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28 Feb 2005

Volume 86, Issue 9, Articles (09xxxx)

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Appl. Phys. Lett. 86, 093101 (2005); http://dx.doi.org/10.1063/1.1870108 (3 pages)

G. S. Paraoanu and A. M. Halvari
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Nucleation and growth processes of α-Fe nanocrystals in amorphous NdFeBCoDy: In situ x-ray diffraction studies

W. Li, X. H. Li, H. Y. Sun, J. W. Zhang, and X. Y. Zhang

Appl. Phys. Lett. 86, 092501 (2005); http://dx.doi.org/10.1063/1.1871338 (3 pages) | Cited 6 times

Online Publication Date: 23 February 2005

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For a detailed understanding of formation processes of nanocrystals in an amorphous matrix, the study of crystal nucleation and growth processes is of basic interest. In the present study, these processes of α-Fe nanocrystals in amorphous NdFeCoDyB have been studied separately by employing in situ high-temperature x-ray diffraction measurements. A small growth activation energy Eg = (1.8±0.2)eV was determined from the isothermal time dependence of the grain size of α-Fe crystals. By these data together with the nucleation activation energy En = (3.0±0.1)eV, a coarse grain size for the α-Fe phase in the α-Fe/Nd2Fe14B nanocomposites developed from amorphous NdFeBCoDy can be understood.
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75.50.Bb Fe and its alloys
75.50.Tt Fine-particle systems; nanocrystalline materials
81.05.Bx Metals, semimetals, and alloys
81.07.Bc Nanocrystalline materials
64.70.K- Solid-solid transitions
75.50.Kj Amorphous and quasicrystalline magnetic materials
64.60.Q- Nucleation
81.16.-c Methods of micro- and nanofabrication and processing
61.46.-w Structure of nanoscale materials
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder

230% room-temperature magnetoresistance in CoFeB/MgO/CoFeB magnetic tunnel junctions

David D. Djayaprawira, Koji Tsunekawa, Motonobu Nagai, Hiroki Maehara, Shinji Yamagata, Naoki Watanabe, Shinji Yuasa, Yoshishige Suzuki, and Koji Ando

Appl. Phys. Lett. 86, 092502 (2005); http://dx.doi.org/10.1063/1.1871344 (3 pages) | Cited 286 times

Online Publication Date: 23 February 2005

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Magnetoresistance (MR) ratio up to 230% at room temperature (294% at 20 K) has been observed in spin-valve-type magnetic tunnel junctions (MTJs) using MgO tunnel barrier layer fabricated on thermally oxidized Si substrates. We found that such a high MR ratio can be obtained when the MgO barrier layer was sandwiched with amorphous CoFeB ferromagnetic electrodes. Microstructure analysis revealed that the MgO layer with (001) fiber texture was realized when the MgO layer was grown on amorphous CoFeB rather than on polycrystalline CoFe. Since there have been no theoretical studies on the MTJs with a crystalline tunnel barrier and amorphous electrodes, the detailed mechanism of the huge tunneling MR effect observed in this study is not clear at the present stage. Nevertheless, the present work is of paramount importance in realizing high-density magnetoresistive random access memory and read head for ultra high-density hard-disk drives into practical use.
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75.50.Bb Fe and its alloys
75.50.Kj Amorphous and quasicrystalline magnetic materials
75.50.Ss Magnetic recording materials
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
68.65.Ac Multilayers
75.47.Np Metals and alloys
75.75.-c Magnetic properties of nanostructures

Flux pinning enhancement in melt-processed YBa2Cu3O7 with extended nanodefects

L. Shlyk, G. Krabbes, G. Fuchs, and K. Nenkov

Appl. Phys. Lett. 86, 092503 (2005); http://dx.doi.org/10.1063/1.1874307 (3 pages) | Cited 9 times

Online Publication Date: 23 February 2005

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Melt-processed YBa2Cu3O7 (YBCO), containing a random array of Y–Ba–Cu–Ir–O nanostructures (about 20–150 nm in diameter and several μm in length) has been grown. The pinning efficiency of the resulting YBCO nanocomposite is superior to Li-doped or undoped melt-processed YBCO at higher magnetic fields. At 77 K, the irreversibility field increases more than 8 T in the presence of the extended nanodefects. The increase of the effective activation energy of the YBCO nanocomposite is nearly twice that of the Li-doped material.
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74.72.-h Cuprate superconductors
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
81.10.Fq Growth from melts; zone melting and refining
74.62.Dh Effects of crystal defects, doping and substitution

Current-induced metastable resistive state in epitaxial thin films of La1−xCaxMnO3 (x = 0.2, 0.3)

J. Gao and F. X. Hu

Appl. Phys. Lett. 86, 092504 (2005); http://dx.doi.org/10.1063/1.1870128 (3 pages) | Cited 34 times

Online Publication Date: 24 February 2005

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The influence of a transport dc current on electric resistivity has been investigated in epitaxial thin films of La1−xCaxMnO3 (x = 0.2, 0.3). A most prominent finding is the appearance of a remarkable resistive peak at temperatures well below the Curie temperature Tc. Such a resistive peak is developed when the dc current over a critical value was applied in a temperature cycling from 300 to 10 K. The resistance peak turns out to be extremely sensitive to a weak current. Even a very small current could greatly depress the height of the peak. Such a current-induced state with high resistivity is metastable compared to the pristine state. The stability of the induced state has been also studied.
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75.50.Dd Nonmetallic ferromagnetic materials
73.61.Ng Insulators
75.70.Ak Magnetic properties of monolayers and thin films
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Nanoscopic netted structure of compositional modulation in (Sm0.33Eu0.33Gd0.33)Ba2Cu3O7−δ superconductors

A. Hu, I. Hirabayashi, M. Winter, M. R. Koblischka, U. Hartmann, and H. Zhou

Appl. Phys. Lett. 86, 092505 (2005); http://dx.doi.org/10.1063/1.1871348 (3 pages) | Cited 21 times

Online Publication Date: 24 February 2005

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We report a nanoscaled crisscross network in high-quality melt-processed (Sm0.33Eu0.33Gd0.33)Ba2Cu3O7−δ superconductors investigated with atomic force microscopy (AFM) and transmission electron microscopy. In the ten-micrometer scale, such a net was unveiled as a consequence of crossing annular stripes originating from ordering compositional modulation. The AFM topographic images further displayed that this compositional stripe result in a surface modulation with a few-nanometer wavelength and roughness. The forming mechanism of this compositional stripe and its role as strong δTc-type pinning defects were discussed in the context of rare earth∕Ba oscillation around the stoichiometric ratio and the spatial fluctuation of local Tc values.
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74.72.-h Cuprate superconductors
74.70.Dd Ternary, quaternary, and multinary compounds (including Chevrel phases, borocarbides, etc.)
74.62.Bf Effects of material synthesis, crystal structure, and chemical composition
74.62.Dh Effects of crystal defects, doping and substitution
74.78.Na Mesoscopic and nanoscale systems
68.35.B- Structure of clean surfaces (and surface reconstruction)
61.72.Mm Grain and twin boundaries
68.37.Lp Transmission electron microscopy (TEM)
68.37.Ps Atomic force microscopy (AFM)
74.40.-n Fluctuation phenomena
61.66.Bi Elemental solids
61.66.Dk Alloys

Failure analysis of high temperature superconducting films at high microwave power using microsecond time domain measurements

A. J. Purnell, L. F. Cohen, L. Hao, and J. C. Gallop

Appl. Phys. Lett. 86, 092506 (2005); http://dx.doi.org/10.1063/1.1868076 (3 pages)

Online Publication Date: 24 February 2005

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A generic characteristic of resonators can be exploited to determine whether thermal or magnetic mechanisms dominate the failure of high quality, high temperature superconducting thin films at high microwave power. We demonstrate that a transient response can be superimposed on a pulse wave form with a frequency equal to the difference between the driving frequency and the resonant frequency of the resonator. When a thermal failure mechanism dominates, the resonant frequency depends on the temporal separation between pulses. This is in marked contrast to a magnetic failure mechanism, which is insensitive to the pulse separation. For poor quality films the technique cannot differentiate between vortices in weak links and local heating at weak links that happen on a short time scale. We discuss the application of the technique to noncontact microwave bolometry.
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85.25.-j Superconducting devices
74.72.-h Cuprate superconductors
74.78.-w Superconducting films and low-dimensional structures
74.25.N- Response to electromagnetic fields
74.25.F- Transport properties
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)

Large upper critical field and irreversibility field in MgB2 wires with SiC additions

M. D. Sumption, M. Bhatia, M. Rindfleisch, M. Tomsic, S. Soltanian, S. X. Dou, and E. W. Collings

Appl. Phys. Lett. 86, 092507 (2005); http://dx.doi.org/10.1063/1.1872210 (3 pages) | Cited 77 times

Online Publication Date: 25 February 2005

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Resistive transition measurements are reported for MgB2 strands with SiC dopants. The starting Mg powders were 325 mesh 99.9% pure, and the B powders were amorphous, 99.9% pure, and at a typical size of 1–2 μm. The SiC was added as 10 mol % of SiC to 90 mol % of binary MgB2 [(MgB2)0.9(SiC)0.1]. Three different SiC powders were used; the average particle sizes were 200 nm, 30 nm, and 15 nm. The strands were heat treated for times ranging from 5 to 30 min at temperatures from 675 °C to 900 °C. Strands with 200 nm size SiC additions had μ0Hirr and Bc2 which maximized at 25.4 T and 29.7 T after heating at 800 °C for 30 min. The highest values were seen for a strand with 15 nm SiC heated at 725 °C for 30 min which had a μ0Hirr of 29 T and a Bc2 higher than 33 T.
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74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
84.71.Mn Superconducting wires, fibers, and tapes
74.25.F- Transport properties
74.25.Op Mixed states, critical fields, and surface sheaths
81.40.Gh Other heat and thermomechanical treatments
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