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Perpendicular magnetic anisotropy (PMA) in magnetic thin films has attracted much attention due to its potential applications in spintronics devices. Here, we report the continuous strain-mediated PMA in epitaxial (111) CoFe 2 O 4 (CFO) thin films grown on (0001) LiTaO 3 substrates. A large variation in lattice strain (∼ 0.9 %) in a continuous way is realized in the CFO thin films by changing substrate temperature during deposition due to the difference in the thermal expansion coefficient between CFO and LiTaO 3. As a result, the PMA of the (111) CFO thin films can be continuously mediated by the strain with uniaxial magnetic anisotropy energy in the range of 0.12 - 14.69 × 10 6 erg/cm 3. Furthermore, the strain as well as the consequent PMA in the (111) CFO thin films can be maintained within the thickness of 25–205 nm, which is consistent with the scenario of the magnetoelastic effect. Our results reveal that the CFO/LiTaO 3 system can be regarded as an ideal platform to realize robust PMA and its continuous strain tuning in the (111) CFO thin films by virtue of strain-induced magnetic anisotropy. [ABSTRACT FROM AUTHOR]

Recent experimental results showed that an electron gas in an asymmetrical double barrier heterostructure can be effectively cooled down under resonant tunneling condition, thus leading to the realization of an electronic cooler. The cooling process is a multi-parameters phenomenon and it is desirable to handle this problem through a reasonably simple approach, in order to understand the role of each parameter. To this end, we present a rate equation modeling of the electron cooling. We model the resonant tunnel injection of the electrons in the well and their thermionic emission assisted by Longitudinal Optical (LO) phonons absorption and emission. The influence of several parameters on the electronic temperature is discussed. This simple model compares rather well to the predictions of non-equilibrium Green function approach and to the experiments. [ABSTRACT FROM AUTHOR]

CaBi2Nb2O9 ceramics were fabricated via the solid-state reaction method by the addition of B2O3 as a sintering aid at lower sintering temperatures than that prepared without B2O3. Ferroelectricity, piezoelectricity, and resistivity can be still greatly enhanced when the sintering temperature is decreased from 1075 °C for CaBi2Nb2O9 to 975 °C for CaBi2Nb2O9–0.25 wt. % B2O3. The high resistivity and weak c-axis texture ensure that the ceramics can be fully polarized in a high enough electric field. The increased spontaneous ferroelectric and remanent polarization result in a significant enhancement in the piezoelectric properties of CaBi2Nb2O9–0.25 wt. % B2O3. The sample sintered at 975 °C possesses a high piezoelectric coefficient d33 of ∼15.0 pC/N and resistivity of 1.4 × 106 Ω cm (at 600 °C) along with a high ferroelectric Curie temperature TC of ∼954 °C. This work is beneficial for the preparation of high-temperature piezoelectric ceramics with excellent electrical properties at a low sintering temperature. [ABSTRACT FROM AUTHOR]

We report a cluster spin glass behavior, multiferroicity and magnetoelectric (ME) effects in the single crystals of 0.46 BiFeO 3 -0.54 Bi 0.5 (K 0.31 Na 0.69) 0.5 TiO 3. The crystals are found to possess a pseudo-cubic structure with a weak tetragonal distortion and show ferroelectricity at room temperature with a ferroelectric curie temperature of T C ∼ 630.1 K. The cluster spin glass state in the crystal is evidenced by detailed dc and ac magnetic experiments, including thermo-remnant magnetization, aging effect, memory effect, etc. Magnetodielectric effects, poling enhanced magnetism, and electric field induced acceleration of magnetization relaxation in the spin glass state are observed and ascribed mainly to spin-lattice coupling. These results might suggest an effective route to improve ferromagnetism and ME effects by constructing a spin glass state in BiFeO 3 -based antiferromagnetic multiferroics. [ABSTRACT FROM AUTHOR]

Transparent conducting oxides (TCOs), combining the mutually exclusive functionalities of high electrical conductivity and high optical transparency, lie at the center of a wide range of technological applications. The current design strategy for n-type TCOs, making wide bandgap oxides conducting through degenerately doping, obtains successful achievements. However, the performances of p-type TCOs lag far behind the n-type counterparts, primarily owing to the localized nature of the O 2 p -derived valence band (VB). Modulation of the VB to reduce the localization is a key issue to explore p-type TCOs. This Perspective provides a brief overview of recent progress in the field of design strategy for p-type TCOs. First, the introduction to principle physics of TCOs is presented. Second, the design strategy for n-type TCOs is introduced. Then, the design strategy based on the concept of chemical modulation of the valence band for p-type TCOs is described. Finally, through the introduction of electron correlation in strongly correlated oxides for exploring p-type TCOs, the performance of p-type TCOs can be remarkably improved. The design strategy of electron correlation for p-type TCOs could be regarded as a promising material design approach toward the comparable performance of n-type TCOs. [ABSTRACT FROM AUTHOR]

In this paper, we report the generation of Ar plasmas by using an electron beam passing through a diamond window. It is observed that plasma brightness varies with the electron beam energy and the duty cycle. The transmission properties of the electrons in a 5-μm-thick diamond film are investigated by both experiments and Monte Carlo simulation. The measured transmittance of the electron beam increases from 20% to 80% by increasing the incident energy of the electron beam on the diamond window from 32.5 keV to 50 keV. The diamond window was checked using a CCD camera and SEM after plasma generation experiments. It is found that the focused electron beam passes through the central region of the diamond window, from the incident plane to the exit plane, and the cross section of the diamond window gradually changes from textured morphology to a featureless structure, indicating that the main energy loss of electrons in the diamond window occurs in the later stage of the transmitting journey. The interaction between the electron and the diamond causes the structure change of the diamond. The simulated transmittances of the electron beam with respect to its incident energy are in agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Resistive random access memories with self-rectifying behaviors, in which the sneak-path issue in passive crossbar arrays can be alleviated without additional access devices, have been investigated recently. The applications of p-type transparent oxide semiconductors as a memory medium will pave the way for realizing all-transparent memories and integrating on the complementary metal-oxide-semiconductor devices. Here, Ag-based p-type delafossite AgCr 1 − x Mg x O 2 thin films were prepared onto n-Si wafers to investigate the resistive switching (RS) performance. Bipolar RS with self-rectifying behaviors were observed in the Au / AgCr 0.92 Mg 0.08 O 2 / n − Si sandwich structure, showing improved rectification ratio, On/Off ratio, and switching durability. The change of the oxygen vacancy concentration resulting from Mg doping plays a key role in determination of the RS. The intrinsic rectifying behavior in the low resistance state is attributed to the existence of a Schottky-like barrier in the AgCr 1 − x Mg x O 2 / n − Si p-n heterojunction. The RS behavior originates from the modification of the barrier, which is induced by the trapping/detrapping of charge carriers in oxygen vacancies at the interface. The results will provide novel RS devices based on p-type transparent delafossite thin films with a self-rectifying feature. [ABSTRACT FROM AUTHOR]

We have investigated the structural, magnetic, and thermal transport properties of Co 1 − x Ni x Cr 2 O 4 (0 ≤ x ≤ 1) to check the evolution of structural and magnetic properties upon the introduction of the Jahn–Teller active ion Ni 2 + . The x ≤ 0.95 samples can be indexed with a cubic lattice with the space group Fd3m, whereas the x = 0.98 sample undergoes a cubic-tetragonal structural transition, and the x > 0.98 samples belong to a tetragonal lattice with the space group I41-amd at room temperature. All these samples undergo a paramagnetic–ferrimagnetic transition at T C. A spiral ferrimagnetic ordering at T S is suppressed when the amount of Ni is increased, and the abnormal magnetization at T S for the x = 0.6 and 0.8 samples is not pronounced. The measurements of magnetic hysteresis reveal that the x = 0.4 and 0.6 samples possess a rather large coercivity H C compared with the x = 0 and 1 samples. Specifically, the x = 0.4 and 0.6 samples undergo a short-range antiferromagnetic transition above T C. The magnetic properties of the Ni-doped samples can be understood in terms of the magnetic structure model consisting of longitudinal and transverse magnetic sublattices. [ABSTRACT FROM AUTHOR]

The influence of growth rate and substrate temperature on the two-dimensional electron gas (2DEG) of LaVO 3 / SrTiO 3 interfaces has been investigated. It is found that both growth rate and substrate temperature can modulate the structural and electrical properties of the LaVO 3 / SrTiO 3 interfaces through oxygen substrate-to-film transfer. When the LaVO 3 thin films are deposited at a low substrate temperature and a high growth rate, the LaVO 3 / SrTiO 3 interfaces exhibit weak oxygen substrate-to-film transfer and resultant low density of oxygen vacancies in the SrTiO 3 substrate. As a result, the intrinsic effect (polar discontinuity and/or dielectric screening) dominates the interfacial conduction, while the oxygen vacancies play a minor role. In contrast, the oxygen substrate-to-film transfer (oxygen vacancies in the SrTiO 3 substrate) can be enhanced (increased) by depositing the LaVO 3 thin films at lower growth rates and/or higher substrate temperatures. In this case, the contribution of oxygen vacancies to the interfacial conduction would prevail over the intrinsic effect. Our results elucidate the origins of 2DEG in the LaVO 3 / SrTiO 3 interfaces and may also be important for other SrTiO 3 -based heterointerfaces. [ABSTRACT FROM AUTHOR]

A ferroelectric film with slim polarization-electric (P-E) hysteresis loops, showing small remanent polarization (P r) and large saturated polarization (P s), is desired to obtain high recoverable energy density (U r e ) and efficiency (η) in thin film capacitors. Here, small P r and large P m values are achieved in BaBi 4 Ti 4 O 15 thin films through modulating film grain size. A large U r e of 44.3 J / cm 3 as well as a high η of 87.1% is obtained. In addition, the derived BaBi 4 Ti 4 O 15 thin films show excellent energy storage performance in wide frequency range, thermal stability, and fatigue endurance. These results suggest that BaBi 4 Ti 4 O 15 films can be considered as a candidate for dielectric energy storage capacitors, and the route through grain size optimization is a promising strategy to improve the capacitive performance of ferroelectric materials. [ABSTRACT FROM AUTHOR]

La2Mo2O9 (LMO) thin films have been deposited on Pt/Ti/SiO2/Si substrates by pulsed laser deposition and the resistive switching (RS) characteristics of the Au/LMO/Pt devices has been investigated. The Au/LMO/Pt devices show excellent unipolar RS characteristics with high resistance ratio between high resistance state and low resistance state (LRS), good endurance, and retention performances. The results of temperature dependence of resistance and x-ray photoelectron spectroscopy suggest that the observed RS characteristics can be explained by the formation and rupture of conducting filaments composed of oxygen vacancies. Furthermore, the plot of the reset current (IR) as a function of the third harmonic coefficient (B0) caused by Joule heating during the reset process shows scaling behavior with a power law of IR∝ B0-δ. The IR and reset power (PR) can also be scaled to the resistance in LRS (R0), i.e., IR(PR) ∝ R0-α(β). The observed scaling behaviors indicate the importance of the Joule heating for the RS characteristics of Au/LMO/Pt devices. These results demonstrate the potential application of LMO thin film in a nonvolatile memory device. [ABSTRACT FROM AUTHOR]

In this paper, a simple model was developed to predict the dynamics of a spark-induced bubble under different ambient pressures. This work helps in developing a deep-towed plasma sparker, as the model can predict the dynamics of bubbles subjected to very high ambient pressures (about 20 MPa) which normally are difficult to obtain experimentally. Experimental results indicate that the maximum bubble radius for a fixed discharge energy decreases as a power-law function of the ambient pressure up to 1.0 MPa; the bubble period also decreases quickly with increasing ambient pressure. For a constant value of the ratio of bubble energy to discharge energy, the modeling results for both maximum radius and bubble period are in good agreement with the experimental results. Both sets of results indicate that the bubble period is proportional to the maximum radius under different ambient pressures. [ABSTRACT FROM AUTHOR]

We investigate the structural, magnetic, and magnetodielectric properties of DyMn0.5Cr0.5O3. The sample can be indexed with an orthorhombic phase with B site disordered space group Pbnm. The valence state of both Mn and Cr ions are suggested to be +3 based on the results of x-ray photoelectron spectroscopy. Two thermally excited dielectric relaxation at temperatures TN23+/Cr3+ ions associated with electron hopping between them are observed. The absence of any noticeable magnetoresistance effect (MR<0.5%) demonstrates that the observed magnetodielectric effect is an intrinsic behavior. These results suggest that DyMn0.5Cr0.5O3 is a magnetodielectric compound, whose dielectric properties are dependence of the applied magnetic field, which exhibits such effects near room temperature and holds great promise for future device applications. [ABSTRACT FROM AUTHOR]

We prepared V-doped Bi6Fe2Ti3O18 thin films on Pt/Ti/SiO2/Si (100) substrates by using a chemical solution deposition route and investigated the doping effect on the microstructure, dielectric, leakage, and ferroelectric properties of Bi6Fe2Ti3O18 thin films. The Bi5.97Fe2Ti2.91V0.09O18 thin film exhibits improved dielectric properties, leakage current, and ferroelectric properties. The incorporation of vanadium resulted in a substantially enhanced remnant polarization (2Pr) over 30μC/cm2 in Bi5.97Fe2Ti2.91V0.09O18 thin film compared with 10μC/cm2 in Bi6Fe2Ti3O18 thin film. It is demonstrated that the improved properties may stem from the improvement of crystallinity of the films with the contribution of suppressed oxygen vacancies and decreased mobility of oxygen vacancies caused by the V-doping. The results will provide a guidance to optimize the ferroelectric properties in Bi6Fe2Ti3O18 thin films by chemical solution deposition, which is important to further explore single-phase multiferroics in the n=5 Aurivillius thin films. [ABSTRACT FROM AUTHOR]

Single-phase polycrystalline SrBi5Ti4CrO18 and SrBi5Ti4Cr0.5Co0.5O18 were synthesized by a modified Pechini method. Both samples have an orthorhombic structure with the space group B2cb. The valence state of Cr is suggested to be +3 and the Co ions exist in the form of Co2+ and Co3+ based on the results of x-ray photoelectron spectroscopy. The sample SrBi5Ti4Cr0.5Co0.5O18 exhibits the paramagnetic state, whereas SrBi5Ti4Cr0.5Co0.5O18 undergoes a ferrimagnetic transition at 89K originating from the antiferromagnetic coupling of Cr-based and Co-based sublattices. In addition, SrBi5Ti4Cr0.5Co0.5O18 shows a typical spin-glass behavior below 89K with zν =6.02 and τ0=(1.75±0.33)x10-14 s as evidenced by the results of the frequency dependence of ac susceptibility and magnetic relaxation measurements. In particular, both the dielectric constant and dielectric loss of SrBi5Ti4Cr0.5Co0.5O18 exhibit the characteristics of dielectric relaxation around 89K with the activation energy of (0.14±0.02) eV, which can be ascribed to the electron hopping of Co2+-VO-Co3+ through the bridging oxygen vacancies. [ABSTRACT FROM AUTHOR]

The transition-metal dichalcogenides provided a good platform for the investigation of superconductivity and charge-density-wave (CDW) order. A series of high quality 6R-TaS2-xSex (0≤x≤1.6) single crystals were fabricated by the chemical-vapor-transport method and systematically investigated. The CDW becomes more stable with more S being replaced by Se, which causes expansion of the lattice, and produces a negative chemical pressure in the system. The x dependence of the onset of superconductivity (Tconset) shows a dome-like shape with a maximum of Tconset=2.7K at x=0.8. The magnetic susceptibility (4πχ) of x=0.8 under the hydrostatic pressure P=1.0 GPa was also measured, which showed a small increase of Tc and superconducting volume fraction. A pronounced peak effect was observed in the isothermal hysteresis with magnetic field (H) parallel to c-axis, which may arise from the quenching inhomogeneity and/or the stacking disorder. Finally, the evolution of the electronic states tuned by Se-doping was summarized in the phase diagram. [ABSTRACT FROM AUTHOR]

We investigate the structural, magnetic, dielectric properties, and scaling behaviors of Aurivillius compounds Bi6-x/3Fe2Ti3-2x(WCo)xO18 (0 ⩽ x ⩽ 0.15). The room-temperature weak ferromagnetism is observed for the W/Co co-doped samples. The results of the dielectric constant εr, complex impedance Z", the dc conductivity σdc, and hopping frequency fH manifest that the dielectric relaxation of the x = 0 sample and the doped samples in the dielectric anomaly region (450-750 K) can be ascribed to the trap-controlled ac conduction around the doubly ionized oxygen vacancies and the localized hopping process of oxygen vacancies, respectively. The scaling behaviors reveal that the dynamic process of both electrons in the x = 0 sample and oxygen vacancies in the doped samples is temperature independent. The ferroelectric Curie-temperature Tc decreases slightly from 973K to 947K with increasing the doping level of W/Co. In addition, the dielectric loss exhibits a dielectric relaxation above 800K with the rather large activation energies (1.95 eV ⩽ Ea ⩽ 2.72 eV). [ABSTRACT FROM AUTHOR]

Cuprous oxide films have been deposited on quartz substrates by a sol-gel method under various annealing temperatures. The X-ray diffraction analysis and Raman scattering show that all the films are of pure Cu2O phase. From comparison of photoluminescence with 488 and 325 nm laser excitations, the electronic transition energies and intensities present the annealing-temperature dependent behavior. The electronic band structures of the Cu2O film annealed at 800 °C, especially for the contribution of exciton series and high energy transitions, have been investigated by temperature dependent transmittance. The extracted refraction index and the high frequency dielectric constant both abruptly decrease until the temperature rises up to 100 K. Six transitions can be clearly identified and the red shift trend of Eo3-Eo5 transition energies with increasing the temperature can be found. Moreover, the anomalous behavior takes place at about 200K from the Eo6 transition. The singularities indicate that the change in the crystalline and electronic band structure occurs as the temperature near 100K and 200K for the film. [ABSTRACT FROM AUTHOR]

We have successfully synthesized the Aurivillius compounds SrBi5Ti4MnO18 and SrBi5Ti4Mn0.5 Co0.5O18 using a modified Pechini method. Both samples have an orthorhombic structure with the space group B2cb. The valence state of Mn is suggested to be +3 and the doped Co ions exist in the form of Co2+ and Co3+ based on the results of x-ray photoelectron spectroscopy. The sample SrBi5Ti4MnO18 exhibits a dominant paramagnetic state with the existence of superparamagnetic state as evidenced by the electron paramagnetic resonance results, whereas SrBi5Ti4Mn0.5Co0.5O18 undergoes a ferrimagnetic transition at 161K originating from the antiferromagnetic coupling of Co-based and Mn-based sublattices, and a ferromagnetic transition at 45K arising from the Mn3+-O-Co3+ (low spin) interaction. The sample SrBi5Ti4Mn0.5Co0.5O18 exhibits two dielectric anomalies. One corresponds to a relaxor-like dielectric relaxation which follows the Vogel-Fulcher function and the other dielectric relaxation obeys the Arrhenius law arising from the collective motion of oxygen vacancies. In addition, the sample SrBi5Ti4Mn0.5Co0.5O18 exhibits a magnetodielectric effect caused by the Maxwell-Wagner effect because of the conductivity of the sample. This is demonstrated by the fact that the activation energy in dielectric loss process is close to that for dc conductivity and the magnetodielectric effect is sensitive to the measured frequency. [ABSTRACT FROM AUTHOR]

We investigate the magnetic, dielectric, and magnetodielectric properties of rare-earth-substituted Aurivillius phase Bi6Fe1.4Co0.6Ti3O18. The room-temperature ferromagnetic behavior is observed in all samples, and the rare-earth-substituted samples exhibit an enhanced magnetization. The weak ferromagnetism can be ascribed to the spin canting of the antiferromagnetic coupling of the Fe-based and Co-based sublattices via Dzyaloshinsky-Moriya interaction. The dielectric loss of all samples exhibits two dielectric relaxation peaks corresponding to two different relaxation mechanisms. One relaxation process with Ea = 0.5 eV is related to the hoping process of oxygen vacancies and the other one with Ea = 1.6 eV can be ascribed to the intrinsic conduction. The Gd-doped sample exhibits a remarkable magnetodielectric effect (9.4%) at RT implying this Aurivillius phase may be the potential candidate for magnetodielectric applications. [ABSTRACT FROM AUTHOR]

Spin glass behavior and exchange bias effect have been observed in antiferromagnetic SrMn3O6-x nanoribbons synthesized via a self-sacrificing template process. The magnetic field dependence of thermoremanent magnetization and isothermal remanent magnetization shows that the sample is good correspondence to spin glass and diluted antiferromagnetic system for the applied field H<2 T and H>2 T, respectively. By detailed analysis of training effect using Binek's model, we argue that the observed exchange bias effect in SrMn3O6-x nanoribbons arises entirely from an interface exchange coupling between the antiferromagnetic core and spin glass shell. The present study is useful for understanding the nature of shell layer and the origin of exchange bias effect in other antiferromagnetic nanosystems as well. [ABSTRACT FROM AUTHOR]

We investigate the structural, magnetic, and electrical properties of charge-spin co-doped Ba1-x-yLax+yTi1-xMxO3 (M = Cr, Fe, and Co). For Ba1-x-yLax+yTi1-xCrxO3, carrier doping induces short-range ferromagnetism, and we find a large negative magnetoresistance in these compounds. However, for Ba1-x-yLax+yTi1-xMxO3 (M = Fe and Co), the magnetism does not seem to couple with the charge carriers. The transport properties of Ba1-x-yLax+yTi1-xCrxO3 can be described by a three-dimensional variable-range-hopping model that implies itinerant charge carriers, whereas almost all the samples of Ba1-x-yLax+yTi1-xMxO3 (M = Fe and Co) exhibit high resistivity. It is known that only the itinerant charge carriers can enhance the coupling of two isolated magnetic ions. Therefore, there is almost no long-range ferromagnetism in the studied system. [ABSTRACT FROM AUTHOR]

Oxidation of SiC with the incorporation of Na in the gate oxide was recently found to lead to significantly enhanced electron mobilities in the SiC inversion layer but the underlying mechanism has remained elusive. Here, we report a combination of density functional first-principles calculations and experiments. The new findings demonstrate that neutral Na is essentially a spectator impurity that occupies near interfacial interstitial sites and does not interact with the interface or with interfacial defects. Na ions, however, introduce an effective mass hydrogenic impurity band at the edge of the SiC conduction band that can account for the observed effects. [ABSTRACT FROM AUTHOR]

We report the structural, magnetic, electrical transport properties, and magnetocaloric effect (MCE) of antipervoskite compound AlCMn3. It exhibits a second-order ferromagnetic-paramagnetic phase transition around (TC) 287 K. The electronic resistivity (ρ) shows a good metallic behavior except for a slope change around TC. At lower temperatures (below 130 K), ρ∝T2 indicates that the electron-electron scatterings domain. At evaluated temperatures (130-270 K), ρ is linear dependence on temperature, implying that the phonon scatterings boost up greatly. Furthermore, a broad distribution of the magnetic entropy change (-ΔSM) peak is found to about 100 K with the magnetic field change ΔH=45 kOe. The relative cooling power are ∼137 J/kg and ∼328 J/kg (or ∼68 K2 and ∼162 K2) with ΔH=20 kOe and 45 kOe, respectively. All these values are comparable with the typical MCE associated with a second-order transition. It suggests that AlCMn3 may be considered as a candidate material for near room-temperature magnetic refrigeration because of: (i) the large full width at half peak of the -ΔSM-T curve, (ii) no hysteresis losses, (iii) the near room-temperature working temperature, and (iv) the low-cost and innoxious raw materials. Moreover, it is found that the simple theoretical model which only considering the magnetoelastic and magnetoelectronic couplings couldn't account well for the observed MCE in antiperovskite AlCMn3. [ABSTRACT FROM AUTHOR]

PbO–MgO–Nb2O5–TiO2 (PMNT) pyrochlore thin films were prepared on Pt-coated silicon substrates by radio-frequency magnetron sputtering and postdeposition annealing method. Very interestingly, these pyrochlore-structured PMNT thin films exhibited ultralow dielectric losses, with a typical loss tangent as low as 0.001, and relatively high dielectric constants, typically [variant_greek_epsilon]r∼170. It was found that the relative permittivity slightly but continuously increased upon cooling without any signature of a structural phase transition, displaying a quantum paraelectriclike behavior; meanwhile, the PMNT pyrochlore thin films did not show any noticeable dielectric dispersion in the real part of permittivity over a wide temperature range (77–400 K). Their dielectric responses could, however, be efficiently tuned by applying a dc electric field. A maximum applied bias field of 1 MV/cm resulted in a ∼20% tunability of the dielectric permittivity, giving rise to a fairly large coefficient of the dielectric nonlinearity, ∼2.5×109 J C-4 m-5. Moreover, the PMNT pyrochlore films exhibited superior electrical insulation properties with a relatively high breakdown field (Ebreakdown∼1.5 MV/cm) and a very low leakage current density of about 8.2×10-7 A/cm2 obtained at an electric field intensity as high as 500 kV/cm. [ABSTRACT FROM AUTHOR]

Tunable second harmonic generation of pulsed CO2 laser radiation in a GaSe crystal was demonstrated. The nonlinear optical properties of GaSe and phase-matching conditions were investigated, and the best fitted dispersion relationships for this crystal were also determined. External conversion efficiency of 0.1% with unfocused pumped radiation had been achieved and maximum single pulse energy of 0.19 mJ was generated at 5.3 μm. [ABSTRACT FROM AUTHOR]

The giant magnetoresistance (GMR) characteristics of hybrid perpendicular/Cu/in-plane magnetic anisotropy [Pd/Co]/Cu/Co single spin valves and [Pd/Co]/Cu/Co/Cu/[Co/Pd] dual spin valves with applied field perpendicular and parallel to the film plane are studied. Linear and nonhysteretic GMR behaviors are observed for the two spin valves in perpendicular-to-plane applied fields due to the coherent rotation of the Co layer driven by the in-plane shape anisotropy. The GMR value of the dual spin valve is 1.8 times as large as that of the single spin valve. The correlations between the magnetic configuration of the devices and the resistance evolvement are analyzed. In-plane GMR curves with maximum applied field of 30 kOe deviate from the linear behavior and can be well described by the Stoner–Wohlfarth model including high order anisotropy terms. Moreover, obvious hysteresis behavior is observed in the in-plane GMR curve of the dual spin valve. This phenomenon results from the hindered rotation of the Co/Pd moments due to the magnetostatic coupling between the top and bottom Co/Pd multilayers in the dual spin valve. [ABSTRACT FROM AUTHOR]

We present our investigation on the critical behavior of the spinel compound CdCr2S4 via the ac susceptibility and dc magnetization measurements. The analysis of ac susceptibility in terms of scaling behavior yields exponent values of δ=5.38±0.01, γ+β=1.77±0.03 and γ=1.44±0.02. Both δ and γ are slightly larger than that predicted by Heisenberg model. Detailed analyses of the inverse of the susceptibility and the electron spin resonance experiment reveal that the existence of correlated magnetic polarons has a key impact on the ferromagnetism of the spinel CdCr2S4. The abnormal critical exponent value of δ of CdCr2S4 may be attributed to the formation of the correlated magnetic polarons and abnormal spin-phonon coupling. [ABSTRACT FROM AUTHOR]

Thermal history dependent photoconductivity has been observed in the thermal hysteresis region of Pr0.5Sr0.5MnO3 thin film. In the cooling process, the film shows an upward resistance relaxation in darkness. Under light illumination, the transient photoconductivity effect is observed. However, it exhibits a downward resistance relaxation without illumination and persistent photoconductivity (PPC) behavior with light illumination in the warming process. The PPC ratio is 18.1% at 110 K with a light density of 1.25 mW mm-2. It is found that the external magnetic field plays a similar role as the light illumination. The persistent magnetoresistance ratio reaches 44.4% at 110 K with a low magnetic field of 0.25 T. The underlying mechanism is discussed based on phase competition and phase stability switch between ferromagnetic metal and charge- and orbital-ordered insulator states. The results may be important for practical applications in photo-/magnetic field sensitive and memory devices. [ABSTRACT FROM AUTHOR]

Dielectric characteristics of Ba6CoNb9O30 tetragonal tungsten bronze ceramics have been evaluated over a broad temperature and frequency range. A broad dielectric peak with strong frequency dispersion is observed at 140–225 K, which well follows the Vogel–Fulcher relationship and indicates the relaxor ferroelectric nature in the present ceramics. The P-E hysteresis loops varying with temperature are consistent with this dielectric relaxation process and provide the further evidence on the relaxor ferroelectric nature. The nonlinear magnetic hysteresis curve is observed in Ba6CoNb9O30 ceramics at 5 K, indicating the soft magnetic behavior which is related to the tetragonal tungsten bronze crystal structure containing magnetic ions of Co3+. [ABSTRACT FROM AUTHOR]

Dielectric properties of Ba0.7Sr0.3TiO3 (BST) thin films, which were prepared on silicon-based substrates by ion beam sputtering and postdeposition annealing method, were systematically investigated in different electrode configurations of metal-insulator-metal and coplanar interdigital capacitors. It was found that a large dielectric anisotropy exists in the films with better in-plane dielectric properties (higher dielectric permittivity and tunability) than those along the out-of-plane direction. The observed anisotropic dielectric responses are explained qualitatively in terms of a thermal strain effect that is related to dissimilar film strains along the in-plane and out-of-plane directions. Another reason for the dielectric anisotropy is due to different influences of the interfacial low-dielectric layer between the BST film and the substrate (metal electrode). [ABSTRACT FROM AUTHOR]

The temperature dependences of magnetization, electrical transport, and thermal transport properties of antiperovskite compound SnCMn3 have been investigated systematically. A positive magnetoresistance (∼11%) is observed around the ferrimagnetic-paramagnetic transition (TC∼280 K) in the field of 50 kOe, which can be attributed to the field-induced magnetic phase transition. The abnormalities of resistivity, Seebeck coefficient, normal Hall effect, and thermal conductivity near TC are suggested to be associated with an abrupt reconstruction of electronic structure. Further, our results indicate an essential interaction among lattice, spin, and charge degrees of freedom around TC. Such an interaction among various degrees of freedom associated with sudden phase transition is suggested to be characteristic of Mn-based antiperovskite compounds. [ABSTRACT FROM AUTHOR]

Dielectric characteristics of Ba4Nd2Ti4Nb6O30 tungsten bronze ceramics have been investigated over a broad range of temperature and frequency. The real part of the dielectric constant ([variant_greek_epsilon]′) shows a sharp peak at 417 K indicating the ferroelectric phase transition. Two dielectric relaxations are observed in the lower temperature range, which are very obvious in the dielectric loss (tan δ) and the imaginary part of the dielectric constant ([variant_greek_epsilon]″). The differential scanning calorimetry (DSC), Raman scattering, and ultrasonic studies of the present ceramics have also been conducted in a broad temperature range covering the ferroelectric transition and/or the dielectric relaxation temperatures. The endothermal peak in the DSC curve around the ferroelectric transition temperature confirms the first-order structural transition with the transition enthalpy of 5.72 kJ/mol. Steplike anomalies in the wave number of the Raman vibrations and abnormal drop of the intensity of the Raman spectra are observed accompanying the ferroelectric phase transition. The dielectric relaxations in the temperature range below Tc are attributed to the structure fluctuations associated with the coupling between the polarizability in the ab plane and the c axis displacement, as well as the remaining random field effects, which are suggested as the possible origin of the anomalous reduction of the Raman intensity below Tc as well as the anomalies in the ultrasonic velocity and attenuation measurements. Freezing of these structure fluctuations are detected by the ultrasonic anomaly at about 120 K and the increasing Raman intensity below 150 K. [ABSTRACT FROM AUTHOR]

The effect of periodic rectangular grooves on vacuum multipactor has been theoretically and experimentally investigated. Dynamic calculation is applied to research the electron trajectory and impact energy under groove surface. Two-dimensional electromagnetic particle-in-cell simulation is used to analyze and compare multipactor scenario, statistic energy, and secondary emission yield on the flat surface with that on the corrugated surface. It has been found by computational and simulative analysis that grooved surface can explicitly suppress multipactor in the developmental stage of multipactor. S-band high power microwave (HPM) dielectric breakdown experiment under vacuum, with microsecond pulse length was conducted. It was confirmed by experiment that periodic grooves perpendicular to the major electric field can effectively increase transmitted power. [ABSTRACT FROM AUTHOR]

The magnetic and magnetocaloric properties of Ga1-xCMn3+x have been investigated. Reversible magnetocaloric effect (MCE) occurs near the Curie temperature TC. With increasing x, we find that the magnetic entropy change -ΔSM decreases while TC and magnetization increase. Meanwhile, the temperature span of -ΔSM versus T plot becomes well broadened. Due to the competition between the broadening temperature span and decreasing -ΔSM, the relative cooling power (RCP) increases initially and then decreases with increasing x further. The largest RCP (2.1 J/cm3 in a magnetic field of 45 kOe) observed at x=0.07 (TC=296.5 K) is comparable with the contemporary magnetic refrigerant materials. Considering the reversible MCE, inexpensive and innoxious raw materials, our result suggests that Ga1-xCMn3+x can be a promising candidate for magnetic refrigeration around room temperature. [ABSTRACT FROM AUTHOR]

Ba0.7Sr0.3TiO3 (BST) thick films with thickness up to 1 μm were deposited on Pt-coated silicon substrates by ion beam sputtering, followed by an annealing treatment. It is demonstrated that pure well-crystallized perovskite phase could be obtained in thick BST films by a low temperature process (535 °C). The BST thick films show highly tunable dielectric properties with tunability (at 800 kV/cm) up to 51.0% and 66.2%, respectively, for the 0.5 and 1 μm thick films. The relationship between strains and dielectric properties was systematically investigated in the thick films. The results suggest that a comparatively larger tensile thermal in-plane strain (0.15%) leads to the degradation in dielectric properties of the 0.5 μm thick film; besides, strong defect-related inhomogeneous strains (∼0.3%) make the dielectric peaks smearing and broadening in the thick films, which, however, preferably results in high figure-of-merit factors over a wide operating temperature range. Moreover, the leakage current behavior in the BST thick films was found to be dominated by the space-charge-limited-current mechanism, irrespective of the film thickness. [ABSTRACT FROM AUTHOR]

It was experimentally found that the evolution of dielectric loss with processing temperature displays a common trend in ferroelectric and dielectric thin oxide films; first an increase and then a decrease in dielectric loss when the processing temperature is gradually raised. Such a dielectric response of ferroelectric/dielectric thin films has been theoretically addressed in this work. We propose that at the initial stage of the crystallization process in thin films, the transformation from amorphous to crystalline phase should substantially increase the dielectric loss. Then, with further increase in the processing temperature, the coalescent growth of small crystalline grains into big ones could be helpful in reducing the dielectric loss by lowering grain boundary densities. The obtained experimental data for (Ba,Sr)TiO3 thin films 500 nm in thickness were analyzed in terms of the model developed and shown to be in reasonable agreement with the theoretical results. [ABSTRACT FROM AUTHOR]

We investigate the exchange bias phenomenon in the layered cobaltite Sr1.5Pr0.5CoO4 by using magnetometry. The exchange bias is strongly dependent on the cooling magnetic field as well as the measuring field. These results suggest that the exchange coupling at the interfaces between the ferromagnetic (FM) regions and the spin-glass (SG) regions may induce an exchange anisotropy. The observed exchange bias effect is suggested to originate from the pinning to FM moments by frozen spins in the SG regions along the cooling field direction. Our results also expand the concept of the magnetically tunable exchange bias, which can contribute to the potential applications for spintronic devices. [ABSTRACT FROM AUTHOR]

We present the results of systematic study of structure, magnetic properties, and electrical transport in layered cobaltites Sr2-xPrxCoO4 (0.5≤x≤1.5). The variations of Jahn–Teller distortion of CoO6 octahedron reflected by two different Co–O bond lengths indicate that all the samples are mostly accommodated in the t2g orbital states. For x<1 samples, with increasing Pr-doping level, the tolerance factor decreases and the distortion of CoO6 octahedron enhances, which result in the increased bending of Co–O–Co bond, the narrowing of the bandwidth, and the decrease of the mobility of eg electrons caused by the substitution of smaller Pr3+ ions for a larger Sr2+ ion. For x>1 samples, the contribution of carrier density should be considered besides the tolerance factor. [ABSTRACT FROM AUTHOR]

The electronic structure and magnetism of the Mn2FeZ (Z=Al, Ga, Si, Ge, Sb) Heusler alloys have been studied by density functional calculations. Two half-metallic ferromagnets, namely, Mn2FeAl and Mn2FeSb, are predicted. It is found that a small expansion of the crystal lattice can restore the half-metallicity in Mn2FeSi. The calculated total magnetic moments Mtot are 1μB/f.u. for Mn2FeAl and Mn2FeGa, 2μB/f.u. for Mn2FeSi and Mn2FeGe, and 3μB/f.u. for Mn2FeSb, which agree with the Slater–Pauling curve quite well. The moments of Mn (A) and Mn (B) are large and antiparallel to each other, which is indicative of ferrimagnetism in Mn2FeZ alloys. Fe shows only a small moment and its moment is parallel to that of Mn (B). By investigating the effect of lattice distortion on the half-metallicity and magnetic moments of Mn2FeZ, it is found that the half-metallic properties of Mn2FeSb are insensitive to the lattice distortion and a 100% spin polarization can be obtained within the wide range of 5.4–6.05 Å. This is preferable in practical applications. [ABSTRACT FROM AUTHOR]

The structural, magnetic, and transport properties of the electron-doped perovskites cobalt oxides La1-xTexCoO3 (0≤x≤0.25) have been investigated. For the structural properties, the room temperature structural transition from rhombohedra (R3C) to orthorhombic (Pbnm) symmetry is found in these samples with x>=0.10 by the Rietveld refinement of x-ray powder diffraction patterns, which is ascribed to the smaller ionic radius of Te. As for the magnetic properties, firstly the spin state transition of Co3+ ions is observed in all samples, and the transition temperature shifts up to higher temperature with Te doping, which is ascribed to the smaller ionic radius and the strong acidity of Te. Secondly, it is interesting that there exists paramagnetic to ferromagnetic phase transition in the Te-doped samples. It is suggested that such a phase transition results from the introduction of Co2+ ions and the appearance of double exchange interaction between Co2+ and Co3+ due to Te doping. Moreover, Curie temperature TC increases with the increase of the Te doping level x (for x≤0.10), and then decreases with the further increase of x. About the transport properties, the temperature dependence of the resistivity ρ shows that all samples display semiconducting behavior in the whole measured temperature region. And it can be seen that the resistivity first decreases with increasing Te doping level from x=0 to 0.10, then increases as x>=0.15. Such variations of TC and ρ with x are suggested to be related to the structure transition. [ABSTRACT FROM AUTHOR]

The effect of Mn doping on structural, magnetic, electronic transport, and specific heat properties in 4d perovskites SrMo1-xMnxO3(0≤x≤0.20) has been investigated. The Mn doping at the Mo site does not change the space group of the samples, but increases the structural parameter a monotonically. All Mn-doped samples show Curie-Weiss paramagnetic behaviors due to the appearance of localized moment, and the magnetic susceptibility χ increases with increasing the Mn-doping content. Samples (x≥0.03) exhibit semiconducting transport behavior, while x=0.01 shows a metallic behavior with an upturn below T1. The magnitude of resistivity monotonically increases with increasing Mn-doping content. The evolution of the resistive properties is discussed according to the competition among Kondo scattering, electron-electron interaction, and the disorder effect. In addition, the electron-electron interaction intensity increases with the increasing of Mn-doping content according to the study of specific heat properties. [ABSTRACT FROM AUTHOR]

The influence of bias current on resistivity and photoconductivity in oxygen-deficient La2/3Sr1/3MnO3-δ thin films has been investigated. There exists a giant resistance drop induced by dc electrical currents alone especially near the temperature of resistance peak (TP). Moreover, the photoconductivity emerges only when the current exceeds a threshold and this effect is quite different from magnetoresistance induced by magnetic fields. It is found that the magnitude of photoconductivity increases with increasing of bias current and the threshold current increases with decreasing temperatures and increasing magnetic fields. The magnitude of photoconductivity achieves 10.2% at temperature of 100 K with light density of 4.3 mW/cm2 and current of 1 mA. The current-assisted electroresistance and photoconductivity are discussed based on the delocalization of localized state by light illumination and bias current. [ABSTRACT FROM AUTHOR]

Structural, magnetic, and magnetocaloric properties in perovskite manganites La1-xPbxMn1-y-zCuyO3 (x~0.14, y=0, 0.01, 0.02, z=0.02, 0.08, 0.11) single crystals have been investigated. The Curie temperature TC decreases monotonously with increasing Cu-doping concentration. A sharp downturn in the inverse magnetic susceptibility 1/χm vs T plot at Griffiths temperature TG well above TC was observed for the free-doped sample, which can be attributed to the appearance of spin cluster in the paramagnetic state. The temperature interval between TG and TC decreases with increasing Cu-doping level and almost disappears as Cu concentration reaches 0.02. Large magnetic entropy change ΔSM occurs near TC and ΔSM approaches a maximum value for the sample with Cu-doping level y=0.01. The result can be explained based on the competing effect between the double-exchange interaction and the lattice distortion induced by Cu doping. [ABSTRACT FROM AUTHOR]