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Spin-transfer-torque resonant switching and injection locking in presence of a weak external microwave field for spin valves with perpendicular materials  [PDF]
Mario Carpentieri,Giovanni Finocchio,Bruno Azzerboni,Luis Torres
Physics , 2010, DOI: 10.1103/PhysRevB.82.094434
Abstract: The effects of a weak microwave field in the magnetization dynamics driven by spin-transfer-torque in spin-valves with perpendicular materials have been systematically studied by means of full micromagnetic simulations. In the system we studied, depending on the working point (bias field and current) in the dynamical stability diagram, we observe either resonant switching and injection locking. The resonant switching, observed in the switching region, occurs when the field frequency is approaching the frequency of the main pre-switching mode giving rise to an asymmetric power distribution of that mode in the sectional area of the free layer. At the resonant frequency, the switching time is weakly dependent on the relative phase between the instant when the current pulse is applied and the microwave field. The injection locking, observed in the dynamical region, is characterized by the following properties: (i) a locking bandwidth which is linearly dependent on the force locking, and (ii) a locking for integer harmonics of the self-oscillation frequency. We compare our numerical results with analytical theory for non-autonomous non-linear system obtaining a good agreement in the current region where the oscillation frequency and output power are characterized from a linear relationship.
Phase locking of vortex based spin transfer oscillators to a microwave current  [PDF]
A. Dussaux,A. V. Khvalkovskiy,J. Grollier,V. Cros,A. Fukushima,M. Konoto,H. Kubota,K. Yakushiji,S. Yuasa,K. Ando,A. fert
Physics , 2010, DOI: 10.1063/1.3565159
Abstract: Phase locking experiments on vortex based spin transfer oscillators with an external microwave current are performed. We present clear evidence of phase locking, frequency pulling, as well as fractional synchronization in this system, with a minimum peak linewidth of only 3 kHz in the locked state. We find that locking ranges of the order of 1/3 of the oscillator frequency are easily achievable because of the large tunability $\partial f/\partial I_{dc}$ observed in our vortex based systems. Such large locking ranges allow us to demonstrate the simultaneous phase locking of two independent oscillators connected in series with the external source.
Magnetization reversal by injection and transfer of spin: experiments and theory  [PDF]
A. Fert,V. Cros,J. -M. George,J. Grollier,H. Jaffres,A. Hamzic,A. Vaures,G. Faini,J. Ben Youssef,H. Le Gall
Physics , 2003, DOI: 10.1016/j.jmmm.2003.12.1351
Abstract: Reversing the magnetization of a ferromagnet by spin transfer from a current, rather than by applying a magnetic field, is the central idea of an extensive current research. After a review of our experiments of current-induced magnetization reversal in Co/Cu/Co trilayered pillars, we present the model we have worked out for the calculation of the current-induced torque and the interpretation of the experiments.
Simultaneous Echo Refocused Magnetization Transfer Imaging  [PDF]
Claus Kiefer
Open Journal of Medical Imaging (OJMI) , 2014, DOI: 10.4236/ojmi.2014.43021
Abstract: Purpose: To increase the efficiency of densely encoded magnetization transfer imaging of the brain, we time-multiplex multiple slices within the same readout using simultaneous echo refocusing FLASH imaging with magnetization transfer (MT) preparation (MT-SER-FLASH). Materials and Methods: Inefficiency in total scan time results from the number of frequency samples needed for sufficient quality of quantitative parameter maps for a binary spin bath model. We present a highly efficient multiplexing method, simultaneous echo refocused magnetization transfer imaging (MT-SER-FLASH) for reducing the total scan time of MT imaging by one-third. The specific absorption rate (SAR) was also reduced by reducing the number of MT-pulses per volume. Results: 2D-MT-SER-FLASH is performed in 19 minutes rather than 1 hour, acceptable for routine clinical application. The SAR could be reduced to 69% instead of more than 100% with a standard 2D or 3D-FLASH with MT-preparation. Conclusion: The net reduction of scan time and SAR enables the use of quantitative model based magnetization transfer imaging within a clinical environment.
Spin Transfer Torque for Continuously Variable Magnetization  [PDF]
Jiang Xiao,A. Zangwill,M. D. Stiles
Physics , 2006, DOI: 10.1103/PhysRevB.73.054428
Abstract: We report quantum and semi-classical calculations of spin current and spin-transfer torque in a free-electron Stoner model for systems where the magnetization varies continuously in one dimension.Analytic results are obtained for an infinite spin spiral and numerical results are obtained for realistic domain wall profiles. The adiabatic limit describes conduction electron spins that follow the sum of the exchange field and an effective, velocity-dependent field produced by the gradient of the magnetization in the wall. Non-adiabatic effects arise for short domain walls but their magnitude decreases exponentially as the wall width increases. Our results cast doubt on the existence of a recently proposed non-adiabatic contribution to the spin-transfer torque due to spin flip scattering.
Transfer Function Modelling as a Tool for Solving Manufacturing System Dysfunction
A.C. Igboanugo,C.C. Nwobi-Okoye
Research Journal of Applied Sciences, Engineering and Technology , 2012,
Abstract: Wastes, poor quality products and downtimes are serious problems usually encountered during production which are often caused by ineffective maintenance and improper determination of process capability. The sensitivity and versatile applicability of transfer function modelling as an off-line tool for dealing with these types of problems is discussed. This study seeks to investigate the relationship among process variability, maintenance, process noise and controlled output in relation to the transfer function characteristics. A conceptual model of single-input-single-output production system, considering maintenance as one of the inputs, was developed and the transfer function synthesised. A random survey of twenty firms in Nigeria was conducted and the outcomes suggest that maintenance and related production variables contribute significantly to degraded output and process variability. Our result showed that there is a strong relationship between the ratio of controller output θ(s) and process noise θe(s), on the one hand and the transfer function characteristics namely k and τR, which can be manipulated to appreciably reduce product degradation and control process variability. Moreover, we showed that transfer function characteristics can be used to determine the need for process facilities maintenance. The study has ably demonstrated that transfer function is very effective in fault diagnosis.
Modelling Heat Transfer of Carbon Nanotubes  [PDF]
Xin-She Yang
Physics , 2010, DOI: 10.1088/0965-0393/13/6/008
Abstract: Modelling heat transfer of carbon nanotubes is important for the thermal management of nanotube-based composites and nanoelectronic device. By using a finite element method for three-dimensional anisotropic heat transfer, we have simulated the heat conduction and temperature variations of a single nanotube, a nanotube array and a part of nanotube-based composite surface with heat generation. The thermal conductivity used is obtained from the upscaled value from the molecular simulations or experiments. Simulations show that nanotube arrays have unique cooling characteristics due to its anisotropic thermal conductivity.
Distribution of the magnetization reversal duration in sub-ns spin-transfer switching  [PDF]
T. Devolder,C. Chappert,J. A. Katine,M. J. Carey,K. Ito
Physics , 2006, DOI: 10.1103/PhysRevB.75.064402
Abstract: We study the distribution of switching times in spin-transfer switching induced by sub-ns current pulses in pillar-shaped spin-valves. The pulse durations leading to switching follow a comb-like distribution, multiply-peaked at a few most probable, regularly spaced switching durations. These durations reflect the precessional nature of the switching, which occurs through a fluctuating integer number of precession cycles. This can be modeled considering the thermal variance of the initial magnetization orientations and the occurrence of vanishing total torque in the possible magnetization trajectories. Biasing the spin-valve with a hard axis field prevents some of these occurrences, and can provide an almost perfect reproducibility of the switching duration.
Coupling efficiency for phase locking of a spin transfer oscillator to a microwave current  [PDF]
B. Georges,J. Grollier,M. Darques,V. Cros,C. Deranlot,B. Marcilhac,A. Fert,G. Faini
Physics , 2008, DOI: 10.1103/PhysRevLett.101.017201
Abstract: The phase locking behavior of spin transfer nano-oscillators (STNOs) to an external microwave signal is experimentally studied as a function of the STNO intrinsic parameters. We extract the coupling strength from our data using the derived phase dynamics of a forced STNO. The predicted trends on the coupling strength for phase locking as a function of intrinsic features of the oscillators i.e. power, linewidth, agility in current, are central to optimize the emitted power in arrays of mutually coupled STNOs.
Magnetization dynamics with a spin-transfer torque  [PDF]
Z. Li,S. Zhang
Physics , 2003, DOI: 10.1103/PhysRevB.68.024404
Abstract: The magnetization reversal and dynamics of a spin valve pillar, whose lateral size is 64$\times$64 nm$^2$, are studied by using micromagnetic simulation in the presence of spin transfer torque. Spin torques display both characteristics of magnetic damping (or anti-damping) and of an effective magnetic field. For a steady-state current, both M-I and M-H hysteresis loops show unique features, including multiple jumps, unusual plateaus and precessional states. These states originate from the competition between the energy dissipation due to Gilbert damping and the energy accumulation due to the spin torque supplied by the spin current. The magnetic energy oscillates as a function of time even for a steady-state current. For a pulsed current, the minimum width and amplitude of the spin torque for achieving current-driven magnetization reversal are quantitatively determined. The spin torque also shows very interesting thermal activation that is fundamentally different from an ordinary damping effect.
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