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15 Oct 2007

Volume 91, Issue 16, Articles (16xxxx)

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Appl. Phys. Lett. 91, 162101 (2007); http://dx.doi.org/10.1063/1.2794995 (3 pages)

D. J. Reilly, C. M. Marcus, M. P. Hanson, and A. C. Gossard
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Influence of growth temperature on critical current and magnetic flux pinning structures in YBa2Cu3O7−x

D. M. Feldmann, O. Ugurlu, B. Maiorov, L. Stan, T. G. Holesinger, L. Civale, S. R. Foltyn, and Q. X. Jia

Appl. Phys. Lett. 91, 162501 (2007); http://dx.doi.org/10.1063/1.2799875 (3 pages) | Cited 20 times

Online Publication Date: 15 October 2007

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We have studied the critical current density (Jc) as a function of applied magnetic field (H) magnitude and orientation for ∼ 1-μm-thick YBa2Cu3O7−x films grown by pulsed laser deposition at growth temperatures (Tg) from 730 to 870 °C. With changing Tg, alternately a high density of planar defects (YBa2Cu4Ox intergrowths) or columnar defects (Y–Cu–O nanocolumns) are introduced. These defects produce a maximum for Jc(H ∼ 1 T) parallel to the plane of the film or the film normal, respectively. For Tg ≥ 830 °C, we present evidence of a Ba–Cu–O liquid phase during growth, which results in a dramatic change in both the microstructure of the films and magnetic field orientation dependence of Jc.
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74.78.-w Superconducting films and low-dimensional structures
74.72.-h Cuprate superconductors
74.25.Sv Critical currents
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
81.15.Fg Pulsed laser ablation deposition
68.55.A- Nucleation and growth

Off-track margin in bit patterned media

Andreas Moser, Olav Hellwig, Dan Kercher, and Elisabeth Dobisz

Appl. Phys. Lett. 91, 162502 (2007); http://dx.doi.org/10.1063/1.2799174 (3 pages) | Cited 16 times

Online Publication Date: 15 October 2007

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When two dimensional patterns are written into bit patterned media the off-track margin is of great importance. Using a static write/read tester we measure the hard error rate (written-in errors) as a function of the distance from the track center. We observe three different segments. In the track center, the head writes very well and almost no hard errors are observed. At the track edges the hard error rate increases linearly over an interval, which is shown to relate to the off-track margin of the recording system. At larger distances from the track center, no islands are switched.
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02.60.-x Numerical approximation and analysis

X-ray absorption and magnetic circular dichroism characterizations of Mn doped ZnO

P. Thakur, K. H. Chae, J.-Y. Kim, M. Subramanian, R. Jayavel, and K. Asokan

Appl. Phys. Lett. 91, 162503 (2007); http://dx.doi.org/10.1063/1.2794764 (3 pages) | Cited 35 times

Online Publication Date: 15 October 2007

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We report the near-edge x-ray absorption fine structure (NEXAFS) and x-ray magnetic circular dichroism (XMCD) experiments on well characterized Mn doped ZnO thin films that show ferromagnetism at room temperature. The NEXAFS measurements at O K edge clearly exhibit a preedge spectral feature which evolves with Mn doping, similar to one observed in hole-doped cuprates and manganites. The Mn L3,2 edge NEXAFS spectra exhibit divalent Mn apart from mixed valent Mn3+/Mn4+ states. The spectral features of XMCD at Mn L3,2 edge demonstrate that ferromagnetism comes from Mn2+ ions and its dichroism shape is independent of Mn concentration.
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78.70.Dm X-ray absorption spectra
78.20.Ls Magneto-optical effects
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors

Influence of oxygen contents of carbohydrate dopants on connectivity and critical current density in MgB2 tapes

Zhaoshun Gao, Yanwei Ma, Xianping Zhang, Dongliang Wang, Huan Yang, Haihu Wen, and K. Watanabe

Appl. Phys. Lett. 91, 162504 (2007); http://dx.doi.org/10.1063/1.2790381 (3 pages) | Cited 10 times

Online Publication Date: 16 October 2007

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More recently, it has been reported that carbohydrate doping resulted in an effective substitution of carbon for boron in MgB2, and hence achieved significant flux pinning enhancement. However, the authors found that the oxygen amount in carbohydrate additives played an important role in improving Jc-B performance in MgB2. In this work, the effect of maleic anhydride (C4H2O3) and maleic acid (C4H4O4) dopants on the superconducting properties of in situ MgB2 tapes has been studied. Compared to the undoped counterpart, both doped tapes showed an enhancement of Jc values by more than an order of magnitude in high-field region; however, the transport Jc of C4H4O4 doped tapes was apparently lower than that of C4H2O3 added samples. The poor grain connectivity as a result of more MgO and voids formed during the reaction in C4H4O4 doped samples can be responsible for the reduced Jc values.
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74.25.Sv Critical currents
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)

Reconfigurable magnetic logic for all basic logic functions produced by ion bombardment induced magnetic patterning

V. Höink, D. Meyners, J. Schmalhorst, G. Reiss, D. Junk, D. Engel, and A. Ehresmann

Appl. Phys. Lett. 91, 162505 (2007); http://dx.doi.org/10.1063/1.2797949 (3 pages) | Cited 5 times

Online Publication Date: 16 October 2007

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In most common logic gates based on transistors, different logic functions have to be realized by applying a large number of logic gates, which are capable of performing only one function. A promising approach to overcome this obstacle with the additional advantage of a nonvolatile output is a reconfigurable logic based on small arrays of magnetic tunnel junctions. Here, an approach utilizing an ion bombardment induced patterning of the reference layer is proposed where the same logic unit consisting of only two magnetic tunnel junctions can be used for the AND, OR, NAND, NOR, and X(N)OR, functions.
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85.75.Ff Reprogrammable magnetic logic
84.30.Sk Pulse and digital circuits

Nanocontact spin-transfer oscillators based on perpendicular anisotropy in the free layer

G. Consolo, L. Lopez-Diaz, L. Torres, G. Finocchio, A. Romeo, and B. Azzerboni

Appl. Phys. Lett. 91, 162506 (2007); http://dx.doi.org/10.1063/1.2797967 (3 pages) | Cited 8 times

Online Publication Date: 17 October 2007

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Micromagnetic simulations are used to predict the behavior exhibited by spin-transfer oscillators when materials with perpendicular anisotropy are introduced in the “free” layer of nanocontact devices. Under a perpendicular-to-plane bias field, the frequency exhibits nonlinear dependence on the anisotropy field, mostly originated by the exchange-dominated propagating nature of spin-wave modes. The increase of frequency without using large bias fields makes it suitable for potential technological applications. A study of the feasibility of bias-field-free devices has been also performed deriving multiharmonic signals at gigahertz frequencies. Here, the magnetization describes a gyrotropic motion where both vortex-core polarization and rotation sense switch periodically.
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85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
84.30.Ng Oscillators, pulse generators, and function generators

Optimized stray-field-induced enhancement of the electron spin precession by buried Fe gates

L. Meier, G. Salis, N. Moll, C. Ellenberger, I. Shorubalko, U. Wahlen, K. Ensslin, and E. Gini

Appl. Phys. Lett. 91, 162507 (2007); http://dx.doi.org/10.1063/1.2800289 (3 pages) | Cited 1 time

Online Publication Date: 18 October 2007

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The magnetic stray field from Fe gates is used to modify the spin precession frequency of InGaAs/GaAs quantum-well electrons in an external magnetic field. By using an etching process to position the gates directly in the plane of the quantum well, the stray-field influence on the spin precession increases significantly compared with results from previous studies with top-gated structures. In line with numerical simulations, the stray-field-induced precession frequency increases as the gap between the ferromagnetic gates is reduced. The inhomogeneous stray field leads to additional spin dephasing.
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73.63.Hs Quantum wells
72.25.Mk Spin transport through interfaces

Synthesis and superconducting properties of niobium nitride nanowires and nanoribbons

U. Patel, S. Avci, Z. L. Xiao, J. Hua, S. H. Yu, Y. Ito, R. Divan, L. E. Ocola, C. Zheng, H. Claus, J. Hiller, U. Welp, D. J. Miller, and W. K. Kwok

Appl. Phys. Lett. 91, 162508 (2007); http://dx.doi.org/10.1063/1.2800809 (3 pages) | Cited 8 times

Online Publication Date: 18 October 2007

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Superconducting niobium nitride wires and ribbons with transverse dimensions down to tens of nanometers were synthesized by annealing NbSe3 nanostructure precursors in flowing ammonia gas at temperatures up to 1000 °C. Their critical temperatures increase with increasing annealing temperatures and reach 9–11.2 K when sintered at 950 °C or above. X-ray diffraction analyses identified Nb4N5 and Nb5N6 phases, dominating at annealing temperatures below and above 950 °C, respectively. Transport measurements show magnetoresistance oscillations at temperatures near the superconducting transition due to vortex-row confinement effects and voltage jumps in current-voltage characteristics at low temperatures attributed to hot-spot behavior.
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74.78.Na Mesoscopic and nanoscale systems
74.25.F- Transport properties
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
81.16.-c Methods of micro- and nanofabrication and processing

Theory for toggle magnetic random access memory: The asymmetric case

D. C. Worledge

Appl. Phys. Lett. 91, 162509 (2007); http://dx.doi.org/10.1063/1.2801388 (3 pages) | Cited 3 times

Online Publication Date: 19 October 2007

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A single domain model is used to analyze the magnetic switching of two coupled magnetic layers, relevant for toggle magnetic random access memory, for the nonideal case where the layers are not identical. In general, we show that discontinuous transitions occur across the critical switching curve and continuous transitions occur around cusps in the critical switching curve. We illustrate this with examples of thickness imbalance, anisotropy mismatch, and Néel coupling and derive the corresponding switching behavior. Features in the direct-write switching can be used to experimentally distinguish between these various sources of asymmetry.
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85.70.Li Other magnetic recording and storage devices (including tapes, disks, and drums)
75.60.-d Domain effects, magnetization curves, and hysteresis
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.30.Gw Magnetic anisotropy

BaFe12O19 thin films grown at the atomic scale from BaFe2O4 and α-Fe2O3 targets

A. L. Geiler, S. D. Yoon, Y. Chen, C. N. Chinnasamy, Z. Chen, M. Geiler, V. G. Harris, and C. Vittoria

Appl. Phys. Lett. 91, 162510 (2007); http://dx.doi.org/10.1063/1.2800890 (3 pages) | Cited 8 times

Online Publication Date: 19 October 2007

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Hexagonal barium ferrite (BaFe12O19) thin films were grown at the atomic scale by alternating target laser ablation deposition (ATLAD) of orthorhombic BaFe2O4 and rhombohedral α-Fe2O3 on ⟨00l⟩ Al2O3 substrates. Crystallographic, magnetic, and microwave properties of the ATLAD films were determined to be comparable with single crystal quality bulk and films of BaFe12O19 produced by other techniques. The ability to deposit high quality hexagonal ferrite thin films by utilizing multiple targets of different chemical compositions in the deposition routine provides unique opportunities to control the ionic distribution in the unit cell of this important class of ferrite materials.
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75.50.Gg Ferrimagnetics
81.05.-t Specific materials: fabrication, treatment, testing, and analysis
81.15.Fg Pulsed laser ablation deposition
68.55.A- Nucleation and growth
75.70.Ak Magnetic properties of monolayers and thin films
82.80.-d Chemical analysis and related physical methods of analysis
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