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17 Oct 2005

Volume 87, Issue 16, Articles (16xxxx)

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Appl. Phys. Lett. 87, 161907 (2005); http://dx.doi.org/10.1063/1.2105998 (3 pages)

J. Das, K. B. Kim, F. Baier, W. Löser, and J. Eckert
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Enhanced grain surface effect on the temperature-dependent behavior of spin-polarized tunneling magnetoresistance of nanometric manganites

P. Dey and T. K. Nath

Appl. Phys. Lett. 87, 162501 (2005); http://dx.doi.org/10.1063/1.2089179 (3 pages) | Cited 19 times

Online Publication Date: 10 October 2005

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We have investigated the effects of nanometric grain size on magnetoresistance (MR), especially on its temperature-dependent behaviors of single-phase nanocrystalline granular La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3 samples with an average grain size in the nanometric regime (12 and 17 nm). Most interestingly, we observed that the magnitude of low-field MR, arising from spin-polarized tunneling of conduction electrons, as well as of high-field MR remains constant up to a sufficiently high temperature ( ∼ 200 K), and then drops sharply with temperature. With the application of a magnetic field, strong freezing of surface spins occur at the defect sites (having strong pinning strength of spins) of disordered grains surface as a consequence of competitive interactions between grain-boundary pinning strength (k) and magnetic field. Thermal energy (kBT), up to a considerably high temperature, remains unable to flip them from their strained condition, resulting in such a temperature insensitive behavior of MR as well as of surface spin susceptibility (χb).
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.47.Lx Magnetic oxides
75.47.Gk Colossal magnetoresistance
81.05.Rm Porous materials; granular materials
61.46.-w Structure of nanoscale materials
72.25.-b Spin polarized transport
61.72.Mm Grain and twin boundaries
75.30.Cr Saturation moments and magnetic susceptibilities

Influence of top electrode on the current-induced magnetic switching in magnetic nanopillars

T. Yang, J. Hamrle, T. Kimura, and Y. Otani

Appl. Phys. Lett. 87, 162502 (2005); http://dx.doi.org/10.1063/1.2093921 (3 pages) | Cited 2 times

Online Publication Date: 10 October 2005

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Magnetic nanopillars with variable top electrodes were fabricated to clarify the roles of the spin current and the spin accumulation in the current-induced magnetic switching. The critical switching current is significantly increased when the size of the top electrode is comparable to that of the nanopillar. This result implies that the dominant contribution in the current-induced magnetic switching is not the spin accumulation, but the spin current.
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75.60.-d Domain effects, magnetization curves, and hysteresis
72.25.Ba Spin polarized transport in metals
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Cc Other ferromagnetic metals and alloys

Nanoscale recording transducer for perpendicular magnetic recording

R. Chomko, D. Litvinov, and Sakhrat Khizroev

Appl. Phys. Lett. 87, 162503 (2005); http://dx.doi.org/10.1063/1.2099530 (3 pages)

Online Publication Date: 10 October 2005

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A study of a perpendicular magnetic recording transducer with the potential to be utilized at areal densities above 1 Tbit/in.2 is presented. Focused ion beam etching is used to fabricate the transducer via trimming of a regular nanoslider ring head from an air bearing surface. The transducer is compared with a regular ring-type head. With spinstand experiments, it is demonstrated that the overlapping width determines the effective trackwidth in the case of the transducer. With other conditions equal, the transducer indicates substantially higher efficiency. The saturation current values for the transducer and the ring head are measured to be above 350 and 800 mA turn, respectively. The performance of the transducer is tested with two types of perpendicular recording media, with and without a soft underlayer, respectively. The experiments indicate that, contrary to the common belief, the medium without a soft underlayer demonstrates the potential for achieving substantially higher areal densities.
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75.50.Ss Magnetic recording materials
07.07.Mp Transducers
85.70.Li Other magnetic recording and storage devices (including tapes, disks, and drums)
81.65.Cf Surface cleaning, etching, patterning

Effects of Mn doping on the properties of FeCr2−xMnxS4

Zhe Qu, Shun Tan, Kuang He, Jiangtao Xu, and Yuheng Zhang

Appl. Phys. Lett. 87, 162504 (2005); http://dx.doi.org/10.1063/1.2106020 (3 pages)

Online Publication Date: 11 October 2005

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In this letter, the effects of Mn doping on the magnetic properties, resistivity, and magnetoresistance of FeCr2−xMnxS4 (0 ⩽ x ⩽ 0.1) are reported. Both the Curie temperature Tc and the maximum value of the magnetization increase with Mn content, rather than decrease as found in the cases of substituting nonmagnetic ions for Cr. These effects are attributed to the fact that Mn participates in the magnetic interaction networks and strengthens the magnetism of the Cr sublattice. Interestingly, it is found that the resistivity decreases while the maximum magnetoresistance increases and shifts to higher temperature with increasing doping level. These are discussed in the context that more itinerant eg electrons are introduced with substitution of Mn for Cr.
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75.30.Hx Magnetic impurity interactions
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.47.Gk Colossal magnetoresistance
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
61.72.up Other materials
72.20.My Galvanomagnetic and other magnetotransport effects

Overcoming the barrier to 1000 A/cm width superconducting coatings

S. R. Foltyn, H. Wang, L. Civale, Q. X. Jia, P. N. Arendt, B. Maiorov, Y. Li, M. P. Maley, and J. L. MacManus-Driscoll

Appl. Phys. Lett. 87, 162505 (2005); http://dx.doi.org/10.1063/1.2106021 (3 pages) | Cited 90 times

Online Publication Date: 11 October 2005

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Remarkable progress has been made in the development of YBa2Cu3O7−δ (YBCO)-based coated conductors, and the problems of continuous processing of commercially viable tape lengths are being rapidly solved by companies around the world. However, the current carried by these tapes is presently limited to about 100 A for a 1-cm-wide tape, and this is due to a rapid decrease of critical current density (Jc) as the coating thickness is increased. We have now overcome this problem by separating relatively thin YBCO layers with very thin layers of CeO2. Using this multilayer technology, we have achieved Jc values on metal substrates of up to 4.0 MA/cm2 (75 K, self-field) in films as thick as 3.5 μm, for an extrapolated current of 1400 A/cm width.
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84.71.Mn Superconducting wires, fibers, and tapes
74.72.-h Cuprate superconductors
74.25.Sv Critical currents

Resistivity minima and Kondo effect in ferromagnetic GaMnAs films

H. T. He, C. L. Yang, W. K. Ge, J. N. Wang, X. Dai, and Y. Q. Wang

Appl. Phys. Lett. 87, 162506 (2005); http://dx.doi.org/10.1063/1.2108131 (3 pages) | Cited 15 times

Online Publication Date: 11 October 2005

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The temperature dependence of the resistivity of ferromagnetic GaMnAs, as-grown or low-temperature-annealed samples is measured from 2 to 290 K. A resistivity minimum is observed with a corresponding temperature TM around 10 K for each sample. Below TM, the resistivity exhibits logarithmic temperature dependence, as α ln(T), and α is independent of the external magnetic field up to 9 T. Such behavior is explained in terms of the Kondo effect arising from the presence of Mn interstitials in the GaMnAs samples. In addition, a well-defined T-squared dependence of resistivity is found in the temperature range between TM and the Curie temperature (TC), which is attributed to single magnon scattering.
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75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors
75.70.Ak Magnetic properties of monolayers and thin films
73.61.Ey III-V semiconductors
75.30.Mb Valence fluctuation, Kondo lattice, and heavy-fermion phenomena
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Benefits of current percolation in superconducting coated conductors

N. A. Rutter, J. H. Durrell, M. G. Blamire, J. L. MacManus-Driscoll, H. Wang, and S. R. Foltyn

Appl. Phys. Lett. 87, 162507 (2005); http://dx.doi.org/10.1063/1.2093920 (3 pages) | Cited 10 times

Online Publication Date: 11 October 2005

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The critical currents of coated conductors fabricated by metal-organic deposition (MOD) on rolling-assisted biaxially textured substrates (RABiTS) and by pulsed laser deposition (PLD) on ion-beam assisted deposition (IBAD) templates have been measured as a function of magnetic field orientation and compared to films grown on single crystal substrates. By varying the orientation of magnetic field applied in the plane of the film, we are able to determine the extent to which current flow in each type of conductor is percolative. Standard MOD/RABiTS conductors have also been compared to samples whose grain boundaries have been doped by diffusing Ca from an overlayer. We find that undoped MOD/RABiTS tapes have a less anisotropic in-plane field dependence than PLD/IBAD tapes and that the uniformity of critical current as a function of in-plane field angle is greater for MOD/RABiTS samples doped with Ca.
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74.25.F- Transport properties
74.78.-w Superconducting films and low-dimensional structures
74.72.-h Cuprate superconductors
74.25.Sv Critical currents
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
81.65.-b Surface treatments
81.15.Fg Pulsed laser ablation deposition
81.15.Jj Ion and electron beam-assisted deposition; ion plating

Thickness-dependent electronic structure of ultrathin SrRuO3 films studied by in situ photoemission spectroscopy

D. Toyota, I. Ohkubo, H. Kumigashira, M. Oshima, T. Ohnishi, M. Lippmaa, M. Takizawa, A. Fujimori, K. Ono, M. Kawasaki, and H. Koinuma

Appl. Phys. Lett. 87, 162508 (2005); http://dx.doi.org/10.1063/1.2108123 (3 pages) | Cited 24 times

Online Publication Date: 11 October 2005

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In situ thickness-dependent photoemission spectroscopy (PES) has been performed on SrRuO3 (SRO) layers deposited on SrTiO3 substrates to study the structure-induced evolution of the electronic structure. The PES spectra showing the existence of two critical film thicknesses reveal that a metal-insulator transition occurs at a film thickness of 4–5 monolayers (ML) and the evolution of Ru 4d-derived states around the Fermi level (EF) saturates at about 15 ML. The observed spectral behavior well matches the electric and magnetic properties and thickness-dependent evolution of surface morphology of the ultrathin SRO films. These experimental results suggest the importance of the disorder associated with the unique growth-mode transition in SRO films.
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71.20.Ps Other inorganic compounds
79.60.Bm Clean metal, semiconductor, and insulator surfaces
68.55.-a Thin film structure and morphology
75.50.Dd Nonmetallic ferromagnetic materials
75.70.Ak Magnetic properties of monolayers and thin films
71.30.+h Metal-insulator transitions and other electronic transitions
68.35.B- Structure of clean surfaces (and surface reconstruction)
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Magnetic coupling between Cr atoms doped at bulk and surface sites of ZnO

Q. Wang, Q. Sun, P. Jena, and Y. Kawazoe

Appl. Phys. Lett. 87, 162509 (2005); http://dx.doi.org/10.1063/1.2106023 (3 pages) | Cited 18 times

Online Publication Date: 12 October 2005

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Contrary to theoretical prediction that Cr-doped bulk ZnO is ferromagnetic, recent experiments on Cr-doped ZnO thin film reveal the coupling to be antiferromagnetic. Using first-principles calculations based on gradient corrected density functional theory, we show that a possible origin of this disagreement may be associated with the site preference of Cr. In bulk, when Cr substitutes Zn, bond contraction occurs and Cr atoms prefer to cluster around O atoms. The ferromagnetic coupling among Cr atoms is driven by Cr 3d and O 2p exchange interactions as in Cr2O cluster. However, when Cr atoms replace Zn on the surface, due to the different bonding environment, bonds expand preventing Cr atoms from clustering around O atoms. Consequently, the coupling between Cr atoms becomes antiferromagnetic.
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75.50.Pp Magnetic semiconductors
75.30.Et Exchange and superexchange interactions
75.50.Ee Antiferromagnetics

Characterization of magnetically actuated resonant cantilevers in viscous fluids

Cyril Vančura, Jan Lichtenberg, Andreas Hierlemann, and Fabien Josse

Appl. Phys. Lett. 87, 162510 (2005); http://dx.doi.org/10.1063/1.2108130 (3 pages) | Cited 12 times

Online Publication Date: 12 October 2005

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The vibration behavior of magnetically actuated resonant microcantilevers immersed in viscous fluids has been studied. A dependence of the resonance frequency and the quality factor (Q factor) on the fluid properties, such as density and viscosity and on the cantilever geometry is described. Various cantilever geometries are analyzed in pure water and glycerol solutions, and the results are explained in terms of the added displaced fluid mass and the fluid damping force for both the resonance frequency and the quality factor. An in-depth knowledge and understanding of such systems is necessary when analyzing resonant cantilevers as biochemical sensors in liquid environments.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
07.10.Cm Micromechanical devices and systems
66.20.-d Viscosity of liquids; diffusive momentum transport

Fabrication of Ni nanoparticles and their size-selective self-assembly into chains under external magnetic field

Srikanth Singamaneni and Valery Bliznyuk

Appl. Phys. Lett. 87, 162511 (2005); http://dx.doi.org/10.1063/1.2105988 (3 pages) | Cited 19 times

Online Publication Date: 13 October 2005

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In this letter, we report a simple method for the preparation of colloidal solutions of nickel nanoparticles through the ultrasonication of thermally evaporated ultrathin films. Atomic force microscopy imaging revealed that the particles had a narrow size distribution centered at 9.6 nm. Self-assembly of particles into nanochains was observed when the particles in the colloidal solution were subjected to a magnetic field during the solvent evaporation process. In the absence of an external magnetic field, randomly distributed individual nanoparticles were observed. The interplay between isotropic van der Waals forces and anisotropic magnetic forces governs the self-assembly process of the nanoparticles. The nanochains form an electrically percolating network on a solid substrate, which is characterized with linear current-voltage characteristics and a conductivity of 7×10−4 S m−1.
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81.16.Dn Self-assembly
81.07.Bc Nanocrystalline materials
61.46.-w Structure of nanoscale materials
68.37.Ps Atomic force microscopy (AFM)

Piezoelectrically induced resistance modulations in La0.7Sr0.3MnO3/Pb(Zr,Ti)O3 field effect devices

C. Thiele, K. Dörr, L. Schultz, E. Beyreuther, and W.-M. Lin

Appl. Phys. Lett. 87, 162512 (2005); http://dx.doi.org/10.1063/1.2108129 (3 pages) | Cited 30 times

Online Publication Date: 13 October 2005

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Epitaxial ferroelectric-ferromagnetic field effect devices of PbZr0.52Ti0.48O3/La0.7Sr0.3MnO3 (PZT∕LSMO) with narrow manganite channels ( ⩽ 7 nm) show butterflylike hysteretic resistance modulations (ΔR) which are commensurate with strain variation from inverse piezoelectric effect of PZT. Based on the type of observed resistance hysteresis loops, contributions from electric field effect and strain effect have been distinguished for devices with varied channel thickness. The strain-induced ΔR decreases with increasing channel thickness. The ΔR from field effect is low in the LSMO channels with 30% Sr doping.
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85.50.-n Dielectric, ferroelectric, and piezoelectric devices
77.65.-j Piezoelectricity and electromechanical effects
85.30.Tv Field effect devices

From nucleation to coercivity

G. P. Zhao, M. G. Zhao, H. S. Lim, Y. P. Feng, and C. K. Ong

Appl. Phys. Lett. 87, 162513 (2005); http://dx.doi.org/10.1063/1.2108120 (3 pages) | Cited 15 times

Online Publication Date: 13 October 2005

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The demagnetization process of an exchange-coupled hard/soft/hard magnetic system and its coercivity mechanism are investigated within a micromagnetic framework. The calculation reveals that the demagnetization is a three-step process: (i) it begins with the nucleation of a transition region, (ii) the subsequent evolution of the domain wall near the interface, and (iii) the irreversible domain-wall motion from the soft phase into hard phase. As the thickness of the soft interlayer decreases, the dominant mechanism of coercivity switches from pinning to nucleation. However, for very thin interlayer, the three-step process occurs at the same applied field.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Et Exchange and superexchange interactions
75.60.Ch Domain walls and domain structure
64.60.Q- Nucleation
75.50.Ww Permanent magnets

High tunnel magnetoresistance in spin-polarized scanning tunneling microscopy of Co nanoparticles on Pt(111)

S. Rusponi, N. Weiss, T. Cren, M. Epple, and H. Brune

Appl. Phys. Lett. 87, 162514 (2005); http://dx.doi.org/10.1063/1.2077856 (3 pages) | Cited 15 times

Online Publication Date: 14 October 2005

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We employ variable-temperature spin-polarized scanning tunneling microscopy in constant current mode to read the magnetic state of monodomain cobalt nanoparticles on Pt(111). In order to avoid stray fields we use in situ prepared antiferromagnetically (Cr) coated W tips. The contrast in apparent height between nanoparticles with opposite magnetization is typically Δz = 0.20±0.05 Å, but can reach up to 1.1 Å, indicating 80% spin-polarization of the nanoparticles and 850% magnetoresistance of the tip-sample tunnel junction with tip and sample at 300 K and 160 K, respectively. There is no zero-bias anomaly. These results suggest state-selective tunneling which is expected to lead to very high magnetoresistance values.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Cc Other ferromagnetic metals and alloys
75.47.Np Metals and alloys
75.60.Ch Domain walls and domain structure

Imaging of vortex states in mesoscopic superconductors

Goran Karapetrov, Jan Fedor, Maria Iavarone, M. T. Marshall, and R. Divan

Appl. Phys. Lett. 87, 162515 (2005); http://dx.doi.org/10.1063/1.2105993 (3 pages) | Cited 4 times

Online Publication Date: 14 October 2005

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Enhanced vortex pinning in nanoscale-engineered superconductors increases the superconducting critical currents by orders of magnitude. Spatial imaging of vortices in these systems at high magnetic fields would provide further insight into the pinning mechanisms and enable development of high-pinning-strength materials. We have developed a novel method of fabricating atomically flat superconductor surfaces containing periodic array of normal metal pinning centers. Using scanning tunneling microscopy and spectroscopy, we map the local density of states in this heterostructure showing the vortex distribution at different applied magnetic fields. By increasing the applied magnetic field, the normal metal pinning centers accommodate several vortices per center until reaching the saturation point, beyond which new vortices get accommodated in the interstitial superconducting regions. The arrangement and pinning of the interstitial vortices is determined by the periodic pinning potential, and repulsive vortex-vortex interaction.
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74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
74.25.Sv Critical currents

Enhanced coercivity in B-rich nanocomposite α-Fe/(NdPr)2Fe14B/Fe3B hard magnetic alloys

I. Betancourt and H. A. Davies

Appl. Phys. Lett. 87, 162516 (2005); http://dx.doi.org/10.1063/1.2106004 (3 pages) | Cited 15 times

Online Publication Date: 14 October 2005

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The structures and magnetic properties of melt-spun B-rich (10 at. %) and Nb-containing nanocomposite alloys, with compositions based on the formula (Nd0.75Pr0.25)yFe90−y−xNbxB10 (y = 8, 10; x = 0, 2, 4) have been studied. Considerable enhancement of the intrinsic coercivity, (of 657 kA/m) together with excellent energy density (of 113 kJ/m3), were observed for y = 8, x = 4; while for y = 10 and x = 2, values of 912 kA/m and 140 kJ/m3 were attained. Results are interpreted in terms of a grain size refining effect of the Nb addition and to the more complete exchange coupling of the soft grains to the hard phase grains afforded by reduced grain sizes.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Et Exchange and superexchange interactions
75.50.Ww Permanent magnets
75.50.Vv High coercivity materials

Effect of side shields on neighbor-induced transition shift in perpendicular magnetic recording

Manish Kapoor, Sissay G. Yoseph, and R. H. Victora

Appl. Phys. Lett. 87, 162517 (2005); http://dx.doi.org/10.1063/1.2108124 (3 pages) | Cited 1 time

Online Publication Date: 14 October 2005

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The use of side shields is considered for the purpose of countering the neighbor-induced transition shift (NITS) in perpendicular magnetic recording. Two kinds of shield geometries are incorporated in the micromagnetic model. It is shown that, while the shields are ineffective against the worst case scenario, one shield design is highly effective in a less restrictive situation. It is further shown that the benefits ( ∼ 65% reduction of NITS) only occur over a restricted range of shield saturation magnetization (Ms). Optimal shield Ms is found to be in the range of 500–800 emu/cc.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Ss Magnetic recording materials
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