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18 Jul 2011

Volume 99, Issue 3, Articles (03xxxx)

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Appl. Phys. Lett. 99, 032501 (2011); http://dx.doi.org/10.1063/1.3610947 (3 pages)

G. X. Miao, M. D. Mascaro, C. H. Nam, C. A. Ross, and J. S. Moodera
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Domain wall induced magnetoresistance in a superconductor/ferromagnet nanowire

G. X. Miao, M. D. Mascaro, C. H. Nam, C. A. Ross, and J. S. Moodera

Appl. Phys. Lett. 99, 032501 (2011); http://dx.doi.org/10.1063/1.3610947 (3 pages) | Cited 2 times

Online Publication Date: 18 July 2011

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In a nanowire consisting of a ferromagnet/insulator/superconductor multilayer structure, the superconductivity is shown to depend strongly on the configuration of the magnetic domain walls in the neighboring ferromagnetic layer, yielding a high magnetoresistance within a temperature range near the superconducting transition temperature TC. Micromagnetic simulations confirmed that out-of-plane stray magnetic fields from uncompensated magnetic poles play a dominant role in inducing magnetoresistance in this particular system.
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74.25.Ha Magnetic properties including vortex structures and related phenomena
74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
75.50.Bb Fe and its alloys
75.60.Ch Domain walls and domain structure
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
73.40.Rw Metal-insulator-metal structures

Low current density spin-transfer torque effect assisted by in-plane microwave field

Jianbo Wang, Congpu Mu, Weiwei Wang, Bin Zhang, Haiyan Xia, Qingfang Liu, and Desheng Xue

Appl. Phys. Lett. 99, 032502 (2011); http://dx.doi.org/10.1063/1.3611446 (3 pages) | Cited 1 time

Online Publication Date: 19 July 2011

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A strategy is presented to greatly reduce both the critical spin polarized current density and the magnetization switching time in elliptical magnetic spin valve. This method is a combination of microwave field and spin polarized current. The numerical simulation at zero temperature shows that the critical current density and the magnetization switching time are sensitive to the frequency and the amplitude of microwave magnetic field. A 75% reduction in critical current density is observed when the frequency of microwave coincides with the natural ferromagnetic resonance frequency of free layer.
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75.76.+j Spin transport effects
72.25.-b Spin polarized transport
75.40.Mg Numerical simulation studies
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance

Enhancement of microwave emission in magnetic tunnel junction oscillators through in-plane field orientation

Z. M. Zeng, P. Upadhyaya, P. Khalili Amiri, K. H. Cheung, J. A. Katine, J. Langer, K. L. Wang, and H. W. Jiang

Appl. Phys. Lett. 99, 032503 (2011); http://dx.doi.org/10.1063/1.3613965 (3 pages) | Cited 7 times

Online Publication Date: 19 July 2011

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We observe giant enhancement of microwave emission in MgO-based magnetic tunnel junction nano-oscillators through in-plane magnetic field orientation. At an optimal in-plane field angle, the output power reaches up to 240 nW, two orders of magnitude higher compared to the vicinity of the easy axis (∼1 nW). Moreover, in this condition, the linewidth is significantly narrowed (<100 MHz) and the additional precession modes are suppressed. Analysis of the data indicates that the optimal field angle is influenced by the field-like torque. The results demonstrate that controlling the in-plane magnetic field orientation can be an important strategy for developing high-power spin-torque oscillators.
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84.30.Ng Oscillators, pulse generators, and function generators
85.35.-p Nanoelectronic devices

Alternating current magnetic susceptibility of a molecular magnet submonolayer directly patterned onto a micro superconducting quantum interference device

M. J. Martínez-Pérez, E. Bellido, R. de Miguel, J. Sesé, A. Lostao, C. Gómez-Moreno, D. Drung, T. Schurig, D. Ruiz-Molina, and F. Luis

Appl. Phys. Lett. 99, 032504 (2011); http://dx.doi.org/10.1063/1.3609859 (3 pages) | Cited 2 times

Online Publication Date: 20 July 2011

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We report the controlled integration, via dip pen nanolithography, of monolayer dots of ferritin-based CoO nanoparticles (12 μB) into the most sensitive areas of a microSQUID sensor. The nearly optimum flux coupling between these nanomagnets and the microSQUID improves the achievable sensitivity by a factor 102, enabling us to measure the linear susceptibility of the molecular array down to very low temperatures (13 mK). This method opens the possibility of applying ac susceptibility experiments to characterize two-dimensional arrays of single molecule magnets within a wide range of temperatures and frequencies.
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75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Xx Molecular magnets
81.16.Nd Micro- and nanolithography
75.75.Cd Fabrication of magnetic nanostructures

Spin resolved photoelectron microscopy using a two-dimensional spin-polarizing electron mirror

C. Tusche, M. Ellguth, A. A. Ünal, C.-T. Chiang, A. Winkelmann, A. Krasyuk, M. Hahn, G. Schönhense, and J. Kirschner

Appl. Phys. Lett. 99, 032505 (2011); http://dx.doi.org/10.1063/1.3611648 (3 pages) | Cited 10 times

Online Publication Date: 21 July 2011

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We report on an imaging spin-filter for electrons. The specular reflection of low-energy electrons at the surface of a tungsten single crystal is used to project a spin-filtered two-dimensional image onto a position sensitive detector. Spin-filtering is based on the spin-dependent reflectivity of electrons due to spin-orbit coupling in the scattering target, while a two-dimensional field of view, encoded in the angle of incidence, is conserved in the outgoing beam. We characterize the efficiency of the spin-filter by recording photoelectron emission microscopy images of the magnetic domain structure of 8 monolayers cobalt grown on copper (100).
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75.70.Ak Magnetic properties of monolayers and thin films
75.60.Ch Domain walls and domain structure
78.20.Ls Magneto-optical effects
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
72.25.Ba Spin polarized transport in metals

Superconducting quantum interference devices with submicron Nb/HfTi/Nb junctions for investigation of small magnetic particles

J. Nagel, O. F. Kieler, T. Weimann, R. Wölbing, J. Kohlmann, A. B. Zorin, R. Kleiner, D. Koelle, and M. Kemmler

Appl. Phys. Lett. 99, 032506 (2011); http://dx.doi.org/10.1063/1.3614437 (3 pages) | Cited 9 times

Online Publication Date: 21 July 2011

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We investigated, at temperature 4.2 K, electric transport, flux noise, and resulting spin sensitivity of miniaturized Nb direct current superconducting quantum interference devices (SQUIDs) based on submicron Josephson junctions with HfTi barriers. The SQUIDs are either of the magnetometer-type or gradiometric in layout. In the white noise regime, for the best magnetometer we obtain a flux noise SΦ1/2 = 250nΦ0/Hz1/2, corresponding to a spin sensitivity Sμ1/2 ≥ 29μB/Hz1/2. For the gradiometer we find SΦ1/2 = 300nΦ0/Hz1/2 and Sμ1/2 ≥ 44μB/Hz1/2. The devices can still be optimized with respect to flux noise and coupling between a magnetic particle and the SQUID, leaving room for further improvement towards single spin resolution.
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85.25.Dq Superconducting quantum interference devices (SQUIDs)
74.50.+r Tunneling phenomena; Josephson effects
85.25.Cp Josephson devices

Noise properties of a resonance-type spin-torque microwave detector

Oleksandr Prokopenko, Gennadiy Melkov, Elena Bankowski, Thomas Meitzler, Vasil Tiberkevich, and Andrei Slavin

Appl. Phys. Lett. 99, 032507 (2011); http://dx.doi.org/10.1063/1.3612917 (3 pages) | Cited 3 times

Online Publication Date: 22 July 2011

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We analyze performance of a resonance-type spin-torque microwave detector (STMD) in the presence of noise and reveal two distinct regimes of STMD operation. In the high-frequency regime, the minimum detectable microwave power Pmin is limited by the low-frequency Johnson-Nyquist noise and the signal-to-noise ratio (SNR) of STMD is proportional to the input microwave power PRF. In the low-frequency regime, Pmin is limited by the magnetic noise, and the SNR is proportional to math. The developed formalism can be used for the optimization of the practical noise-handling parameters of a STMD.
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85.75.Mm Spin polarized resonant tunnel junctions
84.40.Dc Microwave circuits
02.60.Pn Numerical optimization

Modeling of the spin-transfer torque switching in FePt/MgO-based perpendicular magnetic tunnel junctions: A combined ab initio and micromagnetic simulation study

Wanjiao Zhu, Yaowen Liu, and Chun-Gang Duan

Appl. Phys. Lett. 99, 032508 (2011); http://dx.doi.org/10.1063/1.3615664 (3 pages) | Cited 3 times

Online Publication Date: 22 July 2011

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Combined ab initio and micromagnetic simulations are carried out to build up a direct relationship between critical switching current Jc and geometric structure (in-plane lattice constant a and film thickness) of FePt in magnetic tunnel junctions. Ab initio calculations predict that the strain effect can significantly influence the uniaxial magnetic anisotropy constant Ku, whereas the thickness has less significant impact on Ku. Micromagnetic simulations show that Jc linearly increases with the increasing Ku (the decreasing a) and the switching time decreases with the increasing current at a fixed Ku and is sensitive to the perpendicular spin-torque factor at negative current.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.78.Jp Ultrafast magnetization dynamics and switching
75.76.+j Spin transport effects
75.78.Cd Micromagnetic simulations

Electric field control of magnetism without magnetic bias field in the Ni/Pb(Mg1/3Nb2/3)O3-PbTiO3/Ni composite

H. C. Xuan, L. Y. Wang, Y. X. Zheng, Y. L. Li, Q. Q. Cao, S. Y. Chen, D. H. Wang, Z. G. Huang, and Y. W. Du

Appl. Phys. Lett. 99, 032509 (2011); http://dx.doi.org/10.1063/1.3616137 (3 pages)

Online Publication Date: 22 July 2011

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This paper reports on the electric field control of magnetism without magnetic bias field in a Ni/Pb(Mg1/3Nb2/3)O3-PbTiO3/Ni composite prepared by electrochemical deposition. The converse magnetoelectric effect, which was measured by an induction method, shows a peak value of 0.45 G/V at the resonance frequency of 102 kHz. Without magnetic bias field, the magnetization of the Ni layers can be controlled by an applied dc electric field in a reversible and reproducible way and shows an analogous on-off behavior with the electric field switching on and off alternatively.
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75.85.+t Magnetoelectric effects, multiferroics
75.60.Jk Magnetization reversal mechanisms
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)

Microwave coupled electron tunneling measurement of Co nanoparticles

W. Jiang, F. Tijiwa Birk, and D. Davidović

Appl. Phys. Lett. 99, 032510 (2011); http://dx.doi.org/10.1063/1.3614554 (3 pages) | Cited 2 times

Online Publication Date: 22 July 2011

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We study electron tunneling through Co nanoparticles in the presence of repeated microwave pulses at 4.2 K. While individual pulses are too weak to affect the magnetic switching field, repeated microwave pulses start to reduce the magnetic switching field at 10 μs spacing. We use I-V curve as a thermometer to show that the microwave pulses do not heat the sample, showing that magnetization in Co nanoparticles is directly excited by microwave pulses, and the relaxation time of the excitation energy is in the range of microsecond.
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73.63.Bd Nanocrystalline materials
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.75.-c Magnetic properties of nanostructures
73.40.Gk Tunneling

Proximity effect in electron-doped cuprate Josephson junctions

S. Charpentier, G. Roberge, S. Godin-Proulx, and P. Fournier

Appl. Phys. Lett. 99, 032511 (2011); http://dx.doi.org/10.1063/1.3613999 (3 pages)

Online Publication Date: 22 July 2011

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We study the proximity effect in Josephson junctions made of electron-doped cuprates Pr2–xCexCuO4 (PCCO). With superconducting electrodes at a fixed doping x = 0.15, we explore the variation of the Josephson coupling with the doping and the thickness of the barrier. We observe Josephson coupling for any type of barrier: antiferromagnetic (AF) metal, AF insulator, or paramagnetic metal. For an x = 0.05 barrier, we can estimate the normal coherence length, ξn = 12 ± 3 nm. Comparing this value with the theoretical prediction, ξn,max = 4.5 nm, we conclude that the proximity effect through a barrier of PCCO x = 0.05 is stronger than expected.
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74.45.+c Proximity effects; Andreev reflection; SN and SNS junctions
74.50.+r Tunneling phenomena; Josephson effects
74.25.Ha Magnetic properties including vortex structures and related phenomena
74.72.Ek Electron-doped
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