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Now showing items 1 - 16 of 4341

  • Nonlinear magnetohydrodynamics of edge localized mode precursors

    Guo, Z. B.   Wang, Lu   Wang, X. G.  

    A possible origin of edge-localized-mode (ELM) precursors based on nonlinear ideal peeling-ballooning mode is reported. Via nonlinear variational principle, a nonlinear evolution equation of the radial displacement is derived and solved, analytically. Besides an explosive growth in the initial nonlinear phase, it is found that the local displacement evolves into an oscillating state in the developed nonlinear phase. The nonlinear frequency of the ELM precursors scales as omega(pre) similar to x(1/3) (xi) over cap (2/3)(psi,in)n, with x position in radial direction, (xi) over cap (psi,in) strength of initial perturbation, and n toroidal mode number. (C) 2015 AIP Publishing LLC.
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  • Turbulence elasticity—A new mechanism for transport barrier dynamics

    Guo, Z. B.   Diamond, P. H.   Kosuga, Y.   Gürcan, ?. D.  

    We present a new, unified model of transport barrier formation in ldquoelasticrdquo drift wave-zonal flow (DW-ZF) turbulence. A new physical quantity-the delay time (i.e., the mixing time for the DW turbulence)-is demonstrated to parameterize each stage of the transport barrier formation. Quantitative predictions for the onset of limit-cycle-oscillation (LCO) among DW and ZF intensities (also denoted as I-mode) and I-mode to high-confinement mode (H-mode) transition are also given. The LCO occurs when the ZF shearing rate (|langvrang ZF'|) enters the regime Deltaomega k<|langVrang ZF'|tau cr-1, where the mean E times B shear flow driven by ion pressure ldquolocksrdquo the DW-ZF system to the H-mode by reducing the delay time below the threshold value.
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  • Small scale coherent vortex generation in drift wave-zonal flow turbulence

    Guo, Z. B.   Hahm, T. S.   Diamond, P. H.  

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  • Elasticity in drift-wave–zonal-flow turbulence

    Guo, Z. B.   Diamond, P. H.   Kosuga, Y.   Gürcan, ?. D.  

    We present a theory of turbulent elasticity, a property of drift-wave-zonal-flow (DW-ZF) turbulence, which follows from the time delay in the response of DWs to ZF shears. An emergent dimensionless parameter vertical bar < v >'vertical bar/Delta omega(k) is found to be a measure of the degree of Fickian flux-gradient relation breaking, where vertical bar < v >'vertical bar is the ZF shearing rate and Delta omega(k) is the turbulence decorrelation rate. For vertical bar < v >'vertical bar/Delta omega(k) > 1, we show that the ZF evolution equation is converted from a diffusion equation, usually assumed, to a telegraph equation, i.e., the turbulent momentum transport changes from a diffusive process to wavelike propagation. This scenario corresponds to a state very close to the marginal instability of the DW-ZF system, e.g., the Dimits shift regime. The frequency of the ZF wave is Omega(ZF) = +/-gamma(1/2)(d)gamma(1/2)(modu), where gamma(d) is the ZF friction coefficient and gamma(modu) is the net ZF growth rate for the case of the Fickian flux-gradient relation. This insight provides a natural framework for understanding temporally periodic ZF structures in the Dimits shift regime and in the transition from low confined mode to high confined mode in confined plasmas.
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  • Turbulence elasticity-A new mechanism for transport barrier dynamics

    Guo, Z. B.   Diamond, P. H.   Kosuga, Y.   Guercan, Oe. D.  

    We present a new, unified model of transport barrier formation in "elastic" drift wave-zonal flow (DW-ZF) turbulence. A new physical quantity-the delay time (i.e., the mixing time for the DW turbulence)-is demonstrated to parameterize each stage of the transport barrier formation. Quantitative predictions for the onset of limit-cycle-oscillation (LCO) among DW and ZF intensities (also denoted as I-mode) and I-mode to high-confinement mode (H-mode) transition are also given. The LCO occurs when the ZF shearing rate (vertical bar < u >(ZF)'vertical bar) enters the regime Delta omega(k) < vertical bar < V >(ZF)'vertical bar < tau(-1)(cr), where Delta omega(k) is the local turbulence decorrelation rate and tau(cr) is the threshold delay time. In the basic predator-prey feedback system, tau(cr) is also derived. The I-H transition occurs when vertical bar < V >(ExB)'vertical bar > tau(-1)(cr), where the mean E x B shear flow driven by ion pressure "locks" the DW-ZF system to the H-mode by reducing the delay time below the threshold value. (C) 2014 AIP Publishing LLC.
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  • Bistable dynamics of turbulence spreading in a corrugated temperature profile

    Guo, Z. B.   Diamond, P. H.  

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  • Suppression of superconductivity in Nb by IrMn in IrMn/Nb bilayers

    Wu, B. L.   Yang, Y. M.   Guo, Z. B.   Wu, Y. H.   Qiu, J. J.  

    Effect of antiferromagnet on superconductivity has been investigated in IrMn/Nb bilayers. Significant suppression of both transition temperature (T-c) and lower critical field (H-c1) of Nb is found in IrMn/Nb bilayers as compared to a single layer Nb of same thickness; the suppression effect is even stronger than that of a ferromagnet in NiFe/Nb bilayers. The addition of an insulating MgO layer at the IrMn-Nb interface nearly restores T-c to that of the single layer Nb, but H-c1 still remains suppressed. These results suggest that, in addition to proximity effect and magnetic impurity scattering, magnetostatic interaction also plays a role in suppressing superconductivity of Nb in IrMn/Nb bilayers. In addition to reduced T-c and H-c1, the IrMn layer also induces broadening in the transition temperature of Nb, which can be accounted for by a finite distribution of stray field from IrMn. (C) 2013 AIP Publishing LLC.
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  • MAGNETIC RECONNECTION, HELICITY DYNAMICS, AND HYPER-DIFFUSION

    Guo, Z. B.   Diamond, P. H.   Wang, X. G.  

    We examine the influence of noise and Alfven wave turbulence on magnetic reconnection in a reduced magnetohydrodynamics model. We focus on the dynamics of magnetic helicity density. Helicity conservation is then used to calculate the global reconnection rate in terms of the helicity density flux. Two specific scenarios are explored-noisy reconnection and Alfven wave turbulent reconnection. For noisy reconnection, the current sheet is assumed to sit in a noisy state, marginal to plasmoid formation instability. The scaling of the reconnection rate in the presence of noise is proportional to (S-0(2)/ VAL2)(1/11), where S-0(2)/ VAL2 is the relative amplitude of the noise. We obtain this prediction using a symmetry analysis of the helicity density flux. For Alfven wave turbulent reconnection, a mean field closure scheme is applied. A reconnection rate proportional to ( <(B) over tilde2 > / < B >(2))(1/8) is obtained, where <(B) over tilde2 > / < B >(2) and < B > are the relative energy of Alfven wave turbulence and the reconnecting field. The constraint on reconnection rate enforced mean-square magnetic potential conservation is reexamined. A critical magnetic Reynolds number R-m,R- c is identified. For R-m >> R-m,R- c, the reconnection rate becomes independent of Spitzer resistivity and thus can be higher than the Sweet-Parker model prediction. Both cases exhibit a weak dependence of the reconnection rate on the amplitude of the turbulence. Therefore, even noise or weak turbulence can trigger fast reconnection if the system is marginally stable. The important distinction between turbulent reconnection and turbulent dissipation of magnetic energy is also discussed.
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  • Anomalous Hall effect in polycrystalline Ni films

    Guo, Z. B.   Mi, W. B.   Zhang, Q.   Zhang, B.   Aboljadayel, R. O.   Zhang, X. X.  

    We systematically studied the anomalous Hall effect in a series of polycrystalline Ni films with thickness ranging from 4 to 200 nm. It is found that both the longitudinal and anomalous Hall resistivity increased greatly as film thickness decreased. This enhancement should be related to the surface scattering. In the ultrathin films (4-6 nm thick), weak localization corrections to anomalous Hall conductivity were studied. The granular model, taking into account the dominated intergranular tunneling, has been employed to explain this phenomenon, which can explain the weak dependence of anomalous Hall resistivity on longitudinal resistivity as well. (C) 2011 Elsevier Ltd. All rights reserved.
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  • Ion Beam Modification of Exchange Coupling to Fabricate Patterned Media

    Ranjbar, Mojtaba   Piramanayagam, S. N.   Sbiaa, R.   Aung, K. O.   Guo, Z. B.   Chong, T. C.  

    For bit-patterned media, media with low remanent magnetization (M(r)) and high M(r) regions are needed for storing information, which is usually achieved by lithographically defining magnetic and non-magnetic regions. In this work, we have investigated the use of ion beam modification of media surface to define the low and high M(r) states using a medium that is at a low M(r) state to start with. The low M(r) state is achieved by the use of synthetic antiferromagnetic coupling obtained in Co-alloy/Ru/Co-alloy trilayer structured film. Local ion beam modification at 30 keV energy using Ga(+) ions was used to create high M(r) regions. AFM and MFM observations indicated that patterned regions of low and high M(r) can be observed with ion beam irradiation. This technique is a potential method to achieve patterned media without the need of planarization techniques.
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  • Anomalous Hall effect and magnetoresistance behavior in Co/Pd1?xAgx multilayers

    Guo, Z. B.   Mi, W. B.   Manchon, A.   Li, J. Q.   Zhang, B.   Barba, P. G.   Zhang, X. X.  

    \0
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  • Anomalous Hall effect and magnetoresistance behavior in Co/Pd1-xAgx multilayers

    Guo, Z. B.   Mi, W. B.   Manchon, A.   Li, J. Q.   Zhang, B.   Barba, P. G.   Zhang, X. X.  

    In this paper, we report anomalous Hall effect (AHE) correlated with the magnetoresistance behavior in [Co/Pd1-xAgx](n) multilayers. For the multilayers with n = 6, the increase in Ag content from x = 0 to 0.52 induces the change in AHE sign from negative surface scattering-dominated AHE to positive interface scattering-dominated AHE, which is accompanied with the transition from anisotropy magnetoresistance (AMR) dominated transport to giant magnetoresistance (GMR) dominated transport. For n = 80, scaling analysis with R-s proportional to rho(gamma)(xx) yields gamma similar to 3.44 for x = 0.52 which presents GMR-type transport, in contrast to gamma similar to 5.7 for x = 0 which presents AMR-type transport. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4792054]
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  • A differential dual spin valve with high pinning stability

    Han, G. C.   Qiu, J. J.   Wang, C. C.   Ko, V.   Guo, Z. B.  

    A differential dual spin valve (DDSV) is very promising for ultrahigh linear density recording. The antiparallel magnetization configuration of two reference layers in a DDSV has been realized by using a double synthetic antiferromagnetic coupling structure (SAF) in one spin valve and a single SAF in the other. A high pinning field has been obtained using a double SAF with a magnetically balanced structure. The observed resistance transitions at low fields can be ascribed to the inconsistent switching of two free layers. By forming a strong ferromagnetic coupling between two free layers, a consistent switching is achieved. (C) 2010 American Institute of Physics. [doi:10.1063/1.3441412]
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  • Large negative magnetoresistance in reactive sputtered polycrystalline GdNx films

    Mi, W. B.   Guo, Z. B.   Duan, X. F.   Zhang, X. J.   Bai, H. L.  

    Polycrystalline ferromagnetic GdNx films were fabricated at different N-2 flow rates (f(N2)) to modify N-vacancy concentration so as to study its influence on electrotransport. Metal-semiconductor transition appears at Curie temperature (T-C) of similar to 40K. Temperature-dependent magnetoresistance (MR) shows a peak at T-C. The films at f(N2) = 5, 10, 15, and 20 sccm show MR of -38%, -42%, -46%, and -86% at 5K and 50 kOe, respectively. Above 15 K, MR is from colossal MR and from both colossal and tunneling MR below 15K. The enhanced MR at f(N2) = 20 sccm is attributed to large spin polarization of half-metallicity in GdNx with low N vacancies. (C) 2013 AIP Publishing LLC.
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  • Turbulence elasticity: a key concept to a unified paradigm of L -> I -> H transition

    Guo, Z. B.   Diamond, P. H.   Kosuga, Y.   Guercan, O. D.  

    We present a theory of turbulence elasticity, which follows from delayed response of drift waves (DWs) to zonal flow(ZF) shears. It is shown that when vertical bar < V >(ZF)'vertical bar/Delta omega(k) >= 1, with vertical bar < V >(ZF)'vertical bar the ZF shearing rate and Delta omega(k) the local turbulence decorrelation rate, the ZF evolution equation is converted from a diffusion equation to a telegraph equation. This insight provides a natural framework for understanding temporally periodic ZF structures, e.g., propagation of the ZF/turbulence intensity fronts. Furthermore, by incorporating the elastic property of the DW-ZF turbulence, we propose a unified paradigm of low-confinement-mode to intermediate-confinement-mode to high-confinement-mode (L -> I -> H) transitions. In particular, we predict the onset and termination conditions of the limit cycle oscillations, i.e. the I-mode. The transition from an unstable L-mode to I-mode is predicted to occur when Delta omega(k) < vertical bar < V >(ZF)'vertical bar < < V >(cr)', where < V >(cr)' is a critical flow shearing rate and is derived explicitly. If vertical bar < V >(ExB)'vertical bar > < V >(cr)'(< V >(ExB) is mean E x B shear flow driven by edge radial electrostatic field), the I-mode will terminate and spiral into the H-mode.
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  • Exchange bias between NiFe and IrMn/Ru superlattices

    Guo, Z. B.   Wu, Y. H.   Han, G. C.   Qiu, J. J.   Liu, B.   Chantrell, R. W.  

    The letter reports on the exchange bias between NiFe and IrMn/Ru superlattices. Using the exchange bias effect we have demonstrated the presence of exchange coupling in the antiferromagnetic/nonmagnetic superlattice of IrMn/Ru and observed a larger exchange bias in NiFe/[IrMn/Ru](n) than that in NiFe/IrMn bilayers. The enhancement in exchange bias should be attributed to the increase in uncompensated magnetization in IrMn/Ru superlattices. (C) 2008 Elsevier Ltd. All rights reserved.
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