Pulsed excimer laser ablation growth and characterization of Ba(Sn0.1Ti0.9)O3 thin films

Polycrystalline thin films of Ba(Sn_0.1Ti_0.9)O₃ were deposited on Pt coated silicon substrates by pulsed excimer laser ablation technique. The room temperature dielectric constant of the Ba(Sn_0.1Ti_0.9)O₃ films was 350 at a frequency of 100 kHz. The films showed a slightly diffused phase transition in the range of 275–340 K. The polarization hysteresis behavior confirmed the ferroelectric nature of the thin films. Remanent polarization (P_r) and saturation polarization (P_s) were 1.1 and 3.2 μC/cm², respectively. The asymmetric capacitance–voltage curve for Ba(Sn_0.1Ti_0.9)O₃ was attributed to the difference in the nature of the electrodes. Dispersion in both the real (ε′_r) and imaginary (ε″_r) parts of the dielectric constant at low frequencies with increase in temperature was attributed to space charge contribution in the complex dielectric constant.     

Ferroelectric polarization and resistive switching characteristics of ion beam assisted sputter deposited BaTiO3 thin films

In this work, 150 nm thick polycrystalline BaTiO₃ (BTO) films were deposited on Pt/TiO₂/SiO₂/Si substrate by ion beam assisted sputter deposition technique. The bias voltage dependent resistive switching (RS) and ferroelectric polarization characteristics of Au/BTO/Pt devices are investigated. The devices display the stable bipolar RS characteristics without an initial electroforming process. Fittings to current–voltage (I–V) curves suggest that low and high resistance states are governed, respectively, by filamentary model and trap controlled space charge limited conduction mechanism, where the oxygen vacancies act as traps. Presence of oxygen vacancies is evidenced from the photoluminescence spectrum. The devices also display P–V loops with remnant polarization (P_r) of 5.7 μC/cm² and a coercive electric field (E_c) of 173.0 kV/cm. The coupling between the ferroelectric polarization and RS effect in BTO films is demonstrated.     

Effect of Li-substitution on piezoelectric and ferroelectric properties of (Bi0.92Na0.92−xLix)0.5Ba0.06Sr0.02TiO3 lead-free ceramics

New lead-free ceramics (Bi_0.92Na_0.92−xLi_x)_0.5Ba_0.06Sr_0.02TiO₃ have been fabricated by a conventional ceramic technique and their electrical properties have been studied. X-ray diffraction studies reveal that Li⁺, Ba²⁺ and Sr²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure. The partial substitution of Li⁺ for Na⁺ increases the remanent polarization P_r of the ceramics. Because of the large P_r and low coercive field E_c, the ceramics with x = 0.075–0.125 exhibit excellent piezoelectric properties: d₃₃ = 189–235 pC/N, k_p = 33.6–36.3% and k_t = 51.6–54.3%. The ceramics exhibit relaxor behaviors after the substitution of Li⁺ for Na⁺. Our results also suggest that polar and non-polar phases may coexist in the ceramics at temperatures above the depolarization temperature T_d.     

Ferroelectric properties of Pb(Zr1/2Ti1/2)O3–Pb(Zn1/3Nb2/3)O3 ceramics under compressive stress

Effects of compressive stress on the ferroelectric properties of ceramics in PZT–PZN system were investigated. The ceramics with a formula (1−x)Pb(Zr_1/2Ti_1/2)O₃–xPb(Zn_1/3Nb_2/3)O₃ or (1−x)PZT–(x)PZN (x = 0.1–0.5) were prepared by a conventional mixed-oxide method. The ferroelectric properties under the compressive stress of the PZT–PZN ceramics were observed at the stress levels up to 170 MPa using a compressometer in conjunction with a modified Sawyer–Tower circuit. It was found that with increasing compressive stress the area of the ferroelectric hysteresis (P–E) loops, the saturation polarization (P_sat), the remnant polarization (P_r), and the coercive field (E_c) decreased. These results were interpreted through the non-180° ferroelastic domain switching processes.     

Impedance spectroscopy and conduction mechanism of multiferroic (Bi0.6K0.4)(Fe0.6Nb0.4)O3

Polycrystalline (Bi_0.6K_0.4) (Fe_0.6Nb_0.4)O₃ material has been prepared using a mixed-oxide route at 950 °C. It was shown by XRD that at room temperature structure of the compound is of single-phase with hexagonal symmetry. Some electrical characteristics (impedance, modulus, conductivity etc.) were studied over a wide frequency (1 kHz–1 MHz) and temperature (25–500 °C) ranges. The Nyquist plot (i.e., imaginary vs real component of complex impedance) of the material exhibit the existence and magnitude of grain interior and grain boundary contributions in the complex electrical parameters of the material depending on frequency, input energy and temperature. The nature of frequency dependence of ac conductivity follows Joncher׳s power law, and dc conductivity follows the Arrhenius behavior. The appearance of P–E hysteresis loop confirms the ferroelectric properties of the material with remnant polarization (2P_r) of 1.027 µC/cm² and coercive field (2E_c) of 16.633 kV/cm. The material shows very weak ferromagnetism at room temperature with remnant magnetization (2M_r) of 0.035 emu/gm and coercive field (2H_c) of 0.211 kOe.     

Development of perovskite and phase transition in lead cobalt niobate modified lead zirconate titanate system

Ferroelectric lead zirconate titanate–lead cobalt niobate ceramics with the formula (1 − x)Pb(Zr_1/2Ti_1/2)O₃–xPb(Co_1/3Nb_2/3)O₃ where x = 0.0–0.5 were fabricated using a high temperature solid-state reaction method. The formation process, the structure and homogeneity of the obtained powders have been investigated by X-ray diffraction method as well as the simultaneous thermal analysis of both differential thermal analysis (DTA) and thermogravimetry analysis (TGA). It was observed that for the binary system (1 − x)Pb(Zr_1/2Ti_1/2)O₃–xPb(Co_1/3Nb_2/3)O₃, the change in the calcination temperature is approximately linear with respect to the PCoN content in the range x = 0.0–0.5. In addition, X-ray diffraction indicated a phase transformation from a tetragonal to a pseudo-cubic phase when the fraction of PCoN was increased. The dielectric permittivity is remarkably increased by increasing PCoN concentration. The maximum value of remnant polarization P_r (25.3 μC/cm²) was obtained for the 0.5PZT–0.5PCoN ceramic.     

Preparation of YBCO film for microwave filter using a hybrid route

Pure (0 0 l)-textured CeO₂ buffer layers were deposited on single crystal r-plane Al₂O₃ (1–102) substrate by a hybrid process which was combined with magnetron sputtering for the seed layer and metal–organic deposition for the subsequent layer. Strongly c-axis oriented YBCO films were deposited on the CeO₂ buffered r-cut Al₂O₃ (1–102) substrates. Atomic force microscope and scanning electronic microscopy results show that the prepared buffers and YBCO films are relatively dense and smooth. The critical current of the YBCO films exceeds 1.5 MA/cm² at 77 K with the superconducting transition temperature of 90 K. The surface resistivity is as below as 14 μΩ at 1 GHz frequency. The results demonstrate that the hybrid route is a very promising method to prepare YBCO films for microwave application, which can combine the sputtering advantage for preparing of highly c-axis oriented CeO₂ buffer layers and the advantages of metal–organic deposition with rapid processing, low cost and easy preparation of large-area YBCO films.     

The improvement in dielectric and ferroelectric performance of PZT–PZN ceramics by thermal treatment

Pyrochlore-free lead zirconate titanate – lead zinc niobate ceramics have been systematically investigated in the as-sintered condition as well as after annealing. The ceramics were characterized by dielectric spectroscopy and Sawyer–Tower polarization (P–E) measurements. The powders of Pb[(Zr_1/2Ti_1/2)_(1−x)–(Zn_1/3Nb_2/3)_x]O₃, where x = 0.1, 0.3 and 0.5 were prepared using the columbite–(wolframite) precursor method. The general trend seems to indicate that the annealed samples become more normal-ferroelectric-like behavior as opposed to the relaxor-ferroelectric-like behavior observed in the as-sintered state. The as-sintered 0.9PZT–0.1PZN ceramic exhibited weak relaxor-ferroelectric behavior, with a relatively low dielectric constant maximum of 14,000 measured at 1 kHz. Annealing resulted in a transition to normal-ferroelectric-like behavior, a shift in the dielectric maximum temperature from 360 °C to 350 °C, and a dramatic increase in the dielectric constant at 1 kHz to a maximum value of 35,000 for the longer anneal. After thermal annealing at 900 °C for one week a strong enhancement of remanent polarization (P_r) was observed.     

Recombination luminescence of CsI(Tl) under electron pulse irradiation

The luminescence kinetics of CsI(Tl) exposed to an electron pulse irradiation (E_e = 250 keV, t_1/2 = 10 ns, j = 2 ÷ 160 mJ/cm²) has been studied. It has been discovered that the slow emission rise is due to hole V_k–Tl⁰ recombination luminescence at temperature from 100 to 160 K and electron–V_kA recombination, where electrons released from single Tl⁰ at temperature from 180 to 300 K. The effect of Tl concentration on both processes has been investigated.     

Room temperature ferroelectricity of tetragonally strained SrTiO3 thin films on single crystal Rh substrates

Epitaxial SrTiO₃ thin films were deposited on single crystalline Rh substrates by pulsed laser deposition. The tetragonally stained structure of the SrTiO₃ thin films with a c/a ratio of 1.04 was confirmed by x-ray diffraction experiments. The SrTiO₃ thin films exhibited good ferroelectric properties with a high remanent polarization (2P_r) of 8 μC/cm² and a canonical ferroelectric piezoresponse hysteresis loop at room temperature. We estimated a high activation electric field of about 6.4 MV/cm for domain wall creeping. This activation electric field is higher than that of typical ferroelectric materials such as PbTiO₃.     

Effect of oxygen partial pressure on the structure and properties of Cu–Al–O thin films

We have studied the electrical and optical properties of Cu–Al–O films deposited on silicon and quartz substrates by using radio frequency (RF) magnetron sputtering method under varied oxygen partial pressure P_O. The results indicate that P_O plays a critical role in the final phase constitution and microstructure of the films, and thus affects the electrical resistivity and optical transmittance significantly. The electrical resistivity increases with the increase of P_O from 2.4 × 10^?4 mbar to 7.5 × 10^?4 mbar and afterwards it decreases with further increasing P_O up to 1.7 × 10^?3 mbar. The optical transmittance in visible region increases with the increase of P_O and obtains the maximum of 65% when P_O is 1.7 × 10^?3 mbar. The corresponding direct band gap is 3.45 eV.     

Dielectric and ferroelectric properties of 0.8PZT–0.2PCN ceramics under sintering conditions variation

The influences of sintering conditions on electrical properties of the 0.8Pb(Zr_1/2Ti_1/2)O₃–0.2Pb(Co_1/3Nb_2/3)O₃ ceramics have been investigated with sintering temperatures of 1175, 1200, 1225, and 1250 °C and dwell times for 2, 6, and 10 h. The crystal structure of dense specimens showed coexistence between tetragonal, rhombohedral and pseudo cubic phases in all sintering temperatures, while tetragonal-rich phase appeared with increasing dwell times. A maximum dielectric constant was observed at sintering condition of 1200 °C for 2 h, while the transition temperature slightly increased with increasing dwell time. All ceramics also showed diffused phase transition behaviors with a minimum diffusivity at sintering condition of 1200 °C for 2 h. In addition, the polarization–electric field (P–E) hysteresis loops of the ceramic systems also changed significantly with sintering conditions. Interestingly, the ferroelectric parameters; remnant polarization (P_r) and loop squareness (R_sq) tended to increase with increasing sintering temperatures and dwell times.     

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.     

Large spontaneous Hall angle in [Co,CoFe/Pt] multilayer

We have quantitatively investigated the Hall effect in [Co, CoFe/Pt] multilayer films. The [Co, CoFe/Pt] multilayers exhibit large spontaneous Hall resistivity (ρ_H) and Hall angle (ρ_H/ρ). Even though the Hall resistivity in [Co, CoFe/Pt] multilayer films (2.7–4 × 10⁻⁷ Ω cm) is smaller than that of amorphous RE–TM alloy films which show large spontaneous Hall resistivity (<2 × 10⁻⁶ Ω cm), the Hall angle of multilayer (6–8%) is almost twice than that in amorphous rare earth–transition metal alloy films (∼3%). The Hall angle provides evidence of the effects of the exchange interaction of the Hall scattering. The exchange is between conduction electron spins and the localized spins of the transition metal. The large Hall angle of [Co, CoFe/Pt] multilayer can be considered due to the high spin polarization and high Curie temperature of Co and CoFe transition metal layers. Even though the role of interfaces and surfaces in the magnetic properties of multilayer films may dominate that of the bulk, the Hall effects in [Co, CoFe/Pt] multilayer may be mainly dominated by the bulk effect.     

Optical delay with spectral hole burning in Doppler-broadened cesium vapor

The full frequency dependence of the optical delay in the Cs D₁ (6 ²S_1/2 ? 6 ²P_1/2) line has been observed, including all four hyperfine split components. Pulse delays of 1.6 ns to 24.1 ns are obtained by scanning across the hyperfine splitting associated with the lower ²S_1/2 state. Optical control of pulse delays in cesium vapor was demonstrated by pumping the D₂ (6 ²S_1/2 ? 6 ²P_3/2) transition and observing resulting holes in the D₁ delay spectrum. For a pump at four times the saturation intensity, the pulse delays are reduced by a maximum of 78% in a narrow region of 110 MHz. The frequency dependence of the delays of the probe laser in the vicinity of the spectral holes agrees with a Kramers–Kronig model prediction.     

Luminescence study of self-trapped excitons in CdMoO4

Luminescence properties of CdMoO₄ crystals have been investigated in a wide temperature range of T=5–300 K. The luminescence-excitation spectra are examined by using synchrotron radiation as a light source. A broad structureless emission band appears with a maximum at nearly 550 nm when excited with photons in the fundamental absorption region (<350 nm) at T=5 K. This luminescence is ascribed to a radiative transition from the triplet state of a self-trapped exciton (STE) located on a (MoO₄)^2? complex anion. Time-resolved luminescence spectra are also measured under the excitation with 266 nm light from a Nd:YAG laser. It is confirmed that triplet luminescence consists of three emission bands with different decay times. Such composite nature is explained in terms of a Jahn–Teller splitting of the triplet STE state. The triplet luminescence at 550 nm is found to be greatly polarized in the direction along the crystallographic c axis at low temperatures, but change the degree of polarization from positive to negative at T>180 K. This remarkable polarization is accounted for by introducing further symmetry lowering of tetrahedral (MoO₄)^2? ions due to a uniaxial crystal field, in addition to the Jahn–Teller distortion. Furthermore, weak luminescence from a singlet state locating above the triplet state is time-resolved just after the pulse excitation, with a polarization parallel to the c axis. The excited sublevels of STEs responsible for CdMoO₄ luminescence are assigned on the basis of these experimental results and a group-theoretical consideration.     

Analysis of current voltage characteristics of MWIR homojunction photodiodes for uncooled operation

Dark currents n⁺/ν/p⁺ Hg_0.69Cd_0.31Te mid wave infrared photodiodes were measured at room temperature. The diodes exhibited negative differential resistance at room-temperature, but with increasing leakage currents as a function of reverse bias. The current–voltage characteristics were simulated and fitted by incorporating trap assisted tunneling via traps and Shockley–Read–Hall generation recombination process due to dislocations in the carrier transport equations. The thermal suppression of carriers was simulated by taking energy level of trap (E_t), trap density (N_t) and the doping concentrations of n⁺ and ν regions as fitting parameters. Values of E_t and N_t were 0.78E_g and ～6–9 × 10¹⁴ cm^?3 respectively for most of the diodes. Variable temperature current voltage measurements on variable area diode array (VADA) structures confirmed the fact that variation in zero bias resistance area product (R₀A) is related to g–r processes originating from variation in concentration and kind of defects that intersect a junction area.     

Bulk and surface properties of ionic salt CsH5(PO4)2

We report a comparative study of transport and thermodynamic properties of single-crystal and polycrystalline samples of the ionic salt CsH₅(PO₄)₂ possessing a peculiar three-dimensional hydrogen-bond network. The observed potential of electrolyte decomposition ≈ 1.3 V indicates that the main charge carriers in this salt are protons. However, in spite of the high proton concentration, the conductivity appears to be rather low with a high apparent activation energy E_a ≈ 2 eV, implying that protons are strongly bound. The transport anisotropy though is not large, correlates with the crystal structure: the highest conductivity is found in the [001] direction (σ_130 °C ∼ 5.6 × 10^− 6 S cm^− 1) while the minimal conductivity is in the [100] direction (σ_130 °C ∼ 10 ⁻⁶ S cm^− 1). The conductivity of polycrystalline samples appears to exceed the bulk one by 1–3 orders of magnitude with a concomitant decrease of the activation energy (E_a ≈ 1.05 eV), which indicates that a pseudo-liquid layer with a high proton mobility is formed at the surface of grains. Infrared and Raman spectroscopy used to study the dynamics of the hydrogen-bond system in single-crystal and polycrystalline samples have confirmed the formation of such a modified surface layer in the latter. However, no bulk phase transition into the superionic disordered phase is observed in CsH₅(PO₄)₂ up to the melting point T_melt ∼ 151.6 °C, in contrast to its closest relative compound CsH₂PO₄.     

Enhanced ionization of perpendicularly aligned H2 in intense laser fields

Using three-dimensional classical ensembles, we have investigated the enhancement of double ionization of perpendicularly aligned H₂ molecules by a 800 nm laser pulse with intensity ranging from 1 × 10¹⁴ W/cm² to 6 × 10¹⁴ W/cm². The simulated results show that double ionization probability of H₂ strongly depends on R and reaches a maximum at an intensity independent critical distance R_C ≈ 5 a.u. Furthermore, the enhancement of double ionization is more pronounced in the cases of weaker or stronger fields. These results, a well indication of the influence of molecular structures and laser–molecule interactions on double ionization of diatomic molecules, are analyzed in detail and qualitatively explained based on the field-induced barrier suppression model and back analysis.     

XRD and EPR structural investigation of some zinc borate glasses doped with iron ions

Glasses in the system xFe₂O₃·(100?x) [45ZnO·55B₂O₃] (0≤x≤10 mol%) have been prepared by melting at 1200 °C and rapidly cooling at room temperature. The obtained samples were submitted to an additional thermal treatment at 570 °C for 12 h in order to relax the glass structure as well as to improve the local order. The as cast and heat treated samples were investigated using X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) measurements. The XRD patterns of all the studied samples show their vitreous nature. Structural modifications occurring in the heat treated samples compared to the untreated ones have been pointed out. EPR spectra of untreated and heat treated samples revealed resonance absorptions centered at g≈2.0, g≈4.3 and g≈6.4. The compositional variation of the line intensity and linewidth of the absorptions from g≈4.3 and g≈2.0 have been interpreted in terms of the variation in the concentration of the Fe³⁺ ions and the interaction between the iron ions. The EPR spectra of the untreated samples containing 5 mol% Fe₂O₃ have been studied at different temperatures (110–290 K). The line intensity of the resonance signals decreases with increase in temperature whereas the linewidth is found to be independent of temperature. It was also found that the temperature variation of reciprocal line intensity obeys the Boltzmann law.