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25 Aug 2008

Volume 93, Issue 8, Articles (08xxxx)

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Appl. Phys. Lett. 93, 083901 (2008); http://dx.doi.org/10.1063/1.2973167 (3 pages)

R. J. Martín-Palma, C. G. Pantano, and A. Lakhtakia
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Tunable anisotropic nonlinearity in superconductors with asymmetric antidot array

A. Yu. Aladyshkin, J. Van de Vondel, C. C. de Souza Silva, and V. V. Moshchalkov

Appl. Phys. Lett. 93, 082501 (2008); http://dx.doi.org/10.1063/1.2973903 (3 pages) | Cited 3 times

Online Publication Date: 25 August 2008

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The influence of the spatial asymmetry of the pinning potential on the spectral composition of the voltage, induced in perforated superconducting Al bridges by the injection of a sinusoidal bias current, was investigated. The loss of the mirror symmetry of the pinning potential leads to the appearance of even Fourier components in the induced voltage in the vicinity of the superconducting phase transition line on the H-T diagram (H is the external magnetic field, and T is the temperature). Artificially-introduced asymmetry for vortex motion makes it possible to create low-resistive materials, in which nonlinearity depends on the direction of injected electrical currents.
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74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)

Direct visualization of dipolar ferromagnetic domain structures in Co nanoparticle monolayers by electron holography

Kazuo Yamamoto, Sara A. Majetich, Martha R. McCartney, Madhur Sachan, Saeki Yamamuro, and Tsukasa Hirayama

Appl. Phys. Lett. 93, 082502 (2008); http://dx.doi.org/10.1063/1.2973675 (3 pages) | Cited 17 times

Online Publication Date: 25 August 2008

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Electron holography has revealed dipolar ferromagnetic domain structures in ordered monolayer arrays of Co nanoparticles. In zero-field-cooling experiments, we demonstrate the formation of micron-sized magnetically ordered regions with transverse domain walls, and a spatially varying moment order parameter. Truncated vortex structures that can be modified by an applied field are observed near the sample edges. The collectively magnetized state shows soft magnetic properties and long-range order that is stable over times over 1 h at 108 K.
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75.70.Kw Domain structure (including magnetic bubbles and vortices)
75.75.-c Magnetic properties of nanostructures
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Cc Other ferromagnetic metals and alloys
75.50.Tt Fine-particle systems; nanocrystalline materials

Fast domain wall motion in nanostripes with out-of-plane fields

Andrew Kunz and Sarah C. Reiff

Appl. Phys. Lett. 93, 082503 (2008); http://dx.doi.org/10.1063/1.2976678 (3 pages) | Cited 15 times

Online Publication Date: 26 August 2008

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Controlling domain wall motion is important due to the impact on the viability of proposed nanowire devices. One hurdle is slow domain wall speed when driven by fields greater than the Walker field due to nucleation of vortices in the wall. We present simulation results detailing the dynamics of these vortices including the nucleation and subsequent fast ejection of the vortex core leading to fast domain wall speeds. The ejection is due to the reversal of the core moments by an out-of-plane field. The technique can be used to produce domain walls of known orientation, independent of the initial state.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ch Domain walls and domain structure

Magnetic behavior of graphene absorbed with N, O, and F atoms: A first-principles study

M. Wu, En-Zuo Liu, and J. Z. Jiang

Appl. Phys. Lett. 93, 082504 (2008); http://dx.doi.org/10.1063/1.2973209 (3 pages) | Cited 29 times

Online Publication Date: 27 August 2008

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Stable configurations, electronic structures, and magnetic behaviors for N, O, and F absorptions on graphene have been investigated by first-principles calculations. It is found that F atom is located just above the C atom, while both O and N atoms are located above the carbon-carbon bond. Strong interactions between absorbers and C atoms exist. N adatom has an unsaturated electron and its electrons are polarized, inducing 0.84 μB magnetic moment per N atom for graphene, while both F and O adatoms do not have polarized electrons, which results in no net magnetic moment. Possible mechanisms were discussed for different magnetic behaviors of graphene absorbed with N, O, and F atoms. We find that the p-type doping of graphene by N atom can result in strongly spin-polarized impurity states, which may lead to a possible pathway to high temperature magnetic order in graphene.
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75.30.Cr Saturation moments and magnetic susceptibilities
72.25.-b Spin polarized transport
73.20.Hb Impurity and defect levels; energy states of adsorbed species
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Imaging of magnetic colloids under the influence of magnetic field by cryogenic transmission electron microscopy

Jinsong Wu, M. Aslam, and Vinayak P. Dravid

Appl. Phys. Lett. 93, 082505 (2008); http://dx.doi.org/10.1063/1.2976751 (3 pages) | Cited 1 time

Online Publication Date: 28 August 2008

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The application of superparamagnetic nanoparticles for in vivo magnetic resonance imaging (MRI) under external ac magnetic field has attracted considerable research efforts in recent years. However, it is unclear how superparamagnetic nanostructures arrange themselves in fluidic environment under external magnetic field. Here, we report direct visualization of the effect of applied magnetic field to the ferrofluids (about 6 nm superparamagnetic magnetite (Fe3O4) nanoparticle “colloidal” suspension) using the cryogenic transmission electron microscopy (cryo-TEM). While long dipole chains (up to millimeter range) of the magnetite along the magnetic lines are found in samples dried inside the magnetic field, only short dipole chains (within tens of nanometer scale) with random orientations are observed in the wet sample observed by cryo-TEM. In the wet sample, aggregations of medium-length dipole chains (up to hundreds of nanometer) can be observed at the areas where the nanoparticles are “solidified” when phase separation occurs. In situ formation of flux-closure rings is observed at the edge where vitreous ice sublimes due to high-energy electron radiation that leaves magnetite nanoparticles isolated in the vacuum. Such observations may help elucidate the nature of magnetic field-induced assembly in fluidic environment as in the physiological aqueous conditions in MRI and related applications.
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75.75.-c Magnetic properties of nanostructures
82.70.Kj Emulsions and suspensions
75.50.Mm Magnetic liquids
82.70.Dd Colloids
75.50.Tt Fine-particle systems; nanocrystalline materials

Input impedance and gain of a gigahertz amplifier using a dc superconducting quantum interference device in a quarter wave resonator

Lafe Spietz, Kent Irwin, and José Aumentado

Appl. Phys. Lett. 93, 082506 (2008); http://dx.doi.org/10.1063/1.2970967 (3 pages) | Cited 18 times

Online Publication Date: 29 August 2008

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Due to their superior noise performance, superconducting quantum interference devices (SQUIDs) are an attractive alternative to high electron mobility transistors for constructing ultra-low-noise microwave amplifiers for cryogenic use. We describe the use of a lumped element SQUID inductively coupled to a quarter wave resonator. The resonator acts as an impedance transformer and also makes it possible to accurately measure the input impedance and intrinsic microwave characteristics of the SQUID. We present a model for input impedance and gain, compare it to the measured scattering parameters, and describe how to use the model for the systematic design of low-noise microwave amplifiers with a wide range of performance characteristics.
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84.30.Le Amplifiers
85.25.Dq Superconducting quantum interference devices (SQUIDs)
85.30.Tv Field effect devices

Defect dependent ferromagnetism in MgO doped with Ni and Co

J. Narayan, Sudhakar Nori, D. K. Pandya, D. K. Avasthi, and A. I. Smirnov

Appl. Phys. Lett. 93, 082507 (2008); http://dx.doi.org/10.1063/1.2977614 (3 pages) | Cited 15 times

Online Publication Date: 29 August 2008

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We have investigated magnetic properties of MgO single crystals doped with Ni and Co impurities, and studied changes in magnetic properties after heavy ion irradiation. These results are compared with doped single-crystal thin films that contain a higher concentration of trapped defects. The as-grown bulk single crystals, which contain a small equilibrium concentration of vacancies, exhibit a perfect paramagnetic behavior throughout the temperature range and magnetic field. By introducing defects either by ion irradiation or by thin film deposition, which have trapped defects, we are able to achieve defect-mediated ferromagnetic ordering.
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61.72.jd Vacancies
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
61.72.up Other materials
75.50.Dd Nonmetallic ferromagnetic materials
81.15.Fg Pulsed laser ablation deposition
75.20.Ck Nonmetals

Tunnel magnetoresistance of 604% at 300 K by suppression of Ta diffusion in CoFeB/MgO/CoFeB pseudo-spin-valves annealed at high temperature

S. Ikeda, J. Hayakawa, Y. Ashizawa, Y. M. Lee, K. Miura, H. Hasegawa, M. Tsunoda, F. Matsukura, and H. Ohno

Appl. Phys. Lett. 93, 082508 (2008); http://dx.doi.org/10.1063/1.2976435 (3 pages) | Cited 222 times

Online Publication Date: 29 August 2008

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The authors observed tunnel magnetoresistance (TMR) ratio of 604% at 300 K in Ta/Co20Fe60B20/MgO/Co20Fe60B20/Ta pseudo-spin-valve magnetic tunnel junction annealed at 525 °C. To obtain high TMR ratio, it was found critical to anneal the structure at high temperature above 500 °C, while suppressing the Ta diffusion into CoFeB electrodes and in particular to the CoFeB/MgO interface. X-ray diffraction measurement of MgO on SiO2 or Co20Fe60B20 shows that an improvement of MgO barrier quality, in terms of the degree of the (001) orientation and stress relaxation, takes place at annealing temperatures above 450 °C. The highest TMR ratio observed at 5 K was 1144%.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.Pq Other materials
66.30.H- Self-diffusion and ionic conduction in nonmetals
81.40.Gh Other heat and thermomechanical treatments
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