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26 Feb 2007

Volume 90, Issue 9, Articles (09xxxx)

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Appl. Phys. Lett. 90, 093127 (2007); http://dx.doi.org/10.1063/1.2710743 (3 pages)

Y. Q. Li, J. X. Tang, H. Wang, J. A. Zapien, Y. Y. Shan, and S. T. Lee
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Aqueous microdrop manipulation and mixing using ferrofluid dynamics

Woo-Bin Song, Zhenwen Ding, Chulwoo Son, and Babak Ziaie

Appl. Phys. Lett. 90, 092501 (2007); http://dx.doi.org/10.1063/1.2709514 (3 pages) | Cited 9 times

Online Publication Date: 26 February 2007

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In this letter, the authors present a simple method to manipulate free microdroplets using ferrofluid dynamics. For droplet transport, a set of periodic lines of ferrofluid on top of a silicon wafer is created by a single strip magnet and dynamically changed by the rotation of a magnetic stirrer underneath it. It is demonstrated that the speed of droplet movement depends on the rotation speed of the magnetic stirrer as well as the size of the droplet. For better droplet mixing efficiency, a discontinuous pattern at the mixing spots is created by adding a smaller strip magnet to the above setup.
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47.65.Cb Magnetic fluids and ferrofluids
47.55.D- Drops and bubbles
47.51.+a Mixing
47.54.De Experimental aspects
47.61.Jd Multiphase flows
47.85.Np Fluidics

Magnetic characterization of CoFeB/MgO and CoFe/MgO interfaces

Ezana Negusse, A. Lussier, J. Dvorak, Y. U. Idzerda, S. R. Shinde, Y. Nagamine, S. Furukawa, K. Tsunekawa, and D. D. Djayaprawira

Appl. Phys. Lett. 90, 092502 (2007); http://dx.doi.org/10.1063/1.2709619 (3 pages) | Cited 8 times

Online Publication Date: 26 February 2007

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The use of CoFeB ferromagnetic electrodes in place of CoFe has been shown to significantly increase the tunneling magnetoresistance (TMR) of MgO based magnetic tunnel junctions (MTJs). By using soft x-ray scattering techniques, we show that the behavior of the magnetic moments located at the CoFe–MgO interface are drastically different from the rest of the CoFe film, whereas the magnetic response of the CoFeB–MgO interfacial moments is coherent with the film’s bulk. Our results support the view that the high TMR values observed in MgO based MTJs with CoFeB electrodes are due to the uniform magnetic response of the entire CoFeB electrode including the MgO interfacial moments.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.-m Magnetotransport phenomena; materials for magnetotransport
75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Bb Fe and its alloys
78.70.Ck X-ray scattering

Collective spin modes in monodimensional magnonic crystals consisting of dipolarly coupled nanowires

G. Gubbiotti, S. Tacchi, G. Carlotti, N. Singh, S. Goolaup, A. O. Adeyeye, and M. Kostylev

Appl. Phys. Lett. 90, 092503 (2007); http://dx.doi.org/10.1063/1.2709909 (3 pages) | Cited 42 times

Online Publication Date: 26 February 2007

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Magnetization dynamics of dipolarly coupled nanowire arrays has been studied by Brillouin light scattering. Measurements performed in uniformly magnetized wires as a function of the transferred wave vector demonstrated the existence of several discrete collective modes, propagating through the structure with a periodic dispersion curve encompassing several Brillouin zones relative to the artificial spatial periodicity. This experimental evidence has been quantitatively explained by a theoretical model which permits the calculation of the dispersion relation for collective modes in patterned arrays through the numerical solution of an eigenvalue problem for an integral operator.
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75.30.Ds Spin waves
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
78.35.+c Brillouin and Rayleigh scattering; other light scattering

Thermal effects on the critical current of spin torque switching in spin valve nanopillars

M. L. Schneider, M. R. Pufall, W. H. Rippard, S. E. Russek, and J. A. Katine

Appl. Phys. Lett. 90, 092504 (2007); http://dx.doi.org/10.1063/1.2709963 (3 pages) | Cited 11 times

Online Publication Date: 28 February 2007

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In spin valve nanopillars, temperature affects the spin torque reversal of the free magnetic layer. The authors compare values of zero temperature critical switching current Ic0 extrapolated from room temperature pulsed current switching measurements to those of quasistatic current sweeps at 5 K. The values extrapolated from the room temperature pulsed switching probability measurements are always less than or equal to those of the low temperature quasistatic measurements. Further, the room temperature device-to-device variations of the critical switching current are drastically reduced at low temperature, where the Ic0 agrees with theory. Finally, the authors find that Ic0 scales with the free layer volume, as expected.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Tt Fine-particle systems; nanocrystalline materials

Current-induced vortex-vortex switching in a nanopillar comprising two Co nano-rings

T. Yang, A. Hirohata, M. Hara, T. Kimura, and Y. Otani

Appl. Phys. Lett. 90, 092505 (2007); http://dx.doi.org/10.1063/1.2710185 (3 pages) | Cited 13 times

Online Publication Date: 28 February 2007

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The authors fabricated a current-perpendicular-to-plane pseudo-spin-valve nanopillar comprising a thick and a thin Co rings with deep submicron lateral sizes. The dc current can effectively induce the flux-closure vortex states in the rings with desired chiralities. Abrupt transitions between the vortex states are also realized by the dc current and detected with the giant magnetoresistance effect. Both Oersted field and spin-transfer torque are found important to the magnetic transitions, but the former is dominant. They can be designed to cooperate with each other in the vortex-to-vortex transitions by carefully setting the chirality of the vortex state in the thick Co ring.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.60.-d Domain effects, magnetization curves, and hysteresis
75.50.Cc Other ferromagnetic metals and alloys
75.50.Tt Fine-particle systems; nanocrystalline materials
75.47.De Giant magnetoresistance
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Reduction of ac transport and magnetization loss of a high-Tc superconducting tape by placing soft ferromagnetic materials at the edges

F. Gömöry, J. Šouc, M. Vojenčiak, A. K. M. Alamgir, Z. Han, and Ch. Gu

Appl. Phys. Lett. 90, 092506 (2007); http://dx.doi.org/10.1063/1.2710753 (3 pages) | Cited 10 times

Online Publication Date: 1 March 2007

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Reduction of dissipation at the transport of alternating current as well as when exposed to a time varying magnetic field has been observed in the tape from high-temperature superconductor Bi2Sr2Ca2Cu3O10 covered by a Ni sheath at the edges. The improvement of critical current can explain this achievement only in part. Analysis of experimental data shows that also a more favorable pattern of the magnetic flux penetration should take place.
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74.72.-h Cuprate superconductors
74.25.Ha Magnetic properties including vortex structures and related phenomena
74.25.F- Transport properties
74.25.Sv Critical currents
84.71.Mn Superconducting wires, fibers, and tapes

Magnetic properties and tunneling magnetoresistance effect in Fe–CdFeS granular films

K. W. Liu, J. Y. Zhang, D. Z. Shen, X. J. Wu, B. H. Li, B. S. Li, Y. M. Lu, and X. W Fan

Appl. Phys. Lett. 90, 092507 (2007); http://dx.doi.org/10.1063/1.2709892 (3 pages) | Cited 9 times

Online Publication Date: 2 March 2007

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Granular thin film samples composed of Fe clusters embedded in CdFeS were prepared with using low pressure metal organic chemical vapor deposition. Temperature dependence of the resistivity of the granular film follows well the relationship for the tunneling transport between the iron granules. A large negative magnetoresistance of about 2.2% at 0.1 T was observed at room temperature due to the presence of Fe clusters. The hysteresis behavior observed in the magnetoresistance corresponded exactly to those of the hysteresis loop in the magnetization measurement, which evidently showed the correlation between the magnetoresistance and global magnetization in the sample.
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75.70.Ak Magnetic properties of monolayers and thin films
75.47.Pq Other materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Bb Fe and its alloys
75.50.Cc Other ferromagnetic metals and alloys
75.50.Pp Magnetic semiconductors

Quantitative investigation of a terahertz artificial magnetic resonance using oblique angle spectroscopy

T. Driscoll, G. O. Andreev, D. N. Basov, S. Palit, Tong Ren, Jack Mock, Sang-Yeon Cho, Nan Marie Jokerst, and D. R. Smith

Appl. Phys. Lett. 90, 092508 (2007); http://dx.doi.org/10.1063/1.2679766 (3 pages) | Cited 13 times

Online Publication Date: 2 March 2007

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The authors present a spectroscopic analysis of a planar split-ring-resonator (SRR) medium at terahertz frequencies, quantitatively characterizing the associated magnetic resonance. Experimental quantification at terahertz and infrared frequencies of metamaterial optical constants has been primarily absent, largely due to the difficulty of collecting phase information at these frequencies. In this letter, the authors circumvent the need for phase information in the characterization by acquiring the power transmitted through the metamaterial at a series of oblique angles, and relating the multiangle data set to the effective permittivity and permeability through the Fresnel expressions. The resulting measurements reveal the expected resonant permeability of the SRR which exhibits a range of negative values, the minimum value being μ = −0.8 at 1.1 THz.
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78.70.Gq Microwave and radio-frequency interactions
42.70.-a Optical materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

High performance hard magnetic NdFeB thick films for integration into micro-electro-mechanical systems

N. M. Dempsey, A. Walther, F. May, D. Givord, K. Khlopkov, and O. Gutfleisch

Appl. Phys. Lett. 90, 092509 (2007); http://dx.doi.org/10.1063/1.2710771 (3 pages) | Cited 33 times

Online Publication Date: 2 March 2007

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5 μm thick NdFeB films have been sputtered onto 100 mm Si substrates using high rate sputtering (18 μm/h). Films were deposited at ⩽ 500 °C and then annealed at 750 °C for 10 min. While films deposited at temperatures up to 450 °C have equiaxed grains, the size of which decreases with increasing deposition temperature, the films deposited at 500 °C have columnar grains. The out-of-plane remanent magnetization increases with deposition temperature, reaching a maximum value of 1.4 T, while the coercivity remains constant at about 1.6 T. The maximum energy product achieved (400 kJ/m3) is comparable to that of high-quality NdFeB sintered magnets.
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81.15.Cd Deposition by sputtering
75.70.Ak Magnetic properties of monolayers and thin films
75.50.Ww Permanent magnets
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
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