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17 Oct 2005

Volume 87, Issue 16, Articles (16xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 87, 161907 (2005); http://dx.doi.org/10.1063/1.2105998 (3 pages)

J. Das, K. B. Kim, F. Baier, W. Löser, and J. Eckert
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Photovoltaic characteristics of postdeposition iodine-doped amorphous carbon films by microwave surface wave plasma chemical vapor deposition

Ashraf M. M. Omer, Sudip Adhikari, Sunil Adhikary, Hideo Uchida, and Masayoshi Umeno

Appl. Phys. Lett. 87, 161912 (2005); http://dx.doi.org/10.1063/1.2089168 (3 pages) | Cited 14 times

Online Publication Date: 14 October 2005

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The amorphous carbon thin films have been deposited on silicon and quartz substrates by microwave surface wave plasma chemical vapor deposition at low temperature (<100°C) in Ar/CH4 phase gas. Doping of iodine has been done in the postdeposited films by exposing them in iodine vapor. The photovoltaic measurements of the films were carried out before and after iodine doping. The results show dramatic decrease of optical gap from 3.4 to 0.9 eV corresponding to nondoping to iodine doping conditions, respectively. The preliminary photovoltaic characteristics of the film deposited on n-type silicon substrate under light illumination (AM1.5, 100 mW/cm2) reveal a short-circuit current density of 1.15 μA/cm2, open-circuit voltage of 177 mV and fill factor of 21.7%.
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73.61.Cw Elemental semiconductors
73.61.Jc Amorphous semiconductors; glasses
61.72.up Other materials
52.77.Dq Plasma-based ion implantation and deposition
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
72.40.+w Photoconduction and photovoltaic effects
73.50.Pz Photoconduction and photovoltaic effects

Imaging hydrogenous materials with a neutron microscope

J. T. Cremer, M. A. Piestrup, H. Park, C. K. Gary, R. H. Pantell, C. J. Glinka, and J. G. Barker

Appl. Phys. Lett. 87, 161913 (2005); http://dx.doi.org/10.1063/1.2089172 (3 pages) | Cited 1 time

Online Publication Date: 14 October 2005

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Magnified images of materials containing hydrogen, for which the main contrast mechanism for neutrons is incoherent scattering, have been obtained using a microscope employing a neutron compound refractive lens (CRL). The CRL was composed of 100 MgF2 biconcave lenses that produced magnified (22.5×) images of polyethylene and polypropylene (hydrogen-rich) grids and biological specimens using 8.5 Å cold neutrons with a 10% bandwidth. For hydrogenous materials, 98%–99% of the attenuation is by incoherent scattering and 1%–2% from neutron absorption by the hydrogen nuclei. The small angle of acceptance of the CRL discriminates against scattered neutrons from the hydrogenous object, thereby producing the needed contrast for imaging.
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87.64.-t Spectroscopic and microscopic techniques in biophysics and medical physics
07.90.+c Other topics in instruments, apparatus, and components common to several branches of physics and astronomy (restricted to new topics in section 07)

Thermoelastic effect induced by ferroelastic domain switching

Yong Chan Cho, Sang Eon Park, Chae-Ryong Cho, and Se-Young Jeong

Appl. Phys. Lett. 87, 161914 (2005); http://dx.doi.org/10.1063/1.2106003 (3 pages) | Cited 2 times

Online Publication Date: 14 October 2005

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Temperature changes induced by externally applied mechanical stress were observed in the Pb3(PO4)2 ferroelastic crystal. These characteristic features can be explained by the thermoelastic effect as in other thermoelastic martensitic materials. In this study, the mechanical stress dependence of the thermoelastic effect was studied using a modified thermomechanical analysis system. The thermoelastic effect strongly depended on the mechanical stress and was enhanced by ferroelastic domain switching, responsible for the pseudoelastic behavior of ferroelastic materials. The ∂T/∂σsin, the change of sample temperature with varying stress enhanced by pseudoelasticity, was ∼ 9.5×10−2 °C/MPa at 159 °C, ferroelastic temperature region under 1.2 MPa sinusoidal stress. This value is eight to nine times higher than the values obtained under bias stress. This result can be explained by a thermoelastic equation which includes the effective thermal expansion coefficient in the ferroelastic phase.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.F- Deformation and plasticity
62.20.D- Elasticity
64.70.K- Solid-solid transitions

Temperature dependence of the mechanical properties of tetrahedrally coordinated amorphous carbon thin films

David A. Czaplewski, J. P. Sullivan, T. A. Friedmann, and J. R. Wendt

Appl. Phys. Lett. 87, 161915 (2005); http://dx.doi.org/10.1063/1.2108132 (3 pages) | Cited 7 times

Online Publication Date: 14 October 2005

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The complete elastic properties of tetrahedrally coordinated amorphous carbon (ta-C) thin films have been measured in the temperature range of 300–873 K. Flexural and torsional mechanical oscillators were fabricated from ta-C, and using the resonant frequency of the oscillators as a function of temperature, we calculated the temperature-dependent Young’s and shear moduli (658±24 and 271±6.6 GPa, at 300 K, respectively). From these values, we calculated the bulk modulus, Poisson’s ratio, and the elastic stiffness and compliance constants as a function of temperature. In addition, the temperature dependence of the coefficient of thermal expansion of ta-C was determined using a wafer curvature technique.
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68.60.Bs Mechanical and acoustical properties
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.

Barrierless self-assembly of Ge quantum dots on Si(001) substrates with high local vicinality

P. Sutter, E. Sutter, and L. Vescan

Appl. Phys. Lett. 87, 161916 (2005); http://dx.doi.org/10.1063/1.2108133 (3 pages) | Cited 4 times

Online Publication Date: 14 October 2005

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In Ge heteroepitaxy on vicinal Si(001), miscut by 4.8° toward [100], pyramid-shaped faceted quantum dot islands (“huts”) form continuously from individual (105) facets on a wetting layer of coexisting (105) and (001) segments. Via this barrierless kinetic route the first three-dimensional islands rapidly form wherever there are substantial local gradients along ⟨100⟩ in-plane directions.
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81.07.Ta Quantum dots
81.05.Cy Elemental semiconductors
81.16.Dn Self-assembly
81.15.Cd Deposition by sputtering
68.65.Hb Quantum dots (patterned in quantum wells)

Evolution of basal plane dislocations during 4H-silicon carbide homoepitaxy

Z. Zhang and T. S. Sudarshan

Appl. Phys. Lett. 87, 161917 (2005); http://dx.doi.org/10.1063/1.2108109 (3 pages) | Cited 6 times

Online Publication Date: 14 October 2005

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A method based on the combination of molten KOH etching and reactive ion etching was developed to track dislocations from 4H-silicon carbide homoepilayer to the substrate. The conversion of basal plane dislocations (BPDs) to threading edge dislocations (TEDs) was found to occur at the epilayer/substrate interface. The BPDs with dislocation lines parallel (or approximately parallel) to the off-cut direction may propagate as BPDs into the epilayer, while those with dislocation lines forming large angles (>10°) with the off-cut direction will get converted to TEDs. A model is proposed to explain the observations.
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61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
81.65.Cf Surface cleaning, etching, patterning
66.30.Lw Diffusion of other defects
81.05.Hd Other semiconductors
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Controllable charge storage in quantum dots with independent tuning of electric fields

K. S. Gill, N. Moskovitz, L.-C. Wang, M. S. Sherwin, A. Badolato, B. Gerardot, and P. Petroff

Appl. Phys. Lett. 87, 162101 (2005); http://dx.doi.org/10.1063/1.2099519 (3 pages) | Cited 3 times

Online Publication Date: 10 October 2005

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Electrons and holes are loaded into ensembles of InAs/GaAs quantum dots using a sequence of near-infrared optical pulses and voltage biases. The number and sign of charge carriers is completely determined by one of the voltage biases in the sequence. The injected charge remains stored in the dots for at least 10 s for a range of independently varied growth-direction electric fields and is detected by purely electrical means. The storage is robust to temperatures exceeding 80 K.
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73.63.Kv Quantum dots

Contactless measurement of electrical conductivity of Si wafers independent of wafer thickness

Yang Ju, Yo Hirosawa, Hitoshi Soyama, and Masumi Saka

Appl. Phys. Lett. 87, 162102 (2005); http://dx.doi.org/10.1063/1.2105992 (3 pages) | Cited 6 times

Online Publication Date: 11 October 2005

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A method for contactless measurement of electrical conductivity of Si wafers independent of wafer thickness is presented. The effective region within which this proposed technique can be used is discussed in detail, and an equation to determine the boundary of the effective region is introduced. Three groups of Si wafers with different thicknesses were used as samples. A compact microwave instrument working at 94 GHz was used to measure the samples without contact. The reflection coefficient of the microwave signal was obtained and used to determine the electrical conductivity of the wafers. The electrical conductivities of the wafers thus measured were in good agreement with values obtained using a conventional four-point-probe method. This indicates that the proposed technique is a powerful tool for contactless measurement of electrical conductivity of Si as well as other semiconductor wafers independent of wafer thickness.
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84.37.+q Measurements in electric variables (including voltage, current, resistance, capacitance, inductance, impedance, and admittance, etc.)
73.61.Cw Elemental semiconductors

Electron-beam-induced current and cathodoluminescence studies of thermally activated increase for carrier diffusion length and lifetime in n-type ZnO

O. Lopatiuk, L. Chernyak, A. Osinsky, J. Q. Xie, and P. P. Chow

Appl. Phys. Lett. 87, 162103 (2005); http://dx.doi.org/10.1063/1.2106001 (3 pages) | Cited 13 times

Online Publication Date: 11 October 2005

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Temperature dependence of the minority carrier diffusion length and lifetime in bulk n-type ZnO was studied using electron-beam-induced current and cathodoluminescence techniques. The diffusion length was observed to increase exponentially over the temperature range from 25 °C to 125 °C, yielding activation energy of 45±2 meV. Concomitant decrease of the cathodoluminescence intensity for the near-band-edge transition was also observed. The activation energy determined by optical measurements was 58±7 meV. The larger minority carrier diffusion length and smaller luminescence intensity are attributed to the increased lifetime of nonequilibrium holes in the valence band at elevated temperatures.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
78.60.Hk Cathodoluminescence, ionoluminescence
71.20.Nr Semiconductor compounds

Capacitance-voltage characteristics of ZnO/GaN heterostructures

D. C. Oh, T. Suzuki, J. J. Kim, H. Makino, T. Hanada, T. Yao, and H. J. Ko

Appl. Phys. Lett. 87, 162104 (2005); http://dx.doi.org/10.1063/1.2108107 (3 pages) | Cited 9 times

Online Publication Date: 11 October 2005

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We have investigated the electrical properties of ZnO/GaN heterostructures by capacitance-voltage (C-V) measurements. ZnO/GaN heterostructures are fabricated on Ga-polar GaN templates by plasma-assisted molecular-beam epitaxy. The ZnO/GaN heterostructures exhibit a plateau region of 6.5 V in the C-V curves measured at 10 kHz and room temperature. Moreover, it is found that a large electron density is accumulated at the interface of ZnO/GaN, where the concentration approaches ∼ 1018 cm−3. The distinct C-V characteristics are ascribed to large conduction-band discontinuity at the ZnO/GaN heterointerface. It is suggested that the ZnO/GaN heterostructure is a very promising material for the application to heterojunction transistors.
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73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
81.05.Dz II-VI semiconductors
81.05.Ea III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
52.77.-j Plasma applications

Schottky barrier formed by network of screw dislocations in SrTiO3

Xin Guo, Zaoli Zhang, Wilfried Sigle, Eric Wachsman, and Rainer Waser

Appl. Phys. Lett. 87, 162105 (2005); http://dx.doi.org/10.1063/1.2112202 (3 pages) | Cited 3 times

Online Publication Date: 13 October 2005

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A network of screw dislocations was artificially created in SrTiO3, and characterized by transmission electron microscopy (TEM). Slight expansion of the dislocation core was observed by TEM. The composition of the dislocation core was quantified by electron energy loss spectroscopy, which reveals a 13±5% Ti/O ratio increase but no Sr/Ti ratio change in the core, demonstrating an oxygen deficiency or oxygen vacancy surplus in the dislocation core. The vacancy surplus was estimated to be about one vacancy every unit cell along the core. The dislocation core is positively charged; therefore, oxygen vacancies and holes are expelled from the dislocation network, forming a double Schottky barrier, which blocks charge carrier transports across the network. The Schottky barrier height at T = 823 K and PO2 = 2 Pa was determined to be ∼ 0.48 V.
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61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
61.72.J- Point defects and defect clusters
84.37.+q Measurements in electric variables (including voltage, current, resistance, capacitance, inductance, impedance, and admittance, etc.)

In situ hole doping of wide-gap semiconductors by dual-target simultaneous laser ablation: GaN and SiC epitaxial films

Hachizo Muto, Takashi Asano, Rong-Ping Wang, and Takeshi Kusumori

Appl. Phys. Lett. 87, 162106 (2005); http://dx.doi.org/10.1063/1.2105989 (3 pages) | Cited 2 times

Online Publication Date: 14 October 2005

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Apparatus for dual-target simultaneous laser ablation deposition and in situ doping techniques have been developed to achieve p-type doping during epitaxial growth of wide-band-gap semiconductors. The apparatus has two target holders with a target-rotation mechanism and a rotation-axis adjusting mechanism to obtain homogeneously doped films. Mg-doped GaN films have been fabricated on 6H–SiC(0001) and Si(111) substrates in NH3 ambient by simultaneous ablation of GaN and Mg-metal targets using two lasers. Junctions of the films with n-type substrates show a diode curve characteristic of p-n junctions, but not for junction with p-Si, indicating hole doping without further procedures. In situ p-type doping to SiC was also achieved by using SiC and Al4C3 targets.
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61.72.uj III-V and II-VI semiconductors
61.72.up Other materials
81.05.Ea III-V semiconductors
81.05.Hd Other semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
81.15.Fg Pulsed laser ablation deposition
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
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Enhanced grain surface effect on the temperature-dependent behavior of spin-polarized tunneling magnetoresistance of nanometric manganites

P. Dey and T. K. Nath

Appl. Phys. Lett. 87, 162501 (2005); http://dx.doi.org/10.1063/1.2089179 (3 pages) | Cited 19 times

Online Publication Date: 10 October 2005

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We have investigated the effects of nanometric grain size on magnetoresistance (MR), especially on its temperature-dependent behaviors of single-phase nanocrystalline granular La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3 samples with an average grain size in the nanometric regime (12 and 17 nm). Most interestingly, we observed that the magnitude of low-field MR, arising from spin-polarized tunneling of conduction electrons, as well as of high-field MR remains constant up to a sufficiently high temperature ( ∼ 200 K), and then drops sharply with temperature. With the application of a magnetic field, strong freezing of surface spins occur at the defect sites (having strong pinning strength of spins) of disordered grains surface as a consequence of competitive interactions between grain-boundary pinning strength (k) and magnetic field. Thermal energy (kBT), up to a considerably high temperature, remains unable to flip them from their strained condition, resulting in such a temperature insensitive behavior of MR as well as of surface spin susceptibility (χb).
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.47.Lx Magnetic oxides
75.47.Gk Colossal magnetoresistance
81.05.Rm Porous materials; granular materials
61.46.-w Structure of nanoscale materials
72.25.-b Spin polarized transport
61.72.Mm Grain and twin boundaries
75.30.Cr Saturation moments and magnetic susceptibilities

Influence of top electrode on the current-induced magnetic switching in magnetic nanopillars

T. Yang, J. Hamrle, T. Kimura, and Y. Otani

Appl. Phys. Lett. 87, 162502 (2005); http://dx.doi.org/10.1063/1.2093921 (3 pages) | Cited 2 times

Online Publication Date: 10 October 2005

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Magnetic nanopillars with variable top electrodes were fabricated to clarify the roles of the spin current and the spin accumulation in the current-induced magnetic switching. The critical switching current is significantly increased when the size of the top electrode is comparable to that of the nanopillar. This result implies that the dominant contribution in the current-induced magnetic switching is not the spin accumulation, but the spin current.
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75.60.-d Domain effects, magnetization curves, and hysteresis
72.25.Ba Spin polarized transport in metals
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Cc Other ferromagnetic metals and alloys

Nanoscale recording transducer for perpendicular magnetic recording

R. Chomko, D. Litvinov, and Sakhrat Khizroev

Appl. Phys. Lett. 87, 162503 (2005); http://dx.doi.org/10.1063/1.2099530 (3 pages)

Online Publication Date: 10 October 2005

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A study of a perpendicular magnetic recording transducer with the potential to be utilized at areal densities above 1 Tbit/in.2 is presented. Focused ion beam etching is used to fabricate the transducer via trimming of a regular nanoslider ring head from an air bearing surface. The transducer is compared with a regular ring-type head. With spinstand experiments, it is demonstrated that the overlapping width determines the effective trackwidth in the case of the transducer. With other conditions equal, the transducer indicates substantially higher efficiency. The saturation current values for the transducer and the ring head are measured to be above 350 and 800 mA turn, respectively. The performance of the transducer is tested with two types of perpendicular recording media, with and without a soft underlayer, respectively. The experiments indicate that, contrary to the common belief, the medium without a soft underlayer demonstrates the potential for achieving substantially higher areal densities.
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75.50.Ss Magnetic recording materials
07.07.Mp Transducers
85.70.Li Other magnetic recording and storage devices (including tapes, disks, and drums)
81.65.Cf Surface cleaning, etching, patterning

Effects of Mn doping on the properties of FeCr2−xMnxS4

Zhe Qu, Shun Tan, Kuang He, Jiangtao Xu, and Yuheng Zhang

Appl. Phys. Lett. 87, 162504 (2005); http://dx.doi.org/10.1063/1.2106020 (3 pages)

Online Publication Date: 11 October 2005

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In this letter, the effects of Mn doping on the magnetic properties, resistivity, and magnetoresistance of FeCr2−xMnxS4 (0 ⩽ x ⩽ 0.1) are reported. Both the Curie temperature Tc and the maximum value of the magnetization increase with Mn content, rather than decrease as found in the cases of substituting nonmagnetic ions for Cr. These effects are attributed to the fact that Mn participates in the magnetic interaction networks and strengthens the magnetism of the Cr sublattice. Interestingly, it is found that the resistivity decreases while the maximum magnetoresistance increases and shifts to higher temperature with increasing doping level. These are discussed in the context that more itinerant eg electrons are introduced with substitution of Mn for Cr.
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75.30.Hx Magnetic impurity interactions
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.47.Gk Colossal magnetoresistance
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
61.72.up Other materials
72.20.My Galvanomagnetic and other magnetotransport effects

Overcoming the barrier to 1000 A/cm width superconducting coatings

S. R. Foltyn, H. Wang, L. Civale, Q. X. Jia, P. N. Arendt, B. Maiorov, Y. Li, M. P. Maley, and J. L. MacManus-Driscoll

Appl. Phys. Lett. 87, 162505 (2005); http://dx.doi.org/10.1063/1.2106021 (3 pages) | Cited 90 times

Online Publication Date: 11 October 2005

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Remarkable progress has been made in the development of YBa2Cu3O7−δ (YBCO)-based coated conductors, and the problems of continuous processing of commercially viable tape lengths are being rapidly solved by companies around the world. However, the current carried by these tapes is presently limited to about 100 A for a 1-cm-wide tape, and this is due to a rapid decrease of critical current density (Jc) as the coating thickness is increased. We have now overcome this problem by separating relatively thin YBCO layers with very thin layers of CeO2. Using this multilayer technology, we have achieved Jc values on metal substrates of up to 4.0 MA/cm2 (75 K, self-field) in films as thick as 3.5 μm, for an extrapolated current of 1400 A/cm width.
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84.71.Mn Superconducting wires, fibers, and tapes
74.72.-h Cuprate superconductors
74.25.Sv Critical currents

Resistivity minima and Kondo effect in ferromagnetic GaMnAs films

H. T. He, C. L. Yang, W. K. Ge, J. N. Wang, X. Dai, and Y. Q. Wang

Appl. Phys. Lett. 87, 162506 (2005); http://dx.doi.org/10.1063/1.2108131 (3 pages) | Cited 15 times

Online Publication Date: 11 October 2005

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The temperature dependence of the resistivity of ferromagnetic GaMnAs, as-grown or low-temperature-annealed samples is measured from 2 to 290 K. A resistivity minimum is observed with a corresponding temperature TM around 10 K for each sample. Below TM, the resistivity exhibits logarithmic temperature dependence, as α ln(T), and α is independent of the external magnetic field up to 9 T. Such behavior is explained in terms of the Kondo effect arising from the presence of Mn interstitials in the GaMnAs samples. In addition, a well-defined T-squared dependence of resistivity is found in the temperature range between TM and the Curie temperature (TC), which is attributed to single magnon scattering.
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75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors
75.70.Ak Magnetic properties of monolayers and thin films
73.61.Ey III-V semiconductors
75.30.Mb Valence fluctuation, Kondo lattice, and heavy-fermion phenomena
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Benefits of current percolation in superconducting coated conductors

N. A. Rutter, J. H. Durrell, M. G. Blamire, J. L. MacManus-Driscoll, H. Wang, and S. R. Foltyn

Appl. Phys. Lett. 87, 162507 (2005); http://dx.doi.org/10.1063/1.2093920 (3 pages) | Cited 10 times

Online Publication Date: 11 October 2005

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The critical currents of coated conductors fabricated by metal-organic deposition (MOD) on rolling-assisted biaxially textured substrates (RABiTS) and by pulsed laser deposition (PLD) on ion-beam assisted deposition (IBAD) templates have been measured as a function of magnetic field orientation and compared to films grown on single crystal substrates. By varying the orientation of magnetic field applied in the plane of the film, we are able to determine the extent to which current flow in each type of conductor is percolative. Standard MOD/RABiTS conductors have also been compared to samples whose grain boundaries have been doped by diffusing Ca from an overlayer. We find that undoped MOD/RABiTS tapes have a less anisotropic in-plane field dependence than PLD/IBAD tapes and that the uniformity of critical current as a function of in-plane field angle is greater for MOD/RABiTS samples doped with Ca.
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74.25.F- Transport properties
74.78.-w Superconducting films and low-dimensional structures
74.72.-h Cuprate superconductors
74.25.Sv Critical currents
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
81.65.-b Surface treatments
81.15.Fg Pulsed laser ablation deposition
81.15.Jj Ion and electron beam-assisted deposition; ion plating

Thickness-dependent electronic structure of ultrathin SrRuO3 films studied by in situ photoemission spectroscopy

D. Toyota, I. Ohkubo, H. Kumigashira, M. Oshima, T. Ohnishi, M. Lippmaa, M. Takizawa, A. Fujimori, K. Ono, M. Kawasaki, and H. Koinuma

Appl. Phys. Lett. 87, 162508 (2005); http://dx.doi.org/10.1063/1.2108123 (3 pages) | Cited 24 times

Online Publication Date: 11 October 2005

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In situ thickness-dependent photoemission spectroscopy (PES) has been performed on SrRuO3 (SRO) layers deposited on SrTiO3 substrates to study the structure-induced evolution of the electronic structure. The PES spectra showing the existence of two critical film thicknesses reveal that a metal-insulator transition occurs at a film thickness of 4–5 monolayers (ML) and the evolution of Ru 4d-derived states around the Fermi level (EF) saturates at about 15 ML. The observed spectral behavior well matches the electric and magnetic properties and thickness-dependent evolution of surface morphology of the ultrathin SRO films. These experimental results suggest the importance of the disorder associated with the unique growth-mode transition in SRO films.
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71.20.Ps Other inorganic compounds
79.60.Bm Clean metal, semiconductor, and insulator surfaces
68.55.-a Thin film structure and morphology
75.50.Dd Nonmetallic ferromagnetic materials
75.70.Ak Magnetic properties of monolayers and thin films
71.30.+h Metal-insulator transitions and other electronic transitions
68.35.B- Structure of clean surfaces (and surface reconstruction)
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Magnetic coupling between Cr atoms doped at bulk and surface sites of ZnO

Q. Wang, Q. Sun, P. Jena, and Y. Kawazoe

Appl. Phys. Lett. 87, 162509 (2005); http://dx.doi.org/10.1063/1.2106023 (3 pages) | Cited 18 times

Online Publication Date: 12 October 2005

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Contrary to theoretical prediction that Cr-doped bulk ZnO is ferromagnetic, recent experiments on Cr-doped ZnO thin film reveal the coupling to be antiferromagnetic. Using first-principles calculations based on gradient corrected density functional theory, we show that a possible origin of this disagreement may be associated with the site preference of Cr. In bulk, when Cr substitutes Zn, bond contraction occurs and Cr atoms prefer to cluster around O atoms. The ferromagnetic coupling among Cr atoms is driven by Cr 3d and O 2p exchange interactions as in Cr2O cluster. However, when Cr atoms replace Zn on the surface, due to the different bonding environment, bonds expand preventing Cr atoms from clustering around O atoms. Consequently, the coupling between Cr atoms becomes antiferromagnetic.
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75.50.Pp Magnetic semiconductors
75.30.Et Exchange and superexchange interactions
75.50.Ee Antiferromagnetics

Characterization of magnetically actuated resonant cantilevers in viscous fluids

Cyril Vančura, Jan Lichtenberg, Andreas Hierlemann, and Fabien Josse

Appl. Phys. Lett. 87, 162510 (2005); http://dx.doi.org/10.1063/1.2108130 (3 pages) | Cited 12 times

Online Publication Date: 12 October 2005

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The vibration behavior of magnetically actuated resonant microcantilevers immersed in viscous fluids has been studied. A dependence of the resonance frequency and the quality factor (Q factor) on the fluid properties, such as density and viscosity and on the cantilever geometry is described. Various cantilever geometries are analyzed in pure water and glycerol solutions, and the results are explained in terms of the added displaced fluid mass and the fluid damping force for both the resonance frequency and the quality factor. An in-depth knowledge and understanding of such systems is necessary when analyzing resonant cantilevers as biochemical sensors in liquid environments.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
07.10.Cm Micromechanical devices and systems
66.20.-d Viscosity of liquids; diffusive momentum transport

Fabrication of Ni nanoparticles and their size-selective self-assembly into chains under external magnetic field

Srikanth Singamaneni and Valery Bliznyuk

Appl. Phys. Lett. 87, 162511 (2005); http://dx.doi.org/10.1063/1.2105988 (3 pages) | Cited 19 times

Online Publication Date: 13 October 2005

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In this letter, we report a simple method for the preparation of colloidal solutions of nickel nanoparticles through the ultrasonication of thermally evaporated ultrathin films. Atomic force microscopy imaging revealed that the particles had a narrow size distribution centered at 9.6 nm. Self-assembly of particles into nanochains was observed when the particles in the colloidal solution were subjected to a magnetic field during the solvent evaporation process. In the absence of an external magnetic field, randomly distributed individual nanoparticles were observed. The interplay between isotropic van der Waals forces and anisotropic magnetic forces governs the self-assembly process of the nanoparticles. The nanochains form an electrically percolating network on a solid substrate, which is characterized with linear current-voltage characteristics and a conductivity of 7×10−4 S m−1.
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81.16.Dn Self-assembly
81.07.Bc Nanocrystalline materials
61.46.-w Structure of nanoscale materials
68.37.Ps Atomic force microscopy (AFM)

Piezoelectrically induced resistance modulations in La0.7Sr0.3MnO3/Pb(Zr,Ti)O3 field effect devices

C. Thiele, K. Dörr, L. Schultz, E. Beyreuther, and W.-M. Lin

Appl. Phys. Lett. 87, 162512 (2005); http://dx.doi.org/10.1063/1.2108129 (3 pages) | Cited 30 times

Online Publication Date: 13 October 2005

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Epitaxial ferroelectric-ferromagnetic field effect devices of PbZr0.52Ti0.48O3/La0.7Sr0.3MnO3 (PZT∕LSMO) with narrow manganite channels ( ⩽ 7 nm) show butterflylike hysteretic resistance modulations (ΔR) which are commensurate with strain variation from inverse piezoelectric effect of PZT. Based on the type of observed resistance hysteresis loops, contributions from electric field effect and strain effect have been distinguished for devices with varied channel thickness. The strain-induced ΔR decreases with increasing channel thickness. The ΔR from field effect is low in the LSMO channels with 30% Sr doping.
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85.50.-n Dielectric, ferroelectric, and piezoelectric devices
77.65.-j Piezoelectricity and electromechanical effects
85.30.Tv Field effect devices

From nucleation to coercivity

G. P. Zhao, M. G. Zhao, H. S. Lim, Y. P. Feng, and C. K. Ong

Appl. Phys. Lett. 87, 162513 (2005); http://dx.doi.org/10.1063/1.2108120 (3 pages) | Cited 15 times

Online Publication Date: 13 October 2005

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The demagnetization process of an exchange-coupled hard/soft/hard magnetic system and its coercivity mechanism are investigated within a micromagnetic framework. The calculation reveals that the demagnetization is a three-step process: (i) it begins with the nucleation of a transition region, (ii) the subsequent evolution of the domain wall near the interface, and (iii) the irreversible domain-wall motion from the soft phase into hard phase. As the thickness of the soft interlayer decreases, the dominant mechanism of coercivity switches from pinning to nucleation. However, for very thin interlayer, the three-step process occurs at the same applied field.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Et Exchange and superexchange interactions
75.60.Ch Domain walls and domain structure
64.60.Q- Nucleation
75.50.Ww Permanent magnets
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