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6 Feb 2006

Volume 88, Issue 6, Articles (06xxxx)

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Appl. Phys. Lett. 88, 063509 (2006); http://dx.doi.org/10.1063/1.2171834 (3 pages)

M. Feng, N. Holonyak, R. Chan, A. James, and G. Walter
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Simulations of collective synchronization in Josephson junction arrays

K.-T. Kim, M.-S. Kim, Y. Chong, and J. Niemeyer

Appl. Phys. Lett. 88, 062501 (2006); http://dx.doi.org/10.1063/1.2171796 (3 pages) | Cited 9 times

Online Publication Date: 6 February 2006

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We developed a virtual model of a Josephson junction series array based on a rigorous dynamic simulation and investigated coupling effect of Josephson oscillations. Our virtual model for a superconductor-insulator-normal metal-insulator-superconductor junction array reveals that a time-lag junction instability should be avoided for global synchronization in a long array with a large number of junctions. We found that the coupling between the self-generated Josephson oscillations through a microwave transmission line plays an important role in the collective synchronization of the Josephson array.
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74.50.+r Tunneling phenomena; Josephson effects

Combining half-metals and multiferroics into epitaxial heterostructures for spintronics

H. Béa, M. Bibes, M. Sirena, G. Herranz, K. Bouzehouane, E. Jacquet, S. Fusil, P. Paruch, M. Dawber, J.-P. Contour, and A. Barthélémy

Appl. Phys. Lett. 88, 062502 (2006); http://dx.doi.org/10.1063/1.2170432 (3 pages) | Cited 49 times

Online Publication Date: 6 February 2006

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We report on the growth of epitaxial bilayers of the La2/3Sr1/3MnO3 (LSMO) half-metallic ferromagnet and the BiFeO3 (BFO) multiferroic, on SrTiO3(001) by pulsed laser deposition. The growth mode of both layers is two dimensional, which results in unit-cell smooth surfaces. We show that both materials keep their properties inside the heterostructures, i.e., the LSMO layer (11 nm thick) is ferromagnetic with a Curie temperature of ∼ 330 K, while the BFO films shows ferroelectricity down to very low thicknesses (5 nm). Conductive-tip atomic force microscope mappings of BFO/LSMO bilayers for different BFO thicknesses reveal a high and homogeneous resistive state for the BFO film that can thus be used as a ferroelectric tunnel barrier in tunnel junctions based on a half-metal.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Dd Nonmetallic ferromagnetic materials
77.80.-e Ferroelectricity and antiferroelectricity
81.15.Fg Pulsed laser ablation deposition
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Magnetic and electromagnetic wave absorption properties of α-Fe/Z-type Ba-ferrite nanocomposites

Jiu Rong Liu, Masahiro Itoh, and Ken-ichi Machida

Appl. Phys. Lett. 88, 062503 (2006); http://dx.doi.org/10.1063/1.2170402 (3 pages) | Cited 47 times

Online Publication Date: 7 February 2006

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The saturation magnetization values (Ms) of α-Fe/Ba3Co1.8Fe23.6Cr0.6O41 nanocomposites prepared by mechanically alloying α-Fe with Ba3Co1.8Fe23.6Cr0.6O41 powders increased with increasing the concentration of α-Fe. α-Fe/Ba3Co1.8Fe23.6Cr0.6O41 nanocomposites showed higher coercivity values than α-Fe and Ba3Co1.8Fe23.6Cr0.6O41 because of the effects of shape anisotropy and exchange bias. The resin compacts with 33.5 vol % α-Fe/Ba3Co1.8Fe23.6Cr0.6O41 (38, 70, 85 vol % α-Fe) powders provided good electromagnetic wave absorption performances in ranges of 7.5–16.0, 5.4–10.5, and 4.3–8.3 GHz over the absorber thicknesses of 1.3–2.5, 1.6–3.0, and 1.7–3.2 mm, respectively.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Et Exchange and superexchange interactions
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Gg Ferrimagnetics

Hydrogen-induced ferromagnetism in ZnCoO

H.-J. Lee, C. H. Park, S.-Y. Jeong, K.-J. Yee, C. R. Cho, M.-H. Jung, and D. J. Chadi

Appl. Phys. Lett. 88, 062504 (2006); http://dx.doi.org/10.1063/1.2171789 (3 pages) | Cited 49 times

Online Publication Date: 8 February 2006

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We have investigated experimentally the effects of n-type impurities such as Al and H on the magnetic properties of ZnCoO. The carrier density changes slightly upon the introduction of hydrogen but increases significantly in the case of Al. A measurement of magnetic circular dichroism indicates that, of the two impurities, only H induces ferromagnetism in ZnCoO. Our experimental data suggest that, unlike Al, H plays an important role in the enhancement of ferromagnetic spin-spin interactions that goes much beyond a carrier-mediated effect.
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78.20.Ls Magneto-optical effects
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors
61.72.-y Defects and impurities in crystals; microstructure

27 ℏ SQUID amplifier operating with high-Q resonant input load

Paolo Falferi, Michele Bonaldi, Massimo Cerdonio, Andrea Vinante, Renato Mezzena, Giovanni Andrea Prodi, and Stefano Vitale

Appl. Phys. Lett. 88, 062505 (2006); http://dx.doi.org/10.1063/1.2168252 (3 pages)

Online Publication Date: 8 February 2006

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We have extended to ultracryogenic temperatures the complete noise characterization of a low-noise two-stage superconducting quantum interference device (SQUID) amplifier developed for resonant gravitational wave detectors. The additive current noise is evaluated from open input measurements. To evaluate the back action voltage noise, the SQUID is strongly coupled to a high-Q macroscopic electrical resonator operating at 11.7 kHz. From these measurements, we estimate a minimum noise temperature of 15 μK, corresponding to 27 times the quantum-limited noise temperature. Implications of this result for the sensitivity of resonant gravitational wave detectors are briefly discussed.
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85.25.Dq Superconducting quantum interference devices (SQUIDs)
84.30.Le Amplifiers
95.55.Ym Gravitational radiation detectors; mass spectrometers; and other instrumentation and techniques

Imaging of domain wall motion in small magnetic particles through near-field microscopy

Jeroen Schoenmaker, Antonio Domingues dos Santos, Yves Souche, Antonio Carlos Seabra, and Luiz C. Sampaio

Appl. Phys. Lett. 88, 062506 (2006); http://dx.doi.org/10.1063/1.2172016 (3 pages) | Cited 3 times

Online Publication Date: 8 February 2006

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We report magneto-optical scanning near-field optical microscopy images for 4×4×0.08 μm3 and 16×16×0.08 μm3 low magnetic anisotropy Co70.4Fe4.6Si15B10 particles. Measuring magneto-optical differential susceptibility we acquired images of the domain wall movement driven by an applied magnetic field with a spatial resolution better than λ/4. For the 4×4×0.08 μm3 sized particle, a sequence of 27 magneto-optical differential susceptibility images reveals the evolution of the magnetic domain structure between positive and negative saturation fields passing through the four-domain flux-closure magnetization structure. On the 16×16×0.08 μm3 particle, we studied the role of the oscillating driving field on the susceptibility distribution. Comparing the different magneto-optical differential susceptibility sequences, the role of the shape anisotropy on the field induced domain wall movement is evidenced. Micromagnetic simulations were used to provide a better understanding of the domain wall movement.
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68.37.Uv Near-field scanning microscopy and spectroscopy
75.60.Jk Magnetization reversal mechanisms
78.20.Ls Magneto-optical effects

Anisotropies of magnetic Compton profiles in Co/Pd multilayer system

H. Sakurai, M. Ota, F. Itoh, M. Itou, Y. Sakurai, and A. Koizumi

Appl. Phys. Lett. 88, 062507 (2006); http://dx.doi.org/10.1063/1.2172408 (3 pages) | Cited 2 times

Online Publication Date: 8 February 2006

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Anisotropies of spin-projected wave functions are measured on a Pd/Co multilayer system by measuring magnetic Compton profiles. The anisotropies of the wave functions are decomposed into the contributions of Co 3d states and Pd 4d states using an atomic model by a Hartree-Fock calculation assuming uniaxial (cylindrical) symmetry. Perpendicular anisotropy in Pd/Co multilayers is dominated by the anisotropy of Co 3d states; states with both magnetic quantum number ∣m∣ = 2 and ∣m∣ = 1 contribute.
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75.30.Gw Magnetic anisotropy
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Cc Other ferromagnetic metals and alloys
78.70.-g Interactions of particles and radiation with matter

Doping concentration dependence of room-temperature ferromagnetism for Ni-doped ZnO thin films prepared by pulsed-laser deposition

Xiaoxue Liu, Fangting Lin, Linlin Sun, Wenjuan Cheng, Xueming Ma, and Wangzhou Shi

Appl. Phys. Lett. 88, 062508 (2006); http://dx.doi.org/10.1063/1.2170420 (3 pages) | Cited 60 times

Online Publication Date: 8 February 2006

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High-quality Ni-doped ZnO thin films of single phase with preferred c-axis growth orientation were formed on Si (100) substrates by pulsed-laser deposition at room temperature. The films exhibited room-temperature ferromagnetic behaviors with saturation magnetic moment per Ni atom of 0.37μB,0.26μB,0.25μB and 0.21μB for the Ni concentration of 1, 3, 5, and 7 at. %, respectively. The decrease of ferromagnetism with doping concentration demonstrates that ferromagnetism observed at room temperature is an intrinsic property of Ni–ZnO thin films, not from any secondary phase.
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61.72.uj III-V and II-VI semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
68.55.-a Thin film structure and morphology
81.15.Fg Pulsed laser ablation deposition

Engineering periodic arrays of nanoscale twin boundaries in bulk YBa2Cu3O7−δ with RuO2 additions

L. Shlyk, G. Krabbes, G. Fuchs, C. Mickel, B. Rellinghaus, and K. Nenkov

Appl. Phys. Lett. 88, 062509 (2006); http://dx.doi.org/10.1063/1.2172157 (3 pages) | Cited 3 times

Online Publication Date: 8 February 2006

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A periodic array of nano-scale twin boundaries (TBs) was formed inside melt-processed YBa2Cu3O7−δ with RuO2 additions. The arrangement of the regular TBs along c axis improves irreversibility line and critical current density. We found a characteristic kink at Bcr = 2.75 T on the c-axis irreversibility line, indicative of correlated disorder. The field tilt angle below which pinning by the TB is effective is found to be ∼ 40°. We have attempted to reconcile our data with the predictions of the Bose-glass model for correlated disorder.
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74.72.-h Cuprate superconductors
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
61.72.Mm Grain and twin boundaries
74.25.Sv Critical currents

Detection of pico-Tesla magnetic fields using magneto-electric sensors at room temperature

Junyi Zhai, Zengping Xing, Shuxiang Dong, Jiefang Li, and D. Viehland

Appl. Phys. Lett. 88, 062510 (2006); http://dx.doi.org/10.1063/1.2172706 (3 pages) | Cited 86 times

Online Publication Date: 9 February 2006

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The measurement of low-frequency (10−2–103 Hz) minute magnetic field variations (10−12 Tesla) at room temperature in a passive mode of operation would be critically enabling for deployable neurological signal interfacing and magnetic anomaly detection applications. However, there is presently no magnetic field sensor capable of meeting all of these requirements. Here, we present new bimorph and push-pull magneto-electric laminate composites, which incorporate a charge compensation mechanism (or bridge) that dramatically enhances noise rejection, enabling achievement of such requirements.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
07.55.-w Magnetic instruments and components
06.30.Ka Basic electromagnetic quantities

Oscillatory thickness dependence of the coercive field in magnetic three-dimensional antidot arrays

A. A. Zhukov, A. V. Goncharov, P. A. J. de Groot, M. A. Ghanem, P. N. Bartlett, R. Boardman, H. Fangohr, V. Novosad, and G. Karapetrov

Appl. Phys. Lett. 88, 062511 (2006); http://dx.doi.org/10.1063/1.2171792 (3 pages) | Cited 9 times

Online Publication Date: 9 February 2006

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Recent developments in magnetic applications, such as data storage, sensors, and transducers, are stimulating intense research into magnetism on submicrometer-length scales. Emerging self-assembly fabrication techniques have been proposed as viable, low-cost methods to prepare such submicron structures. In this letter we present studies on magnetic nanostructures with 3D architectures, fabricated using a self-assembly template method. We find that the patterning transverse to the film plane, which is a unique feature of this method, governs the magnetic behavior. In particular, the coercive field, a key parameter for magnetic materials, was found to demonstrate an oscillatory dependence on film thickness.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Vv High coercivity materials
75.50.Tt Fine-particle systems; nanocrystalline materials
81.16.Dn Self-assembly
81.16.Rf Micro- and nanoscale pattern formation

Fe/MgO interface engineering for high-output-voltage device applications

C. Tiusan, M. Sicot, M. Hehn, C. Belouard, S. Andrieu, F. Montaigne, and A. Schuhl

Appl. Phys. Lett. 88, 062512 (2006); http://dx.doi.org/10.1063/1.2172717 (3 pages) | Cited 20 times

Online Publication Date: 9 February 2006

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The magnetotransport characteristics of Fe/MgO/Fe epitaxial tunnel junctions are reported. For clean Fe/MgO interfaces, a tunnel magnetoresistance of 150% is measured. However, the magnetoresistance decreases rapidly with the applied voltage. Consequently, the main parameter to optimize for device application, namely the output voltage, remains relatively low. This limitation has been solved by interface engineering through the insertion of carbon impurities at the Fe/MgO interface. Although the tunnel magnetoresistance amplitude is slightly reduced, its variation versus the applied voltage becomes strongly asymmetric with large magnetoresistance maintained up to 1.5 V. This determines a large increase of the tunnel junction output voltage.
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73.40.Gk Tunneling
85.75.−d

Effect of interlayer magnetic coupling on the Jc of YBa2Cu3O7/insulator/YBa2Cu3O7 trilayers

X. Wang and J. Z. Wu

Appl. Phys. Lett. 88, 062513 (2006); http://dx.doi.org/10.1063/1.2173218 (3 pages) | Cited 13 times

Online Publication Date: 9 February 2006

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We have studied critical current density (Jc) in YBa2Cu3O7−x/insulator/YBa2Cu3O7−x trilayers as the interlayer magnetic coupling is reduced by increasing the insulator thickness (tI). Although the thickness of both YBa2Cu3O7−x layers were fixed at 0.25 μm, the self- and weak-field Jc of the trilayer sample increases monotonically with tI from the lowest value for a 0.5-μm-thick YBa2Cu3O7−x film at tI = 0, and saturates at the Jc value for a 0.25-μm-thick YBa2Cu3O7−x film at tI ≈ 20 nm. This result suggests an intimate correlation between Jc and magnetic coupling of vortex segments in two YBCO layers.
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74.78.-w Superconducting films and low-dimensional structures
74.25.Sv Critical currents
74.72.-h Cuprate superconductors
74.78.Fk Multilayers, superlattices, heterostructures
74.25.Ha Magnetic properties including vortex structures and related phenomena

Magnetoresistance of atomic-scale electromigrated nickel nanocontacts

Z. K. Keane, L. H. Yu, and D. Natelson

Appl. Phys. Lett. 88, 062514 (2006); http://dx.doi.org/10.1063/1.2172232 (3 pages) | Cited 20 times

Online Publication Date: 9 February 2006

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We report measurements of the electron transport through atomic-scale constrictions and tunnel junctions between ferromagnetic electrodes. Structures are fabricated using a combination of e-beam lithography and controlled electromigration. Sample geometries are chosen to allow independent control of electrode bulk magnetizations. As junction size is decreased to the single channel limit, conventional anisotropic magnetoresistance (AMR) increases in magnitude, approaching the size expected for tunneling magnetoresistance (TMR) upon tunnel junction formation. Significant mesoscopic variations are seen in the magnitude and sign of the magnetoresistance, and no evidence is found of large ballistic magnetoresistance effects.
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75.47.Np Metals and alloys
73.63.Rt Nanoscale contacts
66.30.Qa Electromigration
75.50.Cc Other ferromagnetic metals and alloys

Nonlinear response of a superconducting filter to a wide band excitation

M. I. Salkola

Appl. Phys. Lett. 88, 062515 (2006); http://dx.doi.org/10.1063/1.2172067 (3 pages)

Online Publication Date: 10 February 2006

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Passive superconducting filters are inherently nonlinear devices that distort information-bearing signals and give rise to spectral broadening. Wide band signals efficiently probe both the causes and effects of filter nonlinearities. In particular, a coherent pulse may evaluate the power spectrum of a distortion signal and the nonlinear transmission coefficient in a single measurement. The nonlinear response depends on the bandwidth of the probing signal and implicitly on the data rate.
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85.25.-j Superconducting devices

Oriented barium hexaferrite thick films with narrow ferromagnetic resonance linewidth

Yajie Chen, Tomokazu Sakai, Taiyang Chen, Soack D. Yoon, Anton L. Geiler, Carmine Vittoria, and Vincent G. Harris

Appl. Phys. Lett. 88, 062516 (2006); http://dx.doi.org/10.1063/1.2173240 (3 pages) | Cited 24 times

Online Publication Date: 10 February 2006

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Hexagonal BaFe12O19 ferrite films, having thicknesses ranging from 200–500 μm, were prepared by a screen printing process followed by sintering heat treatments. Structural, magnetic, and microwave measurements confirmed that the polycrystalline films were suitable for applications in self-biasing microwave devices in that they exhibited a large remanence (4πMr = 3800 G), high hysteresis loop squareness (Mr/Ms = 0.96) and low microwave loss. A derivative linewidth ΔH of 310 Oe was measured at 55.6 GHz. This represents the lowest ΔH measured in polycrystalline hexaferrite materials. ΔH can be further improved by reducing porosity and improving the c-axis orientation of grains in polycrystalline ferrite.
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76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Gg Ferrimagnetics

Plain superconducting films as magnetic field tunable two-dimensional rectifiers

V. V. Pryadun, J. Sierra, F. G. Aliev, D. S. Golubovic, and V. V. Moshchalkov

Appl. Phys. Lett. 88, 062517 (2006); http://dx.doi.org/10.1063/1.2171788 (3 pages) | Cited 4 times

Online Publication Date: 10 February 2006

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Longitudinal and transversal permanent electric fields generated by an ac current through superconducting Pb and Nb thin strips have been studied as the function of the drive frequency (103<f<108 Hz), temperature, and magnetic field. At low frequencies (f<104 Hz) and below the critical temperature, the superconducting strips behave as one-dimensional rectifiers, whereas for higher drive frequencies (f>105 Hz) the rectification becomes two dimensional. The rectification strongly depends on the magnetic field, temperature and ac drive. The unusual dc electric field topology generated by the ac current in the superconducting strips can be explained by a local rectification due the oppositely directed asymmetric edge (Bean-Livingston type) barriers.
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74.78.-w Superconducting films and low-dimensional structures
74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
85.25.-j Superconducting devices
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