ray diffraction studies revealed that all the films were strongly oriented along the (0 0 2) orientation that correspond to the hexagonal wurtzite structure. The crystalline quality of the layers increased with the increase of substrate temperature up to 400 °C and decreased thereafter. The crystallite size of the films varied in the range, 14–24 nm. The surface morphological studies were carried out using atomic force microscope and the layers showed a lower surface roughness of 4.1 nm. The optical band gap of the films was ∼3.35 eV and the electrical resistivity was found to be high, ∼10⁴ Ω cm. The films deposited at higher temperatures (>350 °C) showed ferromagnetic behaviour at 10 K.     

The initial growth structure of Ni1-xFex (x=0.6–0.8) films dc-biased plasma-sputter-deposited on Ni/MgO(0 0 1), and on Fe/MgO(0 0 1)

Cross sectional and plane-view transmission electron microscopy (X- and PV-TEM) were used to investigate the initial growth phase of 5, 10, 20 and 40 nm thick Ni_1-xFe_x (x=0.6–0.8) films, prepared on MgO(0 0 1) covered with a buffer layer of Fe or Ni as well as on naked MgO(0 0 1). The 100 nm thick buffer layers of Fe and Ni were pre-grown on MgO(0 0 1). All of Ni_0.20Fe_0.80, Ni_0.40Fe_0.60, Fe and Ni films could be epitaxially grown at 250°C by dc-biased plasma sputtering at 2.9 kV in pure Ar gas.The films of Ni_0.20Fe_0.80 and Ni_0.40Fe_0.60 were grown in their own stable phase, bcc and fcc on MgO(0 0 1), respectively. However, Ni_0.20Fe_0.80 film could be grown in fcc phase pseudomorphic with Ni(0 0 1) up to 20 nm thick on Ni/MgO(0 0 1), while Ni_0.40Fe_0.60 film in bcc phase pseudomorphic with Fe(0 0 1) up to 10 nm thick on Fe/MgO(0 0 1). With increasing thickness, their growth phases transformed into their own stable phases. Whether or not the pseudomorphic phase may be induced and what its critical thickness may be should depend primarily on the lattice misfit between the crystal planes in contact. The growth mode of Ni_0.40Fe_0.60 film was investigated more in details to be compared with the simulations of the average strain energy versus thickness and with those of the critical thickness of the pseudomorphic films versus the lattice misfit between the contacted crystal planes.     

Characteristics of pulse plated CdxZn1−xSe films

Cd_xZn_1−xSe films (0 ⩽ x ⩽ 1) were deposited for the first time by the pulse plating technique at different duty cycles in the range 6–50% at room temperature from an aqueous bath containing zinc sulphate, cadmium sulphate and selenium oxide. To the author’s knowledge this is the first report on pulse plated Cd_xZn_1−xSe films. The deposition potential was −0.9 V (SCE). The as deposited films exhibited cubic structure. Composition of the films was estimated by Energy Dispersive Analysis of X-ray studies. X-ray photoelectron spectroscopy studies indicated the binding energies corresponding to Zn(2p_3/2), Cd(3d_5/2 and 3d_3/2) and Se(3d_5/2 and 3d_3/2). Optical band gap of the films varied from 1.72 to 2.70 eV as the composition varied from CdSe to ZnSe side. Atomic force microscopy studies indicated grain size in the range of 20–150 nm.     

Surface topographic study of chalcogenide thin films of GexSb(As)40−xS50Te10 glasses

The surface topography and fractal properties of Ge_xSb(As)_40−xS₅₀Te₁₀ (x = 10, 20, 27 at.%) films, evaporated onto glass substrates, have been studied by atomic force microscopic imaging at different scales. The surface of the chalcogenide films is smooth (<5 nm roughness), isotropic and having some particular differences in texture. All films are self-similar with Mean Fractal Dimension in the range of 2.25–2.63. The films with Ge_xSb_40−xS₅₀Te₁₀ composition are more uniform in terms of surface morphology (grains structure) than those with Ge_xAs_40−xS₅₀Te₁₀ composition for which the film surface exhibits a superimposed structure of large particles at x = 10 and 20 at.%.     

Surface stoichiometry of La0.7Sr0.3MnO3 during in vacuo preparation; A synchrotron photoemission study

We present a study of the surface stoichiometry and contamination of La_0.7Sr_0.3MnO₃ thin films following exposure to air and subsequent in vacuo preparation. Samples were studied using both soft X-ray synchrotron photoemission (hν = 150 to 350 eV) and traditional Mg-Kα XPS (hν = 1253.6 eV) whilst annealing incrementally to ≈ 510°C in low pressures of O₂. In all cases, a Mn depleted and Sr rich surface oxide layer is observed, it is of reduced crystalline quality and is charge depleted. This surface layer is weakly affected by subsequent annealing, and is partially reversed by annealing in higher O₂ pressure. Surface carbon contamination is incrementally removed by annealing at increased temperatures, and at 270 °C, it is reduced to ≈ 0.4% of the topmost unit cell. The modification of the surface stoichiometry and electronic properties is consistent with the reported loss of magnetic properties in thin LSMO films.     

The effect of In/N ratio on the optical quality and lasing threshold in GaxIn1 − xAs1 − yNy/GaAs laser structures

We present a review of published work concerning the effect of In and N compositions on the operation wavelength, optical quality and lasing threshold in Ga_xIn_1 − xAs_1 − yN_y/GaAs QW and double heterostructure lasers. We show that the emission wavelength in the range between 1.0 and 1.4 μ m can be obtained for a wide range of In and/or N concentrations. However, in most Fabry–Perot lasers and vertical cavity surface emitting lasers (VCSELs) reported in the literature, the threshold current density plotted as a function of the relative In/N composition (R =  (1  − x) / y) indicate a broad minima for 40  < R <  70, suggesting an optimum relative composition. We also present the results of our studies concerning the optical quality of Ga_xIn_1 − xAs_1 − yN_y/GaAs single quantum wells for R =  15. We show that the optical quality of GaInAsN can be improved while achieving a red shift in the PL spectra. This is unlike the results obtained by rapid thermal annealing or conventional annealing, which are widely employed as post-growth treatment techniques, where any increase in the PL intensity is almost always accompanied by an undesired blue shift.     

Synthesis of Y-doped BaCeO3 nanopowders by a modified solid-state process and conductivity of dense fine-grained ceramics

Powders of BaY_xCe_1 ? xO_3 ? δ (x = 0, 0.1 and 0.15) with specific surface area of 6–8 m²g^? 1 (BET equivalent particle size of 130–160 nm) were prepared by a modified solid-state route using nanocrystalline BaCO₃ and CeO₂ raw materials. These powders showed excellent densification at relatively low temperatures. Dense (96–97% relative density) ceramics with submicron grain size (0–4–0.6 µm) were obtained after sintering at 1250–1280 °C. Ceramics sintered at 1450 °C revealed only a moderate grain growth (grain size ≤ 2 µm), uniform microstructure and very high density (≥ 98%). The total conductivity of the submicron ceramics at 600 °C was comparable with the reference values reported in the literature, meaning that the high number of grain boundaries was not a limiting factor. On lowering temperature, the contribution of the blocking grain boundaries becomes progressively more important and the conductivity decreases in comparison to coarse-grained ceramics. Microscopic conductivities of grain interior and grain boundary are the same irrespective of grain size meaning that the different macroscopic behaviour is only determined by a geometric factor (a trivial size effect).     

Superconducting characteristics and microstructure of polycrystalline Zn-doped In2O3 films

In order to investigate the relation among the superconducting transition T_c, carrier density n, resistivity ρ and the microstructure in the polycrystalline (In₂O₃)_1?x–(ZnO)_x films, we prepared specimen films by post annealing of amorphous films with x = 0.025 at various annealing temperature T_a and for annealing time t_a = 1 h and 4 h. As for microstructures, we have investigated the distribution of elements by scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). We have found followings: (1) The annealed films clearly show the superconductivity of which T_c depends on T_a, t_a and n. This indicates that the superconductivity is determined by the combination of crystallinity and carrier density. (2) The data on STEM–EELS spectra mapping of indium plasmon indicate that droplets of the pure indium phase exist inside a film, where the distribution of these droplets dispersed. Therefore, it seems that droplets do not form an electrical conducting path, that is, it is possible that observed superconductivity is due to intrinsic characteristic of polycrystalline (In₂O₃)_1?x–(ZnO)_x films.     

Infrared transmission of chalcohalide glasses in the Ge–Se–Te–I system

Infrared (IR) transmission spectra in the range from 4000 to 400 cm^?1 have been measured at ambient temperature for the prepared, (Ge₂₂Se₁₄Te₆₄)_100?xI_x, glasses where x = 0, 4, 6, 8 and 10 at.%. The effects of both iodine content and the annealing of glasses at different temperatures on the transmittance and observed absorption peaks have been evaluated. Scanning electron microscope (SEM) micrographs were performed to insure the effect of annealing on the obtained IR transmission spectra.     

Synthesis and critical current density promotion for Nb-doped 2212-BPSCCO HTc-superconducting materials

Although BPSCCO superconducting regime has very low stability under high oxygen pressures as reported in the literature, we managed to synthesize relatively pure 2212-BPSCCO and their Nb-doped samples having general formula Bi_1−xNb_xPbSr₂CaCu₂O₈, where x = 0.1, 0.2, 0.4 and 0.6 mole, respectively, at moderate oxygen pressure (∼30 bar). The superconducting measurements proved that the best recorded T_c ∼ 69 K was for the undoped 2212-BPSCCO, while the lowest T_c ∼ 58 K was recorded for the maximum doped sample x = 0.6 mole indicating that superconductive transition temperatures T_cs decrease regularly with increasing Nb-dopant concentration from x = 0.1 to 0.6, respectively. The lattice parameter c exhibited a slight length compression as Nb-dopant ratio increases from 0.1 to 0.6 mole, respectively. From SE-microscopic analysis, the average grain size was estimated and found in between 0.44 and 1.6 μm which is considered relatively high to that reported in the literature. The measured J_c’s values were found to be enhanced remarkably as Nb-dopant concentration increases.     

Effect of grain size and magnetocrystalline anisotropy on exchange coupling in nanocomposite two-phase Nd-Fe-B magnets

We have studied the exchange coupling in two-phase nanocrystalline alloy ribbons of the composition Nd_4−xSm_xFe_77.5B_18.5 (x=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5). The grain size of the Fe₃B phase changed from 16 to 23 nm by annealing between 670°C and 760°C for 5 min. From the measurements of the magnetic properties and the reversibility of magnetization, we derived that the mean critical grain size of the Fe₃B phase required for full exchange coupling by the Nd₂Fe₁₄B phase is about 20 nm. A detailed study of the magnetization-reversal process was made on samples in which the magnetocrystalline anisotropy of the Nd₂Fe₁₄B phase was reduced by substitution of Sm for Nd. We showed that the exchange-coupled range can be markedly expanded and that the reduced remanence can be increased with increasing Sm content.