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31 Oct 2005

Volume 87, Issue 18, Articles (18xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 87, 183103 (2005); http://dx.doi.org/10.1063/1.2126118 (3 pages)

X. M. Cai, A. B. Djurišić, M. H. Xie, C. S. Chiu, and S. Gwo
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Comparison of morphology evolution of Ge(001) homoepitaxial films grown by pulsed laser deposition and molecular-beam epitaxy

Byungha Shin, John P. Leonard, James W. McCamy, and Michael J. Aziz

Appl. Phys. Lett. 87, 181916 (2005); http://dx.doi.org/10.1063/1.2108115 (3 pages) | Cited 11 times

Online Publication Date: 28 October 2005

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Using a dual molecular-beam epitaxy (MBE)–pulsed laser deposition (PLD) ultrahigh vacuum chamber, we have conducted the first experiments under identical thermal, background, and surface preparation conditions to compare Ge(001) homoepitaxial growth morphology in PLD and MBE. We find that in PLD with low kinetic energy and in MBE the film morphology evolves in a similar fashion: initially irregularly shaped mounds form, followed by pyramidal mounds with edges of the square-base along the ⟨100⟩ directions; the film roughness and mound separation increase with film thickness. In PLD with high kinetic energy, well-defined pyramidal mounds are not observed and the morphology rather resembles that of an ion-etched Ge(001) surface. The areal feature density is higher for PLD films than for MBE films grown at the same average growth rate and temperature. Furthermore, the dependence upon film thickness of roughness and feature separation differ for PLD and MBE. We attribute these differences to the higher yield of defect generation by energetic species in PLD.
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68.55.-a Thin film structure and morphology
81.05.Cy Elemental semiconductors
81.15.Fg Pulsed laser ablation deposition
68.47.Fg Semiconductor surfaces
68.35.B- Structure of clean surfaces (and surface reconstruction)
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Decreasing the hydrogen desorption temperature of LiNH2 through doping: A first-principles study

H. M. Jin and P. Wu

Appl. Phys. Lett. 87, 181917 (2005); http://dx.doi.org/10.1063/1.2056580 (3 pages) | Cited 11 times

Online Publication Date: 28 October 2005

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We present a theoretical approach to investigate the possibility of reducing desorption temperature of LiNH2 by partial element substitution. The approach was based on a first-principles study of electronic structure of LiNH2 and (Li,Mg)NH2. Results of the LiNH2 study showed that LiNH2 is nonmetallic, the bonding between Li and N is ionic, and the bonding between N and H is strongly covalent. Results of the (Li,Mg)NH2 study showed that the bonding nature of Li–N and N–H is the same as in LiNH2, but the bond strength of N–H was reduced. In addition, the system becomes metal-like after substitution. These two major differences can be used to explain the experimental observation that the hydrogen desorption temperature was reduced by Mg substitution. More important, they might be used as criteria to predict the behavior of other substitution elements.
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68.43.Mn Adsorption kinetics
71.15.-m Methods of electronic structure calculations
61.72.up Other materials
84.60.Ve Energy storage systems, including capacitor banks

The anomalous “stiffness” of biphenydimethyldithiol

D. Q. Feng, P. A. Dowben, R. Rajesh, and J. Redepenning

Appl. Phys. Lett. 87, 181918 (2005); http://dx.doi.org/10.1063/1.2120918 (3 pages) | Cited 2 times

Online Publication Date: 28 October 2005

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Although organic adsorbates and thin films are generally regarded as “soft” materials, the effective Debye temperature, indicative of the dynamic motion of lattice normal to the surface, can be very high. For biphenyldimethyldithiol, the effective Debye temperature, determined from core level photoemission from the all carbon arene rings, is comparable to that of graphite. We associate this rigidity to the stiffness of the benzene rings, and the ordering in the molecular thin film. Measurements on the sulfur of poly(hexylthiophene) show that sulfur in an arene ring is less dominated by soft modes than the pendant sulfur of biphenyldimethyldithiol on the time scale of photoemission.
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68.60.-p Physical properties of thin films, nonelectronic
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
79.60.Dp Adsorbed layers and thin films

Field emission from carbon layers containing very long and sparse nanotubes∕nanofilaments

A. L. Musatov, K. R. Izrael’yants, A. B. Ormont, A. V. Krestinin, N. A. Kiselev, V. V. Artemov, O. M. Zhigalina, and Yu. V. Grigoriev

Appl. Phys. Lett. 87, 181919 (2005); http://dx.doi.org/10.1063/1.2126797 (3 pages) | Cited 3 times

Online Publication Date: 28 October 2005

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Field-emission characteristics of carbon layers with very long (up to several mm) and sparse nanotubes (nanofilaments) have been investigated. For such layers field emission current of 10 μA is registered at very low average electric field Eav = 0.16 V/μm and the values of the field amplification coefficient β reach 45 000. It has been found that, at electric fields corresponding to the onset of the field emission, the emitting nanotubes are stretched towards the anode. At high emission currents (exceeding 30–50 μA), one or several luminous filaments have been observed in the gap between the sample and the anode. These luminous filaments are carbon nanotubes (nanofilaments) heated by the emission current.
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79.70.+q Field emission, ionization, evaporation, and desorption
61.46.-w Structure of nanoscale materials
78.67.Ch Nanotubes
78.55.Hx Other solid inorganic materials
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Charge injection and trapping in silicon nanocrystals

M. A. Rafiq, Y. Tsuchiya, H. Mizuta, S. Oda, Shigeyasu Uno, Z. A. K. Durrani, and W. I. Milne

Appl. Phys. Lett. 87, 182101 (2005); http://dx.doi.org/10.1063/1.2119431 (3 pages) | Cited 23 times

Online Publication Date: 24 October 2005

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The temperature dependence of the conduction mechanism in thin films of ∼ 8 nm diameter silicon nanocrystals is investigated using Al/Si nanocrystal/p‐Si/Al diodes. A film thickness of 300 nm is used. From 300 to 200 K, space charge limited current, in the presence of an exponential distribution of trapping states, dominates the conduction mechanism. Using this model, a trap density Nt = 2.3×1017 cm−3 and a characteristic trap temperature Tt = 1670 K can be extracted. The trap density is within an order of magnitude of the nanocrystal number density, suggesting that most nanocrystals trap single or a few carriers at most.
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81.05.Cy Elemental semiconductors
73.61.Cw Elemental semiconductors
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.63.Bd Nanocrystalline materials
72.20.Ht High-field and nonlinear effects
73.50.Fq High-field and nonlinear effects

Surfactant-mediated epitaxy of relaxed low-doped Ge films on Si(001) with low defect densities

T. F. Wietler, E. Bugiel, and K. R. Hofmann

Appl. Phys. Lett. 87, 182102 (2005); http://dx.doi.org/10.1063/1.2120900 (3 pages) | Cited 11 times

Online Publication Date: 24 October 2005

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Fully relaxed, high-quality Ge layers were grown directly on Si(001) substrates by surfactant-mediated epitaxy at high temperature with large Sb flux. We attribute the low dislocation densities in our films to an abrupt strain relief via the formation of a regular array of 90° dislocations at the interface during the initial, microrough stage of growth. This mechanism of abrupt strain relaxation occurs exclusively under high Sb coverage at temperatures ∼ 700 °C. The high growth temperature also enhances Sb segregation leading to a low background doping level of only (3–4×1016) cm−3. Thus, we regard surfactant-mediated epitaxy of relaxed Ge on Si(001) as a promising candidate for device application.
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81.05.Cy Elemental semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
61.72.uf Ge and Si
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances

Observation of interdot correlation in single pair of electromagnetically coupled quantum dots

Shohgo Yamauchi, Kazuhiro Komori, Isao Morohashi, Keishiro Goshima, Takeyoshi Sugaya, and Toshihide Takagahara

Appl. Phys. Lett. 87, 182103 (2005); http://dx.doi.org/10.1063/1.2120910 (3 pages) | Cited 13 times

Online Publication Date: 24 October 2005

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The interdot correlation in a single pair of InAs/GaAs barrier-coupled quantum dots (QDs) is investigated by microphotoluminescence spectroscopy, in which each QD is individually excited at unique energy levels. Surprisingly, we observe an anomalous increase in the luminescence intensity when the two QDs are excited simultaneously. This remarkable finding can be interpreted in terms of the electromagnetic coupling between QDs with thick barrier layers.
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78.55.Cr III-V semiconductors
73.21.La Quantum dots
78.67.Hc Quantum dots

Terahertz studies of the dielectric response and second-order phonons in a GaSe crystal

B. L. Yu, F. Zeng, V. Kartazayev, R. R. Alfano, and Krishna C. Mandal

Appl. Phys. Lett. 87, 182104 (2005); http://dx.doi.org/10.1063/1.2093944 (3 pages) | Cited 10 times

Online Publication Date: 25 October 2005

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The dielectric function and momentum relaxation time of carriers for a single-crystal GaSe were investigated using terahertz time-domain spectroscopy over the frequency range from 0.4 to 2.4 THz. The key parameters determined from THz data using the Drude model are: the plasma frequency ωp = 2.6±0.2 THz, the average momentum relaxation time 〈τ〉 = 56±2 fs, and the mobility μ = 89 cm2/V s for electrons. The THz absorption spectrum showed resonance structures attributed to the difference frequency combinations associated with acoustical and optical phonons.
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42.65.Tg Optical solitons; nonlinear guided waves

Spatial resolution of ballistic electron emission microscopy measured on metal/quantum-well Schottky contacts

C. Tivarus, J. P. Pelz, M. K. Hudait, and S. A. Ringel

Appl. Phys. Lett. 87, 182105 (2005); http://dx.doi.org/10.1063/1.2120899 (3 pages) | Cited 10 times

Online Publication Date: 25 October 2005

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Au Schottky contacts on cleaved AlGaAs/GaAs/AlGaAs quantum wells (QWs) were used as precise nanometer-scale apertures to quantify the spatial resolution of ballistic electron emission microscopy (BEEM). Both the amplitude and width of the measured average BEEM current profiles showed systematic dependencies on the QW width and Au film thickness, indicating surprisingly large BEEM resolutions of ∼ 12, ∼ 16, and ∼ 22 nm for Au film thicknesses of 4, 7, and 15 nm, respectively, but roughly independent of Au grain size. These measurements are consistent with theoretical models that include multiple hot-electron scattering at interfaces and in the bulk of the metal film.
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68.37.Vj Field emission and field-ion microscopy
72.20.Ht High-field and nonlinear effects

Room-temperature single-electron effects in silicon nanocrystal memories

C. Pace, F. Crupi, S. Lombardo, C. Gerardi, and G. Cocorullo

Appl. Phys. Lett. 87, 182106 (2005); http://dx.doi.org/10.1063/1.2123377 (3 pages) | Cited 7 times

Online Publication Date: 25 October 2005

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In this work, we present an experimental study on the single-electron effects observed at room temperature in silicon nanocrystal memories. The electrical characterization has been performed by means of a purposely designed low noise high bandwidth measurement system. Relevant statistical properties of the threshold voltage shifts induced by single-electron trapping and detrapping in the silicon dots are reported. The kinetics of electron capture and emission is also discussed.
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85.35.-p Nanoelectronic devices
85.35.Gv Single electron devices
73.63.Bd Nanocrystalline materials
71.55.Cn Elemental semiconductors

Programmable logic elements based on ferromagnetic nanodisks containing two antidots

M. Rahm, J. Stahl, and D. Weiss

Appl. Phys. Lett. 87, 182107 (2005); http://dx.doi.org/10.1063/1.2120914 (3 pages) | Cited 17 times

Online Publication Date: 26 October 2005

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Magnetoresistive elements for data storage or logic operations require reliable bistable magnetic switching. Soft magnetic nanodisks containing two antidots, which serve as pinning sites for a magnetic vortex, provide an alternative route for bistable magnetic switching. Here we show by means of micromagnetic simulations that field pulses generated by two orthogonal metallic current lines can switch the magnetic vortex core between antidots on a subnanosecond time scale. Using a third strip line to enable switching of the element’s magnetically hard layer, the logic operations AND, OR, NAND, and NOR can be established.
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85.70.Ec Magnetostrictive, magnetoacoustic, and magnetostatic devices
85.70.Li Other magnetic recording and storage devices (including tapes, disks, and drums)
84.30.Sk Pulse and digital circuits

Formation of misfit dislocations at the thin strained Si∕strain-relaxed buffer interface

Roger Loo, Romain Delhougne, Matty Caymax, and Mike Ries

Appl. Phys. Lett. 87, 182108 (2005); http://dx.doi.org/10.1063/1.2120887 (3 pages) | Cited 6 times

Online Publication Date: 26 October 2005

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Misfit dislocations at the strained Si/SiGe strain-relaxed buffer interface are known to enhance leakage currents of strained Si metal oxide semiconductor devices. As we show in this letter, misfit dislocations at this interface might be generated even for Si thicknesses below the critical thickness for layer relaxation. The important parameter is the degree of relaxation of the strain-relaxed buffer. If the strain-relaxed buffer is not fully relaxed, threading dislocations can glide during thermal treatments. This results in misfit dislocations at the strained Si/SiGe interface, because the threading dislocation segments within the strained Si are blocked by its tensile stress.
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61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
68.35.Ct Interface structure and roughness
68.60.Bs Mechanical and acoustical properties
81.40.Gh Other heat and thermomechanical treatments
85.30.Tv Field effect devices

Moisture induced surface polarization in a poly(4-vinyl phenol) dielectric in an organic thin-film transistor

Taeho Jung, Ananth Dodabalapur, Robert Wenz, and Siddharth Mohapatra

Appl. Phys. Lett. 87, 182109 (2005); http://dx.doi.org/10.1063/1.2117629 (3 pages) | Cited 70 times

Online Publication Date: 27 October 2005

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Surface polarization in a poly(4-vinyl phenol) (PVP) dielectric induced by water molecules has been qualitatively investigated in pentacene thin-film transistors. The magnitudes of drain currents from devices with PVP dielectrics subject to specific surface treatments increased with humidity, whereas the opposite responses were observed from device with SiO2 dielectrics. The increase in drain current is attributed to the accumulation of extra charge carriers induced by the surface polarization in addition to that by the vertical electric field. Such polarization effects should be carefully considered in characterizing organic and polymer thin-film transistors, particularly those with polymeric gate insulators.
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77.84.Jd Polymers; organic compounds
77.22.Ej Polarization and depolarization
77.55.-g Dielectric thin films
85.30.Tv Field effect devices

Self-sustained current oscillations in superlattices and the van der Pol equation

Z. Z. Sun, Sun Yin, X. R. Wang, J. P. Cao, Y. P. Wang, and Y. Q. Wang

Appl. Phys. Lett. 87, 182110 (2005); http://dx.doi.org/10.1063/1.2126149 (3 pages) | Cited 6 times

Online Publication Date: 27 October 2005

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The connection between self-sustained current oscillations in superlattices and the famous van der Pol (vdP) equation is established by mapping a widely used model of self-sustained current oscillations to the vdP equation. Since this equation can describe an inductor-capacitor-resistor circuit with a nonlinear resistor of negative differential conductance, we have obtained an equivalent electric circuit for superlattices in the self-sustained current oscillation regime. The origin of the equivalent inductance of a superlattice is revealed.
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73.63.-b Electronic transport in nanoscale materials and structures
72.70.+m Noise processes and phenomena

GaInN quantum wells grown on facets of selectively grown GaN stripes

Barbara Neubert, Peter Brückner, Frank Habel, Ferdinand Scholz, Till Riemann, Jürgen Christen, Martin Beer, and Joseph Zweck

Appl. Phys. Lett. 87, 182111 (2005); http://dx.doi.org/10.1063/1.2126798 (3 pages) | Cited 31 times

Online Publication Date: 27 October 2005

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Multiple GaInN quantum wells (QWs) were grown on facets with reduced piezoelectric fields (PFs) of selectively grown GaN stripes oriented along the 〈1math00〉 and 〈11math0〉 directions by metalorganic vapor phase epitaxy. We found a higher normalized growth rate for the GaInN QWs on the {1math01} facets compared to the {11math2} facets and the planar grown reference sample on unstructured template. The different luminescence wavelengths observed for the QWs on these different facets can partly be explained by the reduced PFs, but additionally indicate that the In incorporation efficiency depends on the facet type. On stripes with trapezoidal cross section, we found strong interfacet migration of In and Ga changing the local thickness and composition significantly.
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81.07.St Quantum wells
81.05.Ea III-V semiconductors
81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.55.Cr III-V semiconductors
78.67.De Quantum wells
77.65.-j Piezoelectricity and electromechanical effects

Quantum interference effect in single Pt(Ga)/C nanowire

Z. M. Liao, J. Xu, Y. P. Song, Y. Zhang, Y. J. Xing, and D. P. Yu

Appl. Phys. Lett. 87, 182112 (2005); http://dx.doi.org/10.1063/1.2125108 (3 pages) | Cited 5 times

Online Publication Date: 28 October 2005

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The electron transport behavior in single Pt(Ga)/C nanowire fabricated using focused ion beam was studied. The Pt(Ga)/C nanowire consists of <3 nm Pt particles, which are sheathed with Ga+-doped amorphous carbon layers. The temperature dependence of the nanowire resistivity measured under four-probe method indicated the weak localization effect resulting from large disorder in the system. Moreover, low temperature resistivity of the nanowire increased with decrease of its temperature, following a −math law, which could be interpreted by electron-electron interaction in weak localization regime. An anomalous positive magnetoresistance was observed as well, which was ascribed to quantum interference effect arising from Coulomb interactions and weak localization in the strong spin-orbit scattering limit.
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73.63.Nm Quantum wires
73.23.Hk Coulomb blockade; single-electron tunneling
73.20.Fz Weak or Anderson localization

Angle-dependent photovoltaic effect in Al–Si multilayers

A. Kyarad and H. Lengfellner

Appl. Phys. Lett. 87, 182113 (2005); http://dx.doi.org/10.1063/1.2126111 (3 pages) | Cited 2 times

Online Publication Date: 28 October 2005

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Al–Si multilayer stacks have been prepared by an alloying process from aluminum and silicon platelets. Irradiation of a stack with infrared to visible laser radiation generates photovoltaic signals depending on the angle of incidence of the laser beam with respect to the layer planes, with zero signal and a polarity reversal for beam and layers in parallel. Results are explained in terms of photoactive layers connected in series and symmetrically aligned along the stack axis. For light beams inclined with respect to the layer planes, asymmetry is introduced by fractional shadowing of photoactive regions due to the intransparent metallic layers.
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72.40.+w Photoconduction and photovoltaic effects
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)

Photoluminescence spectroscopy of bandgap reduction in dilute InNAs alloys

T. D. Veal, L. F. J. Piper, P. H. Jefferson, I. Mahboob, C. F. McConville, M. Merrick, T. J. C. Hosea, B. N. Murdin, and M. Hopkinson

Appl. Phys. Lett. 87, 182114 (2005); http://dx.doi.org/10.1063/1.2126117 (3 pages) | Cited 24 times

Online Publication Date: 28 October 2005

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Photoluminescence (PL) has been observed from dilute InNxAs1−x epilayers grown by molecular-beam epitaxy. The PL spectra unambiguously show band gap reduction with increasing N content. The variation of the PL spectra with temperature is indicative of carrier detrapping from localized to extended states as the temperature is increased. The redshift of the free exciton PL peak with increasing N content and temperature is reproduced by the band anticrossing model, implemented via a (5×5) k∙p Hamiltonian.
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78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
71.20.Nr Semiconductor compounds
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
71.35.-y Excitons and related phenomena
71.15.-m Methods of electronic structure calculations

Traps in AlGaN/GaN/SiC heterostructures studied by deep level transient spectroscopy

Z.-Q. Fang, D. C. Look, D. H. Kim, and I. Adesida

Appl. Phys. Lett. 87, 182115 (2005); http://dx.doi.org/10.1063/1.2126145 (3 pages) | Cited 25 times

Online Publication Date: 28 October 2005

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AlGaN/GaN/SiC Schottky barrier diodes (SBDs), with and without Si3N4 passivation, have been characterized by temperature-dependent current-voltage and capacitance-voltage measurements, and deep level transient spectroscopy (DLTS). A dominant trap A1, with activation energy of 1.0 eV and apparent capture cross section of 2×10−12 cm2, has been observed in both unpassivated and passivated SBDs. Based on the well-known logarithmic dependence of DLTS peak height with filling pulse width for a line-defect related trap, A1, which is commonly observed in thin GaN layers grown by various techniques, is believed to be associated with threading dislocations. At high temperatures, the DLTS signal sometimes becomes negative, likely due to an artificial surface-state effect.
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85.30.Kk Junction diodes
85.30.Hi Surface barrier, boundary, and point contact devices
71.55.Eq III-V semiconductors
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.61.Ey III-V semiconductors
73.30.+y Surface double layers, Schottky barriers, and work functions
81.65.Rv Passivation
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
61.72.Lk Linear defects: dislocations, disclinations
73.20.At Surface states, band structure, electron density of states
73.20.Hb Impurity and defect levels; energy states of adsorbed species

Suppression of oxidation in nickel germanosilicides by Pt incorporation

Md. Anisur Rahman, Thomas Osipowicz, K. L. Pey, L. J. Jin, W. K. Choi, D. Z. Chi, D. A. Antoniadis, E. A. Fitzgerald, and D. M. Isaacson

Appl. Phys. Lett. 87, 182116 (2005); http://dx.doi.org/10.1063/1.2120902 (3 pages) | Cited 3 times

Online Publication Date: 28 October 2005

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The effect of oxidation of 10 nm Ni/Si0.75Ge0.25 and 10 nm Ni(10 at. %Pt)/Si0.75Ge0.25 thin films at annealing temperatures ranging from 400 to 800 °C has been studied in detail by Rutherford backscattering spectrometry analysis, cross-sectional transmission electron microscopy, energy dispersive x-ray, and sheet resistance measurements. It is observed that for the films without Pt incorporation, almost two-thirds of the germanosilicide is oxidized. The incorporation of a Pt(10 at. %) into Ni not only dramatically reduces the oxidation of the germanosilicides, but also improves the interfacial roughness and morphology. The integral amount of oxygen found in the germanosilicide in the Ni(10 at. %Pt)Si0.75Si0.25 films [(1.1±0.17)×1017 at./cm2] is approximately four times less than that of NiSi0.75Si0.25 [(4.0±0.28)×1017 at./cm2]. This result is explained in terms of the roles of the higher melting point and bond energy of PtSi in NiSi and NiGe, and much lower free energy of the formation of platinum oxide.
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81.65.Mq Oxidation
73.40.Ns Metal-nonmetal contacts
61.72.Cc Kinetics of defect formation and annealing
82.80.Yc Rutherford backscattering (RBS), and other methods of chemical analysis
68.37.Lp Transmission electron microscopy (TEM)
82.80.Ej X-ray, Mössbauer, and other γ-ray spectroscopic analysis methods
68.35.Ct Interface structure and roughness
64.70.D- Solid-liquid transitions
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Bending of magnetic avalanches in MgB2 thin films

J. Albrecht, A. T. Matveev, M. Djupmyr, G. Schütz, B. Stuhlhofer, and H.-U. Habermeier

Appl. Phys. Lett. 87, 182501 (2005); http://dx.doi.org/10.1063/1.2123395 (3 pages) | Cited 12 times

Online Publication Date: 24 October 2005

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The penetration of magnetic flux into a superconductor above the lower critical field does not necessarily happen in a regular way. It is found that under particular conditions, a chaotic penetration in form of magnetic avalanches occurs. These avalanches are closely related to the so-called thermomagnetic instability, which identifies local heating due to flux line movement as the origin of the effect. In case of MgB2 thin films, these avalanches are found only below T = 10 K and are suppressed by a covering metallic layer with high thermal conductivity and sufficient thickness. These avalanches are observed in the case of partly gold covered MgB2 films by the magneto-optical Faraday effect. The investigation of avalanches propagating into a gold-covered region revealed a change of the propagation direction depending on the incident angle of these avalanches.
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74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
74.78.-w Superconducting films and low-dimensional structures
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
74.25.F- Transport properties
74.25.Ha Magnetic properties including vortex structures and related phenomena
74.25.Gz Optical properties

Heat capacity at the field-induced ferromagnetic transition in Eu0.58Sr0.42MnO3

G. J. Liu, J. R. Sun, Y. W. Xie, D. J. Wang, C. M. Xiong, H. W. Zhang, T. Y. Zhao, and B. G. Shen

Appl. Phys. Lett. 87, 182502 (2005); http://dx.doi.org/10.1063/1.2125127 (3 pages) | Cited 6 times

Online Publication Date: 27 October 2005

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Eu0.58Sr0.42MnO3 is found to own a wide variety of magnetic structures, and two subsequent magnetic transitions, a PM-AFM (paramagnetic-antiferromagnetic) transition followed by an AFM-FM (ferromagnetic) transition, are observed under proper magnetic field. It is found that the thermal response of heat capacity to the AFM-FM transition is negligibly small when this transition is well below the PM-AFM transition, and grows rapidly as the Curie temperature sweeps through the broad PM-AFM transition under the driving of magnetic field. This result indicates that the magnetic anomaly in heat capacity comes exclusively from the PM-FM transition, and the field-induced AFM-FM transition has essentially no thermal effects. A qualitative explanation for this result has been given based on the mean field theory.
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75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)

Weakening of charge order and antiferromagnetic to ferromagnetic switch over in Pr0.5Ca0.5MnO3 nanowires

S. S. Rao, K. N. Anuradha, S. Sarangi, and S. V. Bhat

Appl. Phys. Lett. 87, 182503 (2005); http://dx.doi.org/10.1063/1.2125129 (3 pages) | Cited 53 times

Online Publication Date: 27 October 2005

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We have prepared crystalline nanowires (diameter ∼ 50 nm, length ∼ a few microns) of the charge-ordering manganite Pr0.5Ca0.5MnO3 using a low reaction temperature hydrothermal method and characterized them using x-ray diffraction, transmission electron microscopy, superconducting quantum interference device (SQUID) magnetometry and electron magnetic resonance measurements. While the bulk sample shows a charge ordering transition at 245 K and an antiferromagnetic transition at 175 K, SQUID magnetometry and electron magnetic resonance experiments reveal that in the nanowires phase, a ferromagnetic transition occurs at ∼ 105 K. Further, the antiferromagnetic transition disappears and the charge ordering transition is suppressed. This result is particularly significant since the charge order in Pr0.5Ca0.5MnO3 is known to be very robust, magnetic fields as high as 27 T being needed to melt it.
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81.07.Bc Nanocrystalline materials
81.07.Vb Quantum wires
61.46.-w Structure of nanoscale materials
68.65.La Quantum wires (patterned in quantum wells)
75.50.Ee Antiferromagnetics
75.50.Dd Nonmetallic ferromagnetic materials
73.21.Hb Quantum wires
73.22.-f Electronic structure of nanoscale materials and related systems
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
71.45.-d Collective effects
68.37.Lp Transmission electron microscopy (TEM)
76.30.Da Ions and impurities: general

Effect of heating rates on superconducting properties of pure MgB2, carbon nanotube- and nano-SiC-doped in situ MgB2/Fe wires

S. K. Chen, K. S. Tan, B. A. Glowacki, W. K. Yeoh, S. Soltanian, J. Horvat, and S. X. Dou

Appl. Phys. Lett. 87, 182504 (2005); http://dx.doi.org/10.1063/1.2126148 (3 pages) | Cited 15 times

Online Publication Date: 27 October 2005

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The influence of heating rates and annealing temperatures on the transition temperatures (Tc) and critical current densities (Jc) of pure MgB2, carbon nanotube- and nano-SiC-doped in situ monofilamentary MgB2/Fe wires was investigated. It was found that higher Jc was obtained for pure MgB2 samples when heat treated with slower heating rates. SiC-doped samples also have higher Jc with slower heating rates, but the Jc is less sensitive to annealing temperatures. However, the Jc of the carbon nanotube-doped wire was found to be insensitive to heating rates. The variation in Tc and Jc with heating rate, and the different behaviors of differently doped MgB2/Fe wires, make it essential to carefully select the optimum heating rates for heat treatment.
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74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
74.25.Sv Critical currents
74.62.Dh Effects of crystal defects, doping and substitution

Technique to measure sub-microsecond magnetic field pulses using magnetic (CoPt) thin films

W. Syed, R. B. van Dover, J. R. Petrie, M. D. Mitchell, and D. A. Hammer

Appl. Phys. Lett. 87, 182505 (2005); http://dx.doi.org/10.1063/1.2126109 (3 pages)

Online Publication Date: 27 October 2005

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We introduce a technique to measure the maximum magnetic field of a submicrosecond duration pulse using magnetic CoPt thin films. In the present experiment, this technique yields a lower limit for the field intensity and reveals the sense of that peak field. The time-varying magnetic field was generated by an exploding wire array plasma called an X pinch. Using a Quantum Design Superconducting Quantum Interference Device magnetometer, two thin films were initialized with remnant magnetization along a specific direction. The two films were then placed near an X-pinch plasma column with magnetizations in opposite directions. The current driven through the X pinch induced a change in magnetization in the films, which implied a lower bound of 17 kOe for the magnitude of the maximum magnetic field to which the film was exposed.
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07.55.Ge Magnetometers for magnetic field measurements
75.70.Ak Magnetic properties of monolayers and thin films
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
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