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The unique optoelectronic properties of single layer graphene (SLG) are ideal for the development of photonic devices across a broad range of frequencies from x rays to microwaves. In the terahertz (THz) frequency range (0.1–10 THz), this has led to the development of optical modulators, nonlinear sources, and photodetectors with state-of-the-art performances. A key challenge is the integration of SLG-based active elements with pre-existing technological platforms in a scalable way, while maintaining performance level unperturbed. Here, we report room temperature THz detectors made of large-area SLG, grown by chemical vapor deposition (CVD) and integrated in antenna-coupled field effect transistors. We selectively activate the photo-thermoelectric detection dynamics, and we employ different dielectric configurations of SLG on Al2O3 with and without large-area CVD hexagonal boron nitride capping to investigate their effect on SLG thermoelectric properties underpinning photodetection. With these scalable architectures, response times ∼5 ns and noise equivalent powers (NEPs) ∼1 nW Hz−1/2 are achieved under zero-bias operation. This shows the feasibility of scalable, large-area, layered material heterostructures for THz detection. [ABSTRACT FROM AUTHOR]

The Kapitza resistance across a Si bicrystal interface was measured using a pump probe optical technique. This approach, termed time resolved thermal wave microscopy (TRTWM), uses ultrafast laser pulses to image lateral thermal transport in bare semiconductors. The sample geometry is that of a Si bicrystal with the vertically oriented boundary intersecting the sample surface. High resolution transmission electron microscopy of the boundary region revealed a thin SiO2 layer at the interface. By comparing experimental results with a continuum thermal transport model the Kapitza resistance between the Si and SiO2 was estimated to be 2.3 × 10-9 m2K/W. [ABSTRACT FROM AUTHOR]

We use ultrashort optical pulses to microscopically image carrier and thermal diffusion in two spatial dimensions in pristine and mechanically polished surfaces of crystalline silicon. By decomposing changes in reflectivity in the latter sample into a transient component that varies with delay time and a steady-state component that varies with pump chopping frequency, the influence of thermal diffusion is isolated from that of carrier diffusion and recombination. Additionally, studies using carrier injection density as a parameter are used to clearly identify the carrier recombination pathway. [ABSTRACT FROM AUTHOR]

Temperature and field dependence of magnetization of Dy-substituted Gd5Si2Ge2 compounds has been studied in the temperature range of 5–330 K. The (DyxGd5-x)Si2Ge2 compounds with x=0.5, 1.5, 3.0, and 3.5 are found to order ferromagnetically with magnetic ordering temperature TC ranging between ∼270 and 80 K. The Dy substitution in Gd5Si2Ge2 leads to a lowering of TC, an increase in coercivity, and also to a loss of the first order magnetic phase transition seen in the parent compound. The magnetic entropy change ΔSM associated with the magnetic transition has been computed using the magnetization-field isotherm data. The maximum magnetic entropy changes of ∼8.8, 13.9, and 6.9 J/kg K are obtained for x=0.5, 3.0, and 5 compounds, respectively, for a field change of 55 kOe. The ΔSM is spread around TC over a few tens of Kelvin and this feature makes these materials promising for possible use in Ericsson-cycle magnetic refrigerator applications. [ABSTRACT FROM AUTHOR]

We have studied the physical properties of epitaxial La[sub 0.67-x]Pr[sub x]Ca[sub 0.33]MnO[sub 3](LPCMO) (x=0.13, 0.20, 0.27) thin films under different type and degree of substrate induced biaxial strain. Films with thickness of 300, 600, and 1000 Å grown on crystalline NdGaO[sub 3] (negligible strain), LaAlO[sub 3] (compressive strain), and SrTiO[sub 3] (tensile strain) by pulsed laser deposition are examined. Our data suggest occurrence of metastable phase mixtures in these films, with the volume fractions controlled by strain. In particular, melting of the charge ordered phase is seen to occur in thin films on SrTiO[sub 3] at fields as low as 1 T, in the low temperature regime. The deviation of ρ(T) from the effective medium theory further indicates that LPCMO thin films cannot be described as simple metal–insulator mixtures. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Thin films of Sr[sub 2]FeMoO[sub 6] have been deposited on single-crystalline and polycrystalline SrTiO[sub 3] substrates by pulsed laser deposition from a stoichiometric target. In order to obtain high-quality films, the deposition parameters were systematically optimized. The films grown under optimized conditions show properties comparable to those of bulk Sr[sub 2]FeMoO[sub 6] single crystals. Surprisingly, polycrystalline films obtained by depositing on polycrystalline SrTiO[sub 3] substrates do not show any significant low-field magnetoresistance (MR), contrary to expectations for a spin-polarized material. However, after controlled annealing-induced alteration of the grain-boundary properties, a development of the low-field MR feature is clearly observed. The occurrence of the low-field MR is accompanied by nonlinearity in the current-voltage characteristics. These results are analyzed in the light of the prevalent understanding of the properties of this double perovskite material. [ABSTRACT FROM AUTHOR]

The oxygen content of La[SUB0.5]Ca0.5]MnO[SUBy] was tuned by annealing the samples at high temperatures in flowing nitrogen with graphite powder nearby. The reduction of oxygen content has dramatic effect on the electrical transport and magnetic properties. The samples with y=2.983, 2.83, and 2.803 show an insulator-metal transition, and an unusual temperature and magnetic-field dependence of the magnetoresistance. The paramagnetic-ferromagnetic transition also shifts to lower temperatures and the antiferromagnetic transition at lower temperature is suppressed. The results are discussed in terms of the effect of oxygen vacancies on the various properties of La[SUB0.5]Ca0.5]MnO[SUBy]. [ABSTRACT FROM AUTHOR]

Studies the synthesis by pulsed laser ablation and the magnetic characterization of nanosized powders of iron oxides and strontium ferrite. Synthesis of nanosized powders; Iron oxide; Strontium ferrite; Magnetic properties of the nanosized powders.

Presents information on a study which determined the microstructural, magnetic and microwave properties of pulsed laser deposited epitaxial thin films of barium ferrite on single crystal sapphire substrates. Experimental details; Results and discussion; Conclusions.

Sintered La0.7Sr0.3MnO3 ceramics were prepared by standard solid-state reaction route. Some of these pellets were again heated to 850 °C and quenched to room temperature. These samples were then studied for low-field magnetoresistance (LFMR) effects. Remarkably, upon quenching from high temperature the polycrystalline as well as the crystalline samples are seen to exhibit a dramatic change in LFMR with the emergence of a large positive MR. In the quenched samples a positive LFMR as high as 48% was noted in a field of 200 Oe. This positive MR effect was found to show time-dependent relaxation. [ABSTRACT FROM AUTHOR]

The effect of simultaneous substitution of Nd and Ca in the antiferromagnetic insulator, LaMnO[SUB3], is studied by x-ray diffraction, magnetoresistance, and magnetization measurements. X-ray powder diffraction patterns of La[SUB1-2]Nd[SUBx]Ca[SUBx]MnO[SUB3] polycrystalline bulk samples for x=0.2, 0.33, and 0.4 confirm the single-phase formation of orthorhombic perovskite structure (space group Pbnm). The unit cell volume decreases with increasing Nd/Ca substitution. Changes in the temperature dependence of resistivity from semiconducting to metallic, and an increase of the ferromagnetic transition temperature, are observed in this series with increasing x. For x=0.2, 0.33, and 0.4, the metal-insulator transition temperature (T[SUBp])is 72, 140, and 155 K, respectively, and the magnetoresistance (MR) observed in this system in 2 T applied magnetic field around T[SUBp] is 35%, 99% and 65%, respectively. The MR of 99% for the x=0.33 sample is promising for practical device applications where MR at low fields is of special interest. The magnetic moment values, obtained from the magnetization data in the magnetically ordered state, agree reasonably well with the free ion Mn moments for all the samples. [ABSTRACT FROM AUTHOR]

Epitaxial thin films of ferromagnet/insulator/ferromagnet layers La[SUB0.7]Ca[SUB0.3]MnO[SUB3]/LaAlO[SUB3]/La[SUB0.7]Sr[SUB0.3]MnO[SUB3]) of different thickness were fabricated by pulsed laser deposition. Microstructural analysis shows island growth on the thicker film surface. dc magnetization study demonstrates the strong signature of domain wall freezing for the composite film. An interesting feature is a cusp/ upturn in field-cooled curve in the composite film-1, much below T[SUBB], the blocking temperature. The low temperature cusp disappears in the composite film-2. We suggest that a quasistable charge ordering via spin localization at low temperature, especially for film-1, is the origin of this feature. [ABSTRACT FROM AUTHOR]

We have measured the temperature (T) dependencies of the dc resistances (R[sub dc]) and the microwave loss (R[sub μw]) in a variety of samples of (La[sub l-χ]Pr[sub χ])[sub 0.7]Ca[sub 0.3]MnO[sub 3] while varying x from 0 to 0.4. Whereas both the sets of data exhibit maxima, the ac loss peak is much flatter and, during cooling, appears at a much lower temperature than the peak temperature in R[sub dc]. The discrepancy, which vanishes for x = 0, increases with lowering tolerance factor (t) (or increasing x). Also R[sub μw] vs T exhibits large thermal hysteresis for x = 0.4 indicating that the transition is first order. Cooling in a magnetic field of 9 kOe causes an upward shift of about 20 K in the R[sub μw] peak, in some of the x = 0.4 films, yielding a large magnetoimpedance. Further, once these films are exposed to a magnetic field at low T, they fail to recover their virginal behavior on subsequent cooling from room T. These films could be brought to their original state by annealing at high T. The discrepancy between R[sub dc] and R[sub μw] implies that the system is inhomogeneous at low T, providing, for the first time, microwave absorption evidence that manganites exhibit multiphase behavior. Presumably, disorder and strain (increasing with x) combine to stabilize a mixed phase. [ABSTRACT FROM AUTHOR]

A commercial atomic force microscope/magnetic force microscope (MFM) was modified to cool magnetic samples down to around 100 K under a high vacuum while maintaining its routine imaging functionality. MFM images of a 120 nm thick La[sub 0.7]Ca[sub 0.3]MnO[sub 3] film on a LaAlO[sub 3] substrate at low temperature show the paramagnetic-to-ferromagnetic phase transition. Evolution of magnetic domains and magnetic ripples with decreasing temperature are also observed near the edge of a 20 nm thick patterned Co film on a Si substrate. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents information on a study which characterized the complex dielectric properties of a series of zirconia-alumina ceramic composites. Characteristics of the samples; Technique used for microwave dielectric measurements; Conclusions.

Presents a study that investigated the depolarization of a neutron beam passing through a sample of a high temperature superconductor. Measurements; Discussion; Summary.

Presents a study which examined the magnetic properties of polycrystalline yttrium-barium-copper-oxide semiconductors. Comparison of the magnetic properties of the samples with that of a single crystal; Methods; Results and discussion.

A high-voltage pulse-slicer unit with variable pulse duration has been developed and integrated with a 7 MeV linear electron accelerator (LINAC) for pulse radiolysis investigation. The pulse-slicer unit provides switching voltage from 1 kV to 10 kV with rise time better than 5 ns. Two MOSFET based 10 kV switches were configured in differential mode to get variable duration pulses. The high-voltage pulse has been applied to the deflecting plates of the LINAC for slicing of electron beam of 2 μs duration. The duration of the electron beam has been varied from 30 ns to 2 μs with the optimized pulse amplitude of 7 kV to get corresponding radiation doses from 6 Gy to 167 Gy. [ABSTRACT FROM AUTHOR]

We performed x-ray absorption spectroscopy measurements at the cobalt L2,3 edge and the oxygen K edge of Co-doped SnO2 and Co-doped TiO2. Our measurements confirm that doped cobalt atoms are in the same local environment in both compounds. Furthermore, the results support the idea that cobalt atoms occupy substitutional cation sites. Additionally, the oxygen spectral shapes offer insight into a possible cause for the observed giant magnetic moment of cobalt atoms present in SnO2, but not in TiO2. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

LaxLu1-xO3 thin films were deposited on 300 mm silicon wafers by physical vapor deposition and fabricated into field-effect transistors using a gate-first process flow. The films were characterized using transmission electron microscopy, Rutherford backscattering spectrometry, and synchrotron x-ray diffraction. The results show the films remain amorphous even at temperatures of 1000 °C. The dielectric properties of LaxLu1-xO3 (0.125≤x≤0.875) thin films were evaluated as a function of film composition. The amorphous LaxLu1-xO3 thin films have a dielectric constant (K) of 23 across the composition range. The inversion thickness (Tinv) of the LaxLu1-xO3 thin films was scaled to <1.0 nm. [ABSTRACT FROM AUTHOR]

Electron tunneling induced photon emission in gallium doped (1%) zinc oxide was detected by scanning tunneling microscope in ambient condition. Simultaneously acquired topography and photon maps reveal interesting correlation. Photon maps depict few intense emission spots. Existence of a threshold tunneling current (6 nA) and applied bias (1.8 V), for detectable photon emission was observed. Further analysis of the results suggests Fowler-type photon emission which is ascribed to radiative electron-hole recombination. Localized photon emission spectroscopy exhibits prominent peaks at hν=2.96, 3.43, and 3.80 eV, which match well with the results obtained from photoluminescence studies. [ABSTRACT FROM AUTHOR]

The use of CoFeB ferromagnetic electrodes in place of CoFe has been shown to significantly increase the tunneling magnetoresistance (TMR) of MgO based magnetic tunnel junctions (MTJs). By using soft x-ray scattering techniques, we show that the behavior of the magnetic moments located at the CoFe–MgO interface are drastically different from the rest of the CoFe film, whereas the magnetic response of the CoFeB–MgO interfacial moments is coherent with the film’s bulk. Our results support the view that the high TMR values observed in MgO based MTJs with CoFeB electrodes are due to the uniform magnetic response of the entire CoFeB electrode including the MgO interfacial moments. [ABSTRACT FROM AUTHOR]

We report on the search for ferromagnetism in undoped and cobalt-doped high-k dielectric HfO2 films. Over a broad range of growth conditions, we do not observe ferromagnetism in undoped HfO2 films. On the other hand, we do observe room temperature ferromagnetism in dilutely Co-doped HfO2 films, but the origin of the same appears extrinsic (a Co rich surface layer) at least for the regime of growth conditions explored. [ABSTRACT FROM AUTHOR]

We report on the growth of highly oriented HfO2 thin films of monoclinic phase stabilized by 5% Co doping on (001) yttrium-stabilized zirconia (YSZ) using pulsed-laser deposition at 700 °C at an oxygen partial pressure of 10-4 Torr. On the other hand, pure HfO2 of such quality did not grow on YSZ in wide range of growth parameters. Rutherford backscattering-ion channeling in this film showed a 24% minimum yield (χmin) indicating highly oriented film growth, while hardly any ion channeling was observed in the undoped sample. High-resolution transmission electron microscopy revealed a sharp interface, and no signature of Co clusters. Electron energy loss spectroscopy showed that Co is in the 2+ state. Attempts were also made to grow films on a (001) Si substrate, and the results showed a very low ion channeling yield (∼8%). [ABSTRACT FROM AUTHOR]

A mixture of a colossal magnetoresistance manganite (La5/8Sr3/8MnO3) and a multiferroic (LuMnO3), of potential interest for magnetoelectronics, was found to be bulk immiscible due to its structural/chemical incompatibility. Here, we demonstrate that thin-film epitaxy offers a route to achieve continuous perovskite solid solutions of such a bulk-immiscible mixture, allowing studies of their physical properties. The films grow with (110)- and (001)-preferred orientation on SrTiO3(100) and LaAlO3(100), respectively. The details of structural, magnetotransport, and magnetization data are discussed within the context of metal-insulator phase coexistence. Epitaxial stabilization of chemically immiscible phases clearly widens the scope of research on manganites as well as other materials. [ABSTRACT FROM AUTHOR]

We present the results of Raman and photoluminescence studies on epitaxial anatase phase Ti[sub 1-x]Co[sub x]O[sub 2] films for x=0–0.07, grown by pulsed-laser deposition. The low-doped system (x=0.01 and 0.02) shows a Curie temperature of 700 K in the as-grown state. The Raman spectra from the doped and undoped films confirm their anatase phase. The photoluminescence spectrum is characterized by a broad emission from self-trapped excitons (STE) at 2.3 eV at temperatures below 120 K. This peak is characteristic of the anatase-phase TiO[sub 2] and shows a small blueshift with increasing doping concentration. The Co-doped samples show two spin-flip emission lines at 2.77 and 2.94 eV. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Thin films of 5% Co doped Cu[SUB2]O were grown on single-crystal (001) MgO substrates by pulsed-laser deposition, without and with 0.5% codoping with Al, V, or Zn. Structural, electrical, and magnetic properties were studied. The films showed phase-pure character under the chosen optimum growth conditions. Spin-glass-like behavior was observed in Co-doped films without codoping. A clear ferromagnetic signal at room temperature was found only in the case of Co:Cu[SUB2]O films codoped with Al. [ABSTRACT FROM AUTHOR]

A prominent theme in inorganic materials research is the creation of uniformly flat thin films and heterostructures over large wafers, which can subsequently be lithographically processed into functional devices. This letter proposes an approach that will lead to thin film topographies that are directly counter to the above-mentioned philosophy. Recent years have witnessed considerable research activity in the area of self-assembly of materials, stimulated by observations of self-organized behavior in biological systems. We have fabricated uniform arrays of nonplanar surface features by a spontaneous assembly process involving the oxidation of simple metals, especially under constrained conditions on a variety of substrates, including glass and Si. In this letter we demonstrate the pervasiveness of this process through examples involving the oxidation of Pd, Cu, Fe, and In. The feature sizes can be controlled through the grain size and thickness of the starting metal thin film. Finally, we demonstrate how such submicron scale arrays can serve as templates for the design and development of self-assembled, nanoelectronic devices. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We demonstrate that lanthanum gallate (LaGaO3) has considerable potential as an electronic substrate material for high-temperature superconducting films. It provides a good lattice and thermal expansion match to YBa2Cu3O7-x, can be grown in large crystal sizes, is compatible with high-temperature film processing, and has a reasonably low dielectric constant (ε[bar_over_tilde:_approx._equal_to]25) and low dielectric losses. Epitaxial YBa2Cu3O7-x films grown on LaGaO3 single-crystal substrates by three techniques have zero resistance between 87 and 91 K. [ABSTRACT FROM AUTHOR]

Presents an abstract of the study 'Neutron Depolarization Effects in a High-Tc Superconductor,' by A.C. Nunes, S.J. Pickart, L. Crow, R. Goyette, S. Shinde and T.M. Shaw.

Sintered La0.7Sr0.3MnO3 ceramics were prepared by standard solid-state reaction route. Some of these pellets were again heated to 850 °C and quenched to room temperature. These samples were then studied for low-field magnetoresistance (LFMR) effects. Remarkably, upon quenching from high temperature the polycrystalline as well as the crystalline samples are seen to exhibit a dramatic change in LFMR with the emergence of a large positive MR. In the quenched samples a positive LFMR as high as 48% was noted in a field of 200 Oe. This positive MR effect was found to show time-dependent relaxation. [ABSTRACT FROM AUTHOR]

Growth of MgB[sub 2] thin films by pulsed laser deposition is examined under ex situ and in situ processing conditions. For the ex situ process, boron films grown by pulsed laser deposition were annealed at 900 °C with excess Mg. For the in situ process, different approaches involving ablation from a stoichiometric target under different growth conditions, as well as multilayer deposition involving interposed Mg layers were examined and analyzed. Magnetic measurements on ex situ processed films show T[sub c] of ∼39 K, while the current best in situ films show a susceptibility transition at ∼22 K. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effect of substrate-induced lattice strains on the structural and magnetic properties of epitaxial barium ferrite (BaFe[sub 12]O[sub 19]) thin films is studied. The 5000 Å thick film shows a very narrow ferromagnetic resonance (FMR) line with a width of about 140 Oe. After appropriate annealing, the linewidth measured at 58 GHz decreases to 37 Oe, which to the best of our knowledge is about two times smaller than the best previously reported value for epitaxial thin films. FMR studies show that the presence of strains broadens resonance absorption. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We performed x-ray absorption spectroscopy measurements at the cobalt L2,3 edge and the oxygen K edge of Co-doped SnO2 and Co-doped TiO2. Our measurements confirm that doped cobalt atoms are in the same local environment in both compounds. Furthermore, the results support the idea that cobalt atoms occupy substitutional cation sites. Additionally, the oxygen spectral shapes offer insight into a possible cause for the observed giant magnetic moment of cobalt atoms present in SnO2, but not in TiO2. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on the search for ferromagnetism in undoped and cobalt-doped high-k dielectric HfO2 films. Over a broad range of growth conditions, we do not observe ferromagnetism in undoped HfO2 films. On the other hand, we do observe room temperature ferromagnetism in dilutely Co-doped HfO2 films, but the origin of the same appears extrinsic (a Co rich surface layer) at least for the regime of growth conditions explored. [ABSTRACT FROM AUTHOR]