Influence of oxygen partial pressure on electrical and optical properties of Zn0.93Mn0.07O thin films

The Zn_1−xMn_xO (x = 0.07) thin films were grown on glass substrates by direct current reactive magnetron cosputtering. The influence of oxygen partial pressure on the structural, electrical and optical properties of the films has been studied. X-ray-diffraction measurement revealed that all the films were single phase and had wurtzite structure with c-axis orientation. The experimental results indicated that there was an optimum oxygen partial pressure where the film shows relative stronger texture, better nano-crystallite and lower surface roughness. As the oxygen partial pressure increases, the carrier concentration systematically decreases and photoluminescence peaks related to zinc interstitials gradually diminish. The minimal resistivity of 70.48 Ω cm with the highest Hall mobility of 1.36 cm² V⁻¹ s⁻¹ and the carrier density of 6.52 × 10¹⁶ cm⁻³ were obtained when oxygen partial pressure is 0.4. All films exhibit a transmittance higher than 80% in the visible region, while the deposited films showed a lower transmittance when oxygen partial pressure is 0.4. With the increasing of oxygen partial pressure, the peak of near-band-edge emission has firstly a blueshift and then redshift, which shows a similar trend to the band gap in the optical transmittance measurement.     

Preparation of delafossite-type CuCrO2 films by sol-gel method

High-quality c-axis oriented delafossite-type CuCrO₂ films were successfully prepared by a simple sol-gel method. The microstructure, optical properties as well as room temperature resistivity were studied. It was found that the grain sizes of CuCrO₂ films pretreated with different temperatures are different; the films were smooth and consisted of fine particles. The maximal transmittance of CuCrO₂ films can reach 70% in the visible region. Optical transmission data of CuCrO₂ films indicate a direct band gap and an indirect-gap of about 3.15 eV and 2.66 eV, respectively. The carrier mobility of the films pretreated at 300 °C is smaller than that of the films pretreated at a higher temperature, because of the stronger carrier scattering.     

Exchange biasing in SFMO/SFWO double perovskite multilayer thin films

In the present study, we have studied the exchange bias interaction in ferromagnetic Sr₂FeMoO₆ (SFMO)/antiferromagnetic Sr₂FeWO₆ (SFWO) multilayer thin films deposited on single crystal LaAlO₃ substrates using KrF pulsed laser deposition technique. XRD pattern revealed that SFMO, SFWO and their multilayer thin films were highly oriented along the c-axis. The microstructure studied by atomic force microscopy was found to be uniform, fine, dense and homogenous in nature. The observed magnetization-temperature curves showed Neel temperature T_N ∼ 37 K for SFWO and Curie temperature T_C > 320 K for SFMO thin films. For multilayer, the field cooled magnetization-field curve was shifted horizontally and the direction of the horizontal shift is opposite to that of H_FC, indicating an exchange bias effect. Exchange bias field H_E was found to decrease with increase in temperature and approached to zero at blocking temperature.     

Structural and optical properties of electrodeposited ZnO thin films on conductive RuO2 oxides

ZnO thin films were grown on the 150 nm-thick RuO₂-coated SiO₂/Si substrates by electrochemical deposition in zinc nitrate aqueous solution with various electrolyte concentrations and deposition currents. Crystal orientation and surface structure of the electrodeposited ZnO thin films were characterized by X-ray diffraction (XRD) and scanning electron microscopy, respectively. The XRD results show the as-electrodeposited ZnO thin films on the RuO₂/SiO₂/Si substrates have mixed crystallographic orientations. The higher electrolyte concentration results in the ZnO thin films with a higher degree of c-axis orientation. Moreover, the use of an ultra-thin 5 nm-thick ZnO buffer layer on the RuO₂/SiO₂/Si substrate markedly improves the degree of preferential c-axis orientation of the electrodeposited ZnO crystalline. The subsequent annealing in vacuum at a low temperature of 300 °C reduces the possible hydrate species in the electrodeposited films. The electrodeposited ZnO thin films on the 5 nm-thick ZnO buffered RuO₂/SiO₂/Si substrates grown in 0.02 M electrolyte at −1.5 mA with a subsequent annealing in vacuum at 300 °C had the best structural and optical properties. The UV to visible emission intensity ratio of the film can reach 7.62.     

Effect of Ag-doping on crystal structure and high temperature thermoelectric properties of c-axis oriented Ca3Co4O9 thin films by pulsed laser deposition

Ag-doped Ca₃Co₄O₉ thin films with nominal composition of Ca_3−xAg_xCo₄O₉ (x = 0∼0.4) have been prepared on sapphire (0 0 0 1) substrates by pulsed laser deposition (PLD). Structural characterizations and surface chemical states analysis have shown that Ag substitution for Ca in the thin films can be achieved with doping amount of x ≤ 0.15; while x > 0.15, excessive Ag was found as isolated and metallic species, resulting in composite structure. Based on the perfect c-axis orientation of the thin films, Ag-doping has been found to facilitate a remarkable decrease in the in-plane electrical resistivity. However, if doped beyond the substitution limit, excessive Ag was observed to severely reduce the Seebeck coefficient. Through carrier concentration adjustment by Ag-substitution, power factor of the Ag-Ca₃Co₄O₉ thin films could reach 0.73 mW m⁻¹ K⁻² at around 700 K, which was about 16% higher than that of the pure Ca₃Co₄O₉ thin film.     

Critical behavior of La0.87K0.13MnO3 manganite

The heat capacity of La_0.87K_0.13MnO₃ manganite is measured in temperature intervals from 80 to 350 K. A nature of the ferromagnetic phase transition and critical properties of the heat capacity near Curie temperature are studied. The principles of change in universal critical parameters near a phase transition point are determined. The critical exponents and heat capacity amplitude are calculated with account of correction to the scaling α = −0.13, А⁺/А⁻ = 1.178, and correspond to the critical behavior of 3D Heisenberg model. It is found a critical exponent of the correlation radius υ = 0.71, which is also conform to the Heisenberg universality class.     

Polarization-dependent electrolyte electroreflectance study of Cu2ZnSiS4 and Cu2ZnSiSe4 single crystals

Polarization-dependent electrolyte electroreflectance (EER) measurements were carried out on the oriented Cu₂ZnSiS₄ and Cu₂ZnSiSe₄ single crystals at room temperature. Thin blade single crystals of Cu₂ZnSiS₄ and Cu₂ZnSiSe₄ were grown by chemical vapor transport technique using iodine as a transport agent. Laue pattern normal to the basal plane of the as-grown crystal revealed the formation of orthorhombic structure with the normal along [2 1 0] and the c axis parallel to the long edge of the crystal platelet. The polarized EER spectra in the vicinity of the direct band edge of Cu₂ZnSiS₄ displayed distinct structures associated with transitions from two upper-most valence bands to the conduction band minimum at Γ point. In the E⊥c configuration, the feature designated as E_A ∼ 3.345 eV was detected and for Е‖c, only E_B ∼ 3.432 eV appeared. For Cu₂ZnSiSe₄, three features denoted as E_A, E_B, and E_C at around 2.348, 2.406 and 2.605 eV, respectively, were recorded for E⊥c polarization, whereas in the Е‖c, only E_B and E_C were the allowed transitions. Based on the experimental observations and a recent band-structure calculation by Chen et al. [Phys. Rev. B 82 (2010) 195203], plausible band structures near the direct band edge of Cu₂ZnSiS₄ and Cu₂ZnSiSe₄ have been proposed.     

High-field magnetization of a Dy2Fe14Si3 single crystal

The magnetization of a Dy₂Fe₁₄Si₃ single crystal was measured at 4.2 K in pulsed fields up to 51 T along the principal axes. The compound orders ferrimagnetically at 500 K, has a spontaneous magnetic moment of 8 μ_B/f.u. (at 4.2 K) and exhibits a very large magnetic anisotropy, 〈1 0 0〉 being the easy axis. In fields applied along the 〈1 0 0〉 and 〈1 2 0〉 axes, field-induced phase transitions are observed at 33 T and at 39 T, respectively. The c-axis magnetization curve crosses the easy-axis curve at 19 T. At higher fields, for all directions, the magnetization continues to increase due to further bending of the sublattice moments. Temperature evolution of magnetic anisotropy and magnetic hysteresis are discussed as well.     

Study of bactericidal efficiency of magnetron sputtered TiO2 films deposited at varying oxygen partial pressure

TiO₂ thin films have been deposited at different Ar:O₂ gas ratios (20:80,70:30,50:50,and 40:60 in sccm) by rf reactive magnetron sputtering at a constant power of 200 W. The formation of TiO₂ was confirmed by X-ray photoelectron spectroscopy (XPS). The oxygen percentage in the films was found to increase with an increase in oxygen partial pressure during deposition. The oxygen content in the film was estimated from XPS measurement. Band gap of the films was calculated from the UV-Visible transmittance spectra. Increase in oxygen content in the films showed substantial increase in optical band gap from 2.8 eV to 3.78 eV. The Ar:O₂ gas ratio was found to affect the particle size of the films determined by a transmission electron microscope (TEM). The particle size was found to be varying between 10 and 25 nm. The bactericidal efficiency of the deposited films was investigated using Escherichia coli (E. coli) cells under 1 h UV irradiation. The growth of E. coli cells was estimated through the Optical Density measurement by UV-Visible absorbance spectra. The qualitative analysis of the bactericidal efficiency of the deposited films after UV irradiation was observed through SEM. A correlation between the optical band gap, particle size and bactericidal efficiency of the TiO₂ films at different argon:oxygen gas ratio has been studied.     

Post annealing effect on transport properties of La0.67Ca0.33MnO3 films grown on vicinal cut substrates

La_0.67Ca_0.33MnO₃ thin films have been grown on 10°, 15°, and 20° vicinal cut SrTiO₃ (1 0 0) substrates by pulse laser deposition. The single phase and the least textured growth have been studied by X-ray diffraction analysis. The post annealing effect with high temperature and high oxygen pressure on the transport properties of films has been investigated by resistance versus temperature measurements. Films with post annealing show large enhancement of metal-insulator transition temperature T_p about 20-30 K towards higher temperature and obvious decrease of resistance, which is attributed to the refilling of oxygen, the change of Mn-O-Mn angle and the improvement of crystallinity by the post annealing effect. Specially, film on 20° vicinal cut substrate exhibits the biggest range gap of peak resistance drop, which may originate from more defects caused by steps at this tilt angle and many of these defects are removed after post annealing.     

Roles of humidity and heat treatment temperatures on YBa2Cu3O7-x coated conductors by trifluoroacetic acid-metal organic deposition process

YBa₂Cu₃O_7−x(YBCO) films were fabricated on an LAO substrate using the trifluoroacetic acid-metal organic deposition (TFA-MOD) method and the effects of the humidity and heat treatment temperatures on the microstructure, degree of texture and critical properties of the films were evaluated. In order to understand the combined effects of the humidity and the calcining and firing temperatures on critical properties, heat-treatment was performed at various temperatures with the other processing variables fixed. The films were calcined at 400–430 and fired at 750–800 °C in a 0–12.1% humidified Ar-O₂ atmosphere. The texture was determined by pole-figure analysis. The amount of the BaF₂ phase was effectively reduced and a sharp and strong biaxial texture was formed under a humidified atmosphere, which led to increased critical properties. In addition, the microstructure varied significantly with firing temperature but changed little with calcining temperature. The highest I_C of 40 A/cm-width, which corresponds to J_C value of 1.8 MA/cm², was obtained for the films fired at 775 (in 12.1% humidity) after calcining at 400–430 °C. It is likely that °C the highest I_C value is due to the formation of a more pure YBCO phase, c-axis grains, and a denser microstructure.     

Effect of tin on high-temperature sulphidation of Fe-Cr alloys in H2/H2S atmospheres

The high temperature sulphidation behaviour of Fe-46Cr-xSn (x = 0; 0.2; 0.5; 1; 2) alloys has been studied at temperatures of 1073, 1173 and 1273 K in H₂/H₂S mixtures with different sulphur vapour partial pressures of 10⁻¹, 10⁻³ and 10⁻⁵ Pa. Thermogravimetric studies in combination with scanning electron microscope (SEM), with energy dispersive spectrometer (EDS), and X-ray diffraction (XRD) techniques, have displayed a significant influence of the sulphur partial pressure on the composition and growth rate of the sulphide scale. The results have shown that addition of tin increases the sulphidation rate of Fe-46Cr alloys but not considerably (except at temperatures of 1073 and 1173 K combined with sulphur partial pressure of 10⁻⁵ Pa). The metallic core of the studied samples was enriched in tin and iron, moreover tin was found in the internal layer close to the metallic core as metallic Fe_xSn_y inclusions with tin concentrations of up to 12 at.%.     

Effects of lanthanum doping on the preferred orientation, phase structure and electrical properties of sol-gel derived Pb1-3x/2Lax (Zr0.6Ti0.4)O3 thin films

Pb_1-3x/2La_x (Zr_0.6Ti_0.4)O₃ thin films (0 ≤ x ≤ 0.08) were prepared on the Pt (1 1 1)/Ti/SiO₂/Si (1 0 0) substrates by a sol-gel method. The morphology, preferred orientation, phase structure, dielectric and ferroelectric properties of the films have been investigated. Our results show that lanthanum doping is favorable to enhance crystalline and obtain (1 0 0)-preferred orientation of the films. Meanwhile, it is suggested that the films undergo a structure change from “rhombohedral” phase to monoclinic phase as the lanthanum-doped content is increased to x ≈ 0.05. Results of dielectric properties and ferroelectric properties indicate that lanthanum doping contributes to improve film dielectric constant and dielectric loss while it brings about a striking decrease in remnant polarization value. Possible explanations for the variations of electrical properties have been discussed in terms of preferred orientation, phase structure and large lattice distortion.     

The magnetic, structure and mechanical properties of rapidly solidified (Nd7Y2.5)-(Fe64.5Nb3)-B23 nanocomposite permanent magnet

The Nd₇Y_2.5Fe_64.5Nb₃B₂₃ nanocomposite permanent magnets in the form of rods with 2 mm in diameter have been developed by annealing the amorphous precursors produced by copper mold casting technique. The phase evolution, structure, magnetic and mechanical properties were investigated with X-ray diffractometry, differential scanning calorimetry, electron microscopy, magnetometry and universal uniaxial compression strength techniques. The heat treatment conditions under which the magnets attained maximum magnetic and mechanical properties have been established. The results indicate that magnet properties are sensitive to grain size and volume content of the magnetic phases present in the microstructure. The composite microstructure was mainly composed of soft α-Fe (20-30 nm) and hard Nd₂Fe₁₄B (45-65 nm) magnetic phase grains. The maximum coercivity of 959.18 kA/m was achieved with the magnets annealed at 760 °C whereas the highest remanence of 0.57 T was obtained with the magnets treated at 710 °C. The optimally annealed magnets possessed promising magnetic properties such as _jH_c of 891.52 kA/m, B_r of 0.57 T, M_r/M_s = 0.68, (BH)_max of 56.8 kJ/m³ as well as the micro-Vickers hardness (H_v) of 1138 ± 20 and compressive stress (σ_f) of 239 ± 10 MPa.     

Crystal structures and magnetic properties of iron (III)-based phosphates: Na4NiFe(PO4)3 and Na2Ni2Fe(PO4)3

Crystal structures from two new phosphates Na₄NiFe(PO₄)₃ (I) and Na₂Ni₂Fe(PO₄)₃ (II) have been determined by single crystal X-ray diffraction analysis. Compound (I) crystallizes in a rhombohedral system (S. G: R-3c, Z = 6, a = 8.7350(9) Å, c = 21.643(4) Å, R₁ = 0.041, wR₂=0.120). Compound (II) crystallizes in a monoclinic system (S. G: C2/c, Z = 4, a = 11.729(7) Å, b = 12.433(5) Å, c = 6.431(2) Å, β = 113.66(4)°, R₁ = 0.043, wR₂=0.111). The three-dimensional structure of (I) is closely related to the Nasicon structural type, consisting of corner sharing [(Ni/Fe)O₆] octahedra and [PO₄] tetrahedra forming [NiFe(PO₄)₃]⁴⁺ units which align in chains along the c-axis. The Na⁺ cations fill up trigonal antiprismatic sites within these chains. The crystal structure of (II) belongs to the alluaudite type. Its open framework results from [Ni₂O₁₀] units of edge-sharing [NiO₆] octahedra, which alternate with [FeO₆] octahedra that form infinite chains. Coordination of these chains yields two distinct tunnels in which site Na⁺.The magnetization data of compound (I) reveal antiferromagnetic (AFM) interactions by the onset of deviations from a Curie-Weiss behaviour at low temperature as confirmed by Mössbauer measurements performed at 4.2 K. The corresponding temperature dependence of the reciprocal susceptibility χ⁻¹ follows a typical Curie-Weiss behaviour for T > 105 K. A canted AFM state is proposed for compound (II) below 46 K with a field-induced magnetic transition at H ≈ 19 kOe, revealed in the hysteresis loop measured at 5 K. This transition is most probably associated with a spin-flop transition.     

Pulsed laser deposition of anatase and rutile TiO2 thin films

The investigation deals with the preparation of both anatase and rutile thin films from a sintered rutile target of TiO₂ by pulsed laser ablation technique. Microstructural characterization of the sintered target was carried out using X-ray diffraction and AC impedance spectroscopy. Thin films of titania were deposited on (111) Si substrates at 673 K in the laser energy range 200-600 mJ/pulse at two different conditions: (i) deposition at 3.5 × 10^− 5 mbar of oxygen, and (ii) deposition at an oxygen partial pressure of 0.1 mbar. The influence of laser energy and oxygen addition on the film growth has been studied. X-ray diffraction analysis of the films indicated that the films are single phasic and nano crystalline. Titania films deposited in the energy range 200-600 mJ/pulse at a base pressure of 5 × 10^− 5 mbar are rutile with particle sizes in the range 5-10 nm, whereas the films formed at the oxygen partial pressure 0.1 mbar are anatase with particle sizes in the range 10-24 nm. In addition, at higher energies, a significant amount of particulates of titania are found on the surface of the films. The change in the microstructural features of the films as a function of laser energy and oxygen addition is discussed in relation with the interaction of the ablated species with the background gas.     

Structure, ferroelectric/magnetoelectric properties and leakage current density of (Bi0.85Nd0.15)FeO3 thin films

In this paper, we report on the structure, ferroelectric/magnetoelectric properties and improvement of leakage current density of (Bi_0.85Nd_0.15)FeO₃ (BNFO) thin films deposited on Pt(1 1 1)/Ti/SiO₂/Si substrates from the polymeric precursor method. X-ray patterns and Rietveld refinement indicated that BNFO thin films with a tetragonal structure can be obtained at 500 °C for 2 h in static air. Field emission scanning electron, atomic force and piezoelectric force microscopies showed the microstructure, thickness and domains with polarization-oriented vectors of BNFO thin films. Ferroelectric and magnetoelectric properties are evident by hysteresis loops. The magnetoelectric coefficient measurement was performed to show the magnetoelectric coupling behavior. The maximum magnetoelectric coefficient in the longitudinal direction was close to 12 V/cm Oe. Piezoresponse force microscopy micrographs reveal a polarization reversal with 71° and 180° domain switchings and one striped-domain pattern oriented at 45° besides the presence of some nanodomains with rhombohedral phase involved in a matrix with tetragonal structure. The cluster models illustrated the unipolar strain behavior of BNFO thin films. The leakage current density at 5.0 V is equal to 1.5 × 10⁻¹⁰ A/cm² and the dominant mechanism in the low-leakage current for BNFO thin films was space-charge-limited conduction.     

Physical properties of Gd7NiPd2 single crystal

Investigations of X-ray diffraction, electronic structure, dc-magnetization M(T), ac-magnetic susceptibility χ_ac(T) and magnetocaloric properties for the Gd₇NiPd₂ single crystal were performed. A single crystal of Gd₇NiPd₂ was grown by the Czochralski method from a levitating melt. Anomalies in χ_ac(T) and M(T)-curves establish that Gd₇NiPd₂ undergoes a long-range ferromagnetic-type ordering at T_C = 298 K, followed by a spin-reorientation below 135 K. The magnetization data indicate that there is an excess magnetic moment calculated per Gd³⁺ ions. The measured XPS valence band indicates the hybridization effect between Gd 5d, Ni 3d and Pd 4d states. The calculated values of entropy change ΔSm for the examined compound amount to −6.85 J/K kg for the a-axis and −6.49 for the c-axis at 7 T.     

Rhombohedral-monoclinic phase transition related to grain orientation in Zr-rich Pb(ZrxTi1−x)O3 thin films

Pb(Zr_xTi_1−x)O₃ thin films with mixed orientations of (1 1 1) and (1 0 0) were prepared on Pt/Ti/SiO₂/Si substrate by sol-gel technique. The compositions of PZT thin films are chosen as x = 0.55 and x = 0.58. Both of the compositions are in the rhombohedral phase region of the Pb(Zr_xTi_1−x)O₃ phase diagram, but the former is near the monoclinic phase existence region, and the latter is far from the monoclinic phase existence region. Rhombohedral-monoclinic phase transitions are reported in both of the thin films. The results show that the phase transition is related to the grain orientation. Phase transitions in the films are clearly identified: rhombohedral phase transforms to M_B phase in (1 1 1)-oriented grains, and rhombohedral phase transforms to M_A phase in (1 0 0)-oriented grains. The remnant polarization is determined by the content of (1 1 1)-oriented grains. It is shown that the remnant polarization is greater in the film with higher content of (1 1 1)-oriented grains.     

Effects of Mg substitution on the structural, optical, and electrical properties of CuAlO2 thin films

Mg-doped CuAlO₂ thin films are prepared by the chemical solution method. The XRD results show that the solid solubility of Mg species on Al sites in CuAlO₂ lattice is lower than 2 at.%. When less than 2 at.% of Mg is added to the CuAlO₂ film, the surface roughness of the films was reduced with Mg substitution. Moreover, the c-axis orientation of the films improves because the in-plane fusion between CuAlO₂ crystallites is hindered. Optical and electrical measurements show that substituting Al³⁺ in the films with Mg²⁺ increases both their transmittance in the visible region and their optical band gaps. As well, their electrical conductivity is enhanced. At 300 K, the conductivity of the 1 at.% Mg-doped sample is up to 5.2 × 10⁻³ S/cm. Thus, Mg-doped CuAlO₂ films may have potential applications as transparent conductive oxides.