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27 Oct 2008

Volume 93, Issue 17, Articles (17xxxx)

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Appl. Phys. Lett. 93, 172501 (2008); http://dx.doi.org/10.1063/1.3005561 (3 pages)

Yan Wang, X. F. Han, and X.-G. Zhang
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Effect of Co interlayers in Fe/MgO/Fe magnetic tunnel junctions

Yan Wang, X. F. Han, and X.-G. Zhang

Appl. Phys. Lett. 93, 172501 (2008); http://dx.doi.org/10.1063/1.3005561 (3 pages) | Cited 9 times

Online Publication Date: 27 October 2008

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The effect of Co interlayers in –Fe(001)/Co/MgO/Co/Fe– magnetic tunnel junctions is studied by first-principles calculation. We confirm that the Co layers inserted at the two Fe/MgO interfaces strongly influence the interfacial resonance (IR) states and the tunneling magnetoresistance (TMR). The effect is not monotonic. Strongest IR occurs at Co layer thickness of 0.5 monolayer (ML). With 1 ML Co, the IR is dramatically reduced and TMR ratio is maximized.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)

Evolution of magnetization configuration phase diagram with increasing exchange coupling in synthetic antiferromagnets

Hideo Fujiwara

Appl. Phys. Lett. 93, 172502 (2008); http://dx.doi.org/10.1063/1.2990611 (3 pages) | Cited 6 times

Online Publication Date: 27 October 2008

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Comprehensive analysis of the magnetization response to an in-plane field for a balanced synthetic antiferromagnet has been performed. Evolution of the critical field pseudoastroids and the magnetization configuration phase diagram with increasing exchange coupling are presented. It is found that the nonparallel magnetization configurations are classified into two, one existing for all exchange coupling strengths and the other starting to appear at a critical value. The analytic formulas for the off-anisotropy-axis cusp of the pseudoastroid are also presented.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Ee Antiferromagnetics
75.30.Et Exchange and superexchange interactions

In-plane c-axis oriented barium ferrite films with self-bias and low microwave loss

Young-Yeal Song, Jaydip Das, Zihui Wang, Wei Tong, and Carl E. Patton

Appl. Phys. Lett. 93, 172503 (2008); http://dx.doi.org/10.1063/1.3010374 (3 pages) | Cited 13 times

Online Publication Date: 28 October 2008

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Hybrid pulse laser deposition and liquid phase epitaxy methods have been used to produce in-plane c-axis (IPCA) oriented barium ferrite (BaM) films on a-plane (11math0) sapphire substrates with low microwave loss and a high remanence. Total thicknesses were from 5 to 20 μm. A reasonable compromise for low loss and high remanence was reached at a thickness of 7 μm, with a remanence ratio of 0.84 and a 59 GHz peak-to-peak derivative linewidth of 250 Oe. The 20 μm thick film had a linewidth of 110 Oe, one of the smallest values ever obtained for IPCA BaM films.
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75.70.-i Magnetic properties of thin films, surfaces, and interfaces
68.55.at Other materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Gg Ferrimagnetics

Ferromagnetic mechanism in Ni-doped anatase TiO2

Jun Chen, Guang-Hong Lu, Honghong Cao, Tianmin Wang, and Yun Xu

Appl. Phys. Lett. 93, 172504 (2008); http://dx.doi.org/10.1063/1.3002291 (3 pages) | Cited 6 times

Online Publication Date: 28 October 2008

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The ferromagnetism in Ni-doped anatase TiO2 is studied by a series of supercell density functional calculations. Our results show that O vacancy (VO) plays a key role of ferromagnetism in Ni-doped TiO2, and the magnetic origin is of Ni3+ ion. In the stoichiometric configuration of 2 Ni with one VO, the system shows the ferromagnetic insulating ground state. A vacancy-mediated direct-exchange mechanism is proposed and it reasonably interprets the ferromagnetism in such structure without using the concept of charge carriers.
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75.50.Dd Nonmetallic ferromagnetic materials
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
61.72.jd Vacancies
61.72.up Other materials
61.66.Bi Elemental solids
61.66.Dk Alloys

Iron nanoparticle driven spin-valve behavior in aligned carbon nanotube arrays

J. D. Bergeson, S. J. Etzkorn, M. B. Murphey, L. Qu, J. Yang, L. Dai, and A. J. Epstein

Appl. Phys. Lett. 93, 172505 (2008); http://dx.doi.org/10.1063/1.2999374 (3 pages) | Cited 2 times

Online Publication Date: 28 October 2008

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We report the operation of spin-valve structures formed from arrays of aligned carbon nanotubes. The devices require only one deposited ferromagnetic layer with the embedded iron catalyst nanoparticle serving as the other magnetic electrode. A peak in the resistance occurs clearly as a result of the reversal of the magnetization of the electrodes. Device magnetoresistance ratios reach 25%, yielding an estimate of the spin scattering length of 9 μm at low temperature.
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85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
85.35.Kt Nanotube devices

10 ℏ superconducting quantum interference device amplifier for acoustic gravitational wave detectors

P. Falferi, M. Bonaldi, M. Cerdonio, R. Mezzena, G. A. Prodi, A. Vinante, and S. Vitale

Appl. Phys. Lett. 93, 172506 (2008); http://dx.doi.org/10.1063/1.3002321 (3 pages) | Cited 3 times

Online Publication Date: 28 October 2008

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From measurements of additive and back action noise, we estimate the noise temperature of a two-stage superconducting quantum interference device (SQUID) amplifier developed for the acoustic wide-band gravitational wave detectors of the next generation. The minimum noise temperature evaluated at 8.9 kHz and T ≤ 0.3 K is about 4 μK and corresponds to 10 times the quantum-limited noise temperature. As the high-Q electrical resonator used for the back action noise measurements behaves in many respects like the input load constituted by an acoustic gravitational wave detector, this SQUID amplifier is expected to keep the same noise performance once installed on the detector.
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85.25.Dq Superconducting quantum interference devices (SQUIDs)
04.80.Nn Gravitational wave detectors and experiments
05.40.Ca Noise

Asymmetric critical current of niobium microbridges with ferromagnetic stripe

A. Papon, K. Senapati, and Z. H. Barber

Appl. Phys. Lett. 93, 172507 (2008); http://dx.doi.org/10.1063/1.3009207 (3 pages) | Cited 3 times

Online Publication Date: 28 October 2008

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A highly asymmetric current-voltage (IV) characteristic is reported in long Nb microbridges with a ferromagnetic cobalt overlayer. In the hysteretic regime, asymmetry as high as 100% was observed between forward and reverse critical currents. Such an asymmetry is a result of the competing magnetic fields produced at the edge of the bridge due to the bias current and the magnetic overlayer. While the temperature dependence of the critical current in the shallow branch of hysteretic IV curves (Ic−) followed a typical Ginzburg–Landau (GL) behavior, Ic+, the critical current of the other branch increased more sharply at low temperature. Local heating of the bridge due to the bias current was found to have no significant effect on the nature of critical current asymmetry.
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74.25.Sv Critical currents
74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
85.25.-j Superconducting devices

Asymmetric energy potential of pinned domain walls at constrictions

Peter Warnicke

Appl. Phys. Lett. 93, 172508 (2008); http://dx.doi.org/10.1063/1.3009287 (3 pages)

Online Publication Date: 28 October 2008

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The pinning properties of magnetic vortex domain walls were experimentally investigated in patterned nanowires with artificial constrictions. Domain walls were nucleated and depinned by applied magnetic fields and observed with magnetic force microscopy. It was found that the pinning strength is dependent on the chirality of the vortex wall. Micromagnetic simulations confirm these results and reveal the energy potential of the pinned domain wall, including the width of the pinning site.
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75.60.Ch Domain walls and domain structure
81.16.Rf Micro- and nanoscale pattern formation
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Magnetic field effect on the hydrogen desorption and recombination reaction in disproportionated Sm–Co-type alloys

S. Rivoirard, J. Lyubina, E. Beaugnon, and O. Gutfeisch

Appl. Phys. Lett. 93, 172509 (2008); http://dx.doi.org/10.1063/1.3000660 (3 pages) | Cited 1 time

Online Publication Date: 29 October 2008

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This paper reports on a unique approach to influence the hydrogen desorption and recombination reaction in hydrogen disproportionated SmCo-type alloys upon heating in vacuum (HDDR process) by applying magnetic fields up to 4.1 T. The recombination reaction is affected strongly by the magnetic field. More specifically, hydrogen desorption is shifted to higher temperatures when the field is increased. This influence of magnetic field on the equilibrium of the HDDR reaction is qualitatively explained by its effect on the respective phase stability. It is related to the contribution of the magnetic energy term to the total Gibbs free energy of the reactants.
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68.43.Nr Desorption kinetics
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
81.30.Bx Phase diagrams of metals, alloys, and oxides
65.40.G- Other thermodynamical quantities

Temperature dependent dynamic and static magnetic response in magnetic tunnel junctions with Permalloy layers

J. F. Sierra, V. V. Pryadun, F. G. Aliev, S. E. Russek, M. García-Hernández, E. Snoeck, and V. V. Metlushko

Appl. Phys. Lett. 93, 172510 (2008); http://dx.doi.org/10.1063/1.3005644 (3 pages) | Cited 2 times

Online Publication Date: 29 October 2008

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Ferromagnetic resonance and static magnetic properties of CoFe/Al2O3/CoFe/Py and CoFe/Al2O3/CoFeB/Py magnetic tunnel junctions and of 25 nm thick single-layer Permalloy (Py) films have been studied as a function of temperature down to 2 K. The temperature dependence of the ferromagnetic resonance excited in the Py layers in magnetic tunnel junctions shows “kneelike” enhancement of the resonance frequency accompanied by an anomaly in the magnetization near 60 K. We attribute the anomalous static and dynamic magnetic response at low temperatures to interface stress induced magnetic reorientation transition at the Py interface which could be influenced by dipolar soft-hard layer coupling through the Al2O3 barrier.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Bb Fe and its alloys
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
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.)

Room temperature magnetic stabilization of buried cobalt nanoclusters within a ferromagnetic matrix studied by soft x-ray magnetic circular dichroism

A. T. Hindmarch, K. J. Dempsey, J. P. Morgan, B. J. Hickey, D. A. Arena, and C. H. Marrows

Appl. Phys. Lett. 93, 172511 (2008); http://dx.doi.org/10.1063/1.3012368 (3 pages) | Cited 3 times

Online Publication Date: 29 October 2008

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Single dusting layers of size-selected Co nanoclusters (NCs) of sizes ranging from 1.5–5.5 nm have been deposited by a gas-phase aggregation method in ultrahigh vacuum, and embedded within a NiFe matrix. Magnetic hysteresis loops have been obtained using soft x-ray magnetic circular dichroism, which shows that these Co NCs embedded in NiFe exhibit room temperature ferromagnetism with identical coercivity to the surrounding NiFe film. The strong local exchange field at the interface between NiFe and Co NCs, combined with the magnetic anisotropy of the NiFe film, allows stabilization of NC ferromagnetism which persists to room temperature.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
78.20.Ls Magneto-optical effects
75.50.Cc Other ferromagnetic metals and alloys
75.50.Bb Fe and its alloys

Near-isotropic performance of intrinsically anisotropic high-temperature superconducting tapes due to self-assembled nanostructures

Y. L. Zuev, D. K. Christen, S. H. Wee, A. Goyal, and S. W. Cook

Appl. Phys. Lett. 93, 172512 (2008); http://dx.doi.org/10.1063/1.3009286 (3 pages) | Cited 8 times

Online Publication Date: 29 October 2008

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We report material and operating parameter conditions where prototype high-temperature superconducting tape conductors exhibit in-plane critical current characteristics that are essentially field orientation independent. This phenomenon is observed for specific magnetic field intensities that depend on the operating temperature and in materials having strong flux pinning by extended nanoscale structures aligned roughly along the crystalline c-axis. The effect can be described by a simple model for the field dependence of critical current density, generalized for anisotropic electronic response. This description may provide insight into means to fine tune the material properties for nearly isotropic performance characteristics at a preferred field and temperature.
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84.71.Mn Superconducting wires, fibers, and tapes
74.25.Sv Critical currents
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
74.78.Na Mesoscopic and nanoscale systems
81.16.Dn Self-assembly
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Role of pinning in current driven domain wall motion in wires with perpendicular anisotropy

C. Burrowes, D. Ravelosona, C. Chappert, S. Mangin, Eric E. Fullerton, J. A. Katine, and B. D. Terris

Appl. Phys. Lett. 93, 172513 (2008); http://dx.doi.org/10.1063/1.2998393 (3 pages) | Cited 15 times

Online Publication Date: 30 October 2008

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Spatial and time resolved giant magnetoresistance measurements are used to detect thermally activated domain wall (DW) propagation along wires formed from spin valves having perpendicular magnetic anisotropy. We show that field- and current-induced DW propagation are both strongly influenced by the presence of pinning defects. As a result, the creep motion of DWs over macroscopic length scales is found to be stochastic. Only the combination of a high polarized current density and/or magnetic field allows DWs to propagate freely. These results may help explain low current-driven DW velocities observed in previous experiments.
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75.60.Ch Domain walls and domain structure
75.30.Gw Magnetic anisotropy
75.47.De Giant magnetoresistance

Controlled manipulation of individual vortices in a superconductor

E. W. J. Straver, J. E. Hoffman, O. M. Auslaender, D. Rugar, and Kathryn A. Moler

Appl. Phys. Lett. 93, 172514 (2008); http://dx.doi.org/10.1063/1.3000963 (3 pages) | Cited 10 times

Online Publication Date: 30 October 2008

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We report controlled local manipulation of single vortices by low temperature magnetic force microscopy in a thin film of superconducting Nb. We are able to position the vortices in arbitrary configurations and to measure the distribution of local depinning forces. This technique opens up possibilities for the characterization and use of vortices in superconductors.
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74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
74.78.-w Superconducting films and low-dimensional structures
74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)

High tunneling magnetoresistance in Fe/GaOx/Ga1−xMnxAs with metal/insulator/semiconductor structure

H. Saito, A. Yamamoto, S. Yuasa, and K. Ando

Appl. Phys. Lett. 93, 172515 (2008); http://dx.doi.org/10.1063/1.3013327 (3 pages) | Cited 7 times

Online Publication Date: 31 October 2008

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We fabricated Fe/GaOx/Ga1−xMnxAs magnetic tunnel diodes that had a metal/insulator/semiconductor (MIS) structure. A tunneling magnetoresistance (TMR) ratio up to 58% was observed, which is the highest value yet reported in MIS-type TMR devices. This indicates that GaOx is an excellent tunnel-barrier material for spin-dependent transport between 3d-ferromagnetic metal and GaAs-based semiconductors.
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85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
85.30.Kk Junction diodes
85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)
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