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27 Jul 2009

Volume 95, Issue 4, Articles (04xxxx)

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Appl. Phys. Lett. 95, 043110 (2009); http://dx.doi.org/10.1063/1.3184784 (3 pages)

Felix Loske and Angelika Kühnle
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Direct observation of the controlled magnetization reversal processes in Py/Al/Py asymmetric ring stacks

L. Huang, M. A. Schofield, and Y. Zhu

Appl. Phys. Lett. 95, 042501 (2009); http://dx.doi.org/10.1063/1.3187545 (3 pages) | Cited 3 times

Online Publication Date: 27 July 2009

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Electron holographic experiments were performed to study the magnetization reversal process of patterned Py/Al/Py (20nm/20nm/10nm) asymmetric ring stacks. By changing the in-plane field applied perpendicular to the ring’s symmetric axis, we directly observed the vortex-based magnetization reversal process through controlled domain wall motion and annihilation. The two magnetic layers were found to switch at different critical fields, leading to the existence of various distinct domain state combinations. Quantitative agreement was obtained between measured phase shifts and those derived from micromagnetic calculations, which allows us to resolve the layer-by-layer magnetic behavior as a function of applied external field.
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75.70.Kw Domain structure (including magnetic bubbles and vortices)
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.60.Jk Magnetization reversal mechanisms
75.70.Ak Magnetic properties of monolayers and thin films

Long baseline planar superconducting gradiometer for biomagnetic imaging

C. Granata, A. Vettoliere, C. Nappi, M. Lisitskiy, and M. Russo

Appl. Phys. Lett. 95, 042502 (2009); http://dx.doi.org/10.1063/1.3189285 (3 pages) | Cited 1 time

Online Publication Date: 28 July 2009

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A niobium based dc-superconducting quantum interference device (SQUID) planar gradiometer with a long baseline (50 mm) for biomagnetic applications has been developed. The pickup antenna consists of two integrated rectangular coils connected in series and magnetically coupled to a dc-SQUID in a double parallel washer configuration by two series multiturn input coils. Due to a high intrinsic responsivity, the sensors have shown at T = 4.2 K a white magnetic flux noise spectral density as low as 3 μΦ0/Hz1/2. The spectral density of the magnetic field noise referred to one sensing coil, is 3.0 fT/Hz1/2 resulting in a gradient spectral noise of 0.6 fT/(cm Hz1/2). In order to verify the effectiveness of such sensors for biomagnetic applications, the magnetic response to a current dipole has been calculated and the results have been compared with those of an analogous axial gradiometer. The results show that there is no significant difference. Due to their high intrinsic balance and good performances, planar gradiometers may be the elective sensors for biomagnetic application in a soft shielded environment.
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87.85.Pq Biomedical imaging
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
85.25.Dq Superconducting quantum interference devices (SQUIDs)
87.57.-s Medical imaging

Superconducting double spin valve with extraordinary large tunable magnetoresistance

Francesco Giazotto

Appl. Phys. Lett. 95, 042503 (2009); http://dx.doi.org/10.1063/1.3190498 (3 pages)

Online Publication Date: 28 July 2009

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A superconducting double spin valve device is proposed. Its operation takes advantage of the interplay between the spin-filtering effect of ferromagnetic insulators and superconductivity-induced out-of-equilibrium transport. Depending on the degree of nonequilibrium, extraordinary large tunnel magnetoresistance as large as 102%–106% can be obtained for realistic material parameters, and it can be tuned over several orders of magnitude under proper voltage biasing and temperature. The relevance of this setup for low-temperature applications is further discussed.
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85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
85.70.Ec Magnetostrictive, magnetoacoustic, and magnetostatic devices
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
74.25.F- Transport properties
74.25.Ha Magnetic properties including vortex structures and related phenomena
85.25.-j Superconducting devices

Fast Fourier transform spectrometer readout for large arrays of microwave kinetic inductance detectors

S. J. C. Yates, A. M. Baryshev, J. J. A. Baselmans, B. Klein, and R. Güsten

Appl. Phys. Lett. 95, 042504 (2009); http://dx.doi.org/10.1063/1.3159818 (3 pages) | Cited 13 times

Online Publication Date: 29 July 2009

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Microwave kinetic inductance detectors have great potential for large, very sensitive detector arrays for use in, for example, submillimeter imaging. Being intrinsically readout in the frequency domain, they are particularly suited for frequency domain multiplexing allowing ∼ 1000 s of devices to be readout with one pair of coaxial cables. However, this moves the complexity of the detector from the cryogenics to the warm electronics. We present here the concept and experimental demonstration of the use of fast Fourier transform spectrometer readout, showing no deterioration of the noise performance compared to the low noise analog mixing while allowing high multiplexing ratios.
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07.57.Pt Submillimeter wave, microwave and radiowave spectrometers; magnetic resonance spectrometers, auxiliary equipment, and techniques
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors

Josephson squelch filter for quantum nanocircuits

P. Forn-Díaz, R. N. Schouten, W. A. den Braver, J. E. Mooij, and C. J. P. M. Harmans

Appl. Phys. Lett. 95, 042505 (2009); http://dx.doi.org/10.1063/1.3186047 (3 pages) | Cited 1 time

Online Publication Date: 30 July 2009

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We fabricated and tested a squelch circuit consisting of a copper powder filter with an embedded Josephson junction connected to ground. For small signals (squelch ON), the small junction inductance attenuates strongly from dc to at least 1 GHz, while for higher frequencies dissipation in the copper powder increases the attenuation exponentially with frequency. For large signals (squelch OFF), the circuit behaves as a regular metal powder filter. The measured ON/OFF ratio is larger than 50 dB up to 50 MHz. This squelch can be applied in low temperature measurement and control circuitry for quantum nanostructures, such as superconducting qubits and quantum dots.
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85.35.-p Nanoelectronic devices
84.30.Vn Filters
84.71.Mn Superconducting wires, fibers, and tapes
85.25.Cp Josephson devices

Structural and electrical characterization of SiO2/MgO(001) barriers on Si for a magnetic transistor

A. Kohn, A. Kovács, T. Uhrmann, T. Dimopoulos, and H. Brückl

Appl. Phys. Lett. 95, 042506 (2009); http://dx.doi.org/10.1063/1.3182732 (3 pages) | Cited 3 times

Online Publication Date: 30 July 2009

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We report a structural and electrical study of sputter-deposited SiO2/MgO barriers for developing magnetic Si-based transistors. We propose that SiO2/MgO tunneling barriers may utilize spin-filtering by achieving crystalline MgO (001) while reducing spin-scattering due to the Si/SiO2 interface. We find that MgO (<3 nm thick) crystallizes with (001) preferred orientation on thermally oxidized Si(<2 nm). Typical processing temperatures do not cause significant intermixing with SiO2 or ferromagnetic electrode. Conversely, MgO on Si is amorphous up to 2 nm thick. Capacitance-voltage characteristics of MgO capacitors are influenced significantly by the density of interface-states, as high as 5×1013 cm−2 eV−1 while Si/SiO2/MgO structures are electrically beneficial by reducing to 6×1012 cm−2 eV−1.
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73.40.Gk Tunneling
85.30.Tv Field effect devices
84.32.Tt Capacitors
73.20.At Surface states, band structure, electron density of states
82.45.Fk Electrodes

Robust coupling of superconducting order parameter in a mesoscale NbN–Fe–NbN epitaxial structure

S. K. Bose and R. C. Budhani

Appl. Phys. Lett. 95, 042507 (2009); http://dx.doi.org/10.1063/1.3189284 (3 pages) | Cited 2 times

Online Publication Date: 30 July 2009

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We report an unconventional and promising route to self-assemble distributed superconductor-ferromagnet-superconductor (S-F-S) Josephson junctions on single crystal [100] MgO. These structures consist of [110] epitaxial nanoplaquettes of Fe covered with superconducting NbN films of varying thickness. The S-F-S structures are characterized by a strong magnetoresistance (MR) anisotropy for the in-plane and out-of-plane magnetic fields. The stronger in-plane MR suggests decoherence of S-F-S junctions whose critical current follows a (1−T/Tc) and (1−T/Tc)1/2 dependence for T ≈ Tc and T⪡Tc, respectively, in accordance with the theory of supercurrent transport in such junctions.
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74.50.+r Tunneling phenomena; Josephson effects
74.78.-w Superconducting films and low-dimensional structures
74.25.Sv Critical currents
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