181 research outputs found
Magnetization reversal by injection and transfer of spin: experiments and theory
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
Switching a spin-valve back and forth by current-induced domain wall motion
We have studied the current-induced displacement of a domain wall (DW) in the
permalloy (Py) layer of a Co/Cu/Py spin valve structure at zero and very small
applied field. The displacement is in opposite direction for opposite dc
currents, and the current density required to move DW is only of the order of
10^6 A/cm^2. For H = 3 Oe, a back and forth DW motion between two stable
positions is observed. We also discuss the effect of an applied field on the DW
motion.Comment: 4 pages, 3 figure
Switching the magnetic configuration of a spin valve by current induced domain wall motion
We present experimental results on the displacement of a domain wall by
injection of a dc current through the wall. The samples are 1 micron wide long
stripes of a CoO/Co/Cu/NiFe classical spin valve structure.
The stripes have been patterned by electron beam lithography. A neck has been
defined at 1/3 of the total length of the stripe and is a pinning center for
the domain walls, as shown by the steps of the giant magnetoresistance curves
at intermediate levels (1/3 or 2/3) between the resistances corresponding to
the parallel and antiparallel configurations. We show by electric transport
measurements that, once a wall is trapped, it can be moved by injecting a dc
current higher than a threshold current of the order of magnitude of 10^7
A/cm^2. We discuss the different possible origins of this effect, i.e. local
magnetic field created by the current and/or spin transfer from spin polarized
current.Comment: 3 pages, 3 figure
Dynamics of two coupled vortices in a spin valve nanopillar excited by spin transfer torque
We investigate the dynamics of two coupled vortices driven by spin transfer.
We are able to independently control with current and perpendicular field, and
to detect, the respective chiralities and polarities of the two vortices. For
current densities above , a highly coherent signal
(linewidth down to 46 kHz) can be observed, with a strong dependence on the
relative polarities of the vortices. It demonstrates the interest of using
coupled dynamics in order to increase the coherence of the microwave signal.
Emissions exhibit a linear frequency evolution with perpendicular field, with
coherence conserved even at zero magnetic field
Coupling efficiency for phase locking of a spin transfer oscillator to a microwave current
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
Origin of the spectral linewidth in non linear oscillators based on MgO tunnel junctions
We demonstrate the strong impact of the oscillator agility on the line
broadening by studying spin transfer induced microwave emission in MgO-based
tunnel junctions with current. The linewidth is almost not affected by
decreasing the temperature. At very low currents, a strong enhancement of the
linewidth at low temperature is attributed to an increase of the non linearity,
probably due to the field-like torque. Finally we evidence that the noise is
not dominated by thermal fluctuations but rather by the chaotization of the
magnetization system induced by the spin transfer torque.Comment: 12 pages, 3 figures, published in Phys. Rev. B 80, 060404 (2009
Differential effects of tissue plasminogen activator and streptokinase on infarct size and on rate of enzyme release: influence of early infarct related artery patency: The GUSTO Enzyme Substudy
Background The recent international GUSTO trial of 41 021 patients with acute myocardial infarction demonstrated improved 90-mm infarct related artery patency as well as reduced mortality in patients treated with an accelerated regimen of tissue plasminogen activator, compared to patients treated with streptokinase. A regimen combining tissue plasminogen activator and streptokinase yielded intermediate results. The present study investigated the effects of treatment on infarct size and enzyme release kinetics in a subgroup of these patients. Methods A total of 553 patients from 15 hospitals were enrolled in the study. Four thrombolytic strategies were compared: streptokinase with subcutaneous heparin, streptokinase with intravenous (iv.) heparin, tissue plasminogen activator with iv. heparin, and streptokinase plus tissue plasminogen activator with i.v. heparin. The activity of alpha-hydroxybutyrate dehydrogenase (HBDH) in plasma was centrally analysed and infarct size was defined as cumulative HBDH release per litre of plasma within 72 h of the first symptoms (Q(72)). Patency of the infarct-related vessel was determined by angiography in 159 patients, 90 mm after treatment. Results Infarct size was 3·72 g-eq . 1−1 in patients with adequate coronary perfusion (TIMI-3) at the 90 mm angi-ogram and larger in patients with TIMI-2 (4·35 g-eq . 1−1) or TIMI 0-1 (5·07 g-eq . 1−1)flow (P=0·024). In this subset of the GUSTO angiographic study, early coronary patency rates (TIMI 2+3) were similar in the two streptokinase groups (53 and 46%). Higher, but similar, patency rates were observed in the tissue plasminogen activator and combination therapy groups (87 and 90%). Median infarct size for the four treatment groups, expressed in gram- equivalents (g-eq) of myocardium, was 4·4, 4·5, 3·9 and 3·9 g-eq per litre of plasma (P=0·04 for streptokinase vs tissue plasminogen activator). Six hours after the first symptoms, respectively 5·3, 6·6, 14·0 and 13·6% of total HBDH release was complete (P<0·000l for streptokinase vs tissue plasminogen activator). Conclusions Rapid and complete coronary reperfusion salvages myocardial tissue, resulting in limitation of infarct size and accelerated release of proteins from the myocardium. Treatment with tissue plasminogen activator, resulting in earlier reperfusion was more effective in reducing infarct size than the streptokinase regimens, which contributes to the differences in survival between treatment groups in the GUSTO tria
Optimizing magneto-dipolar interactions for synchronizing vortex based spin-torque nano-oscillators
We report on a theoretical study about the magneto-dipolar coupling and
synchronization between two vortex-based spin-torque nano-oscillators. In this
work we study the dependence of the coupling efficiency on the relative
magnetization parameters of the vortices in the system. For that purpose, we
combine micromagnetic simulations, Thiele equation approach, and analytical
macro-dipole approximation model to identify the optimized configuration for
achieving phase-locking between neighboring oscillators. Notably, we compare
vortices configurations with parallel (P) polarities and with opposite (AP)
polarities. We demonstrate that the AP core configuration exhibits a coupling
strength about three times larger than in the P core configuration.Comment: 8 pages, 11 figure
Identification and selection rules of the spin-wave eigen-modes in a normally magnetized nano-pillar
We report on a spectroscopic study of the spin-wave eigen-modes inside an
individual normally magnetized two layers circular nano-pillar
(PermalloyCopperPermalloy) by means of a Magnetic Resonance Force
Microscope (MRFM). We demonstrate that the observed spin-wave spectrum
critically depends on the method of excitation. While the spatially uniform
radio-frequency (RF) magnetic field excites only the axially symmetric modes
having azimuthal index , the RF current flowing through the
nano-pillar, creating a circular RF Oersted field, excites only the modes
having azimuthal index . Breaking the axial symmetry of the
nano-pillar, either by tilting the bias magnetic field or by making the pillar
shape elliptical, mixes different -index symmetries, which can be excited
simultaneously by the RF current. Experimental spectra are compared to
theoretical prediction using both analytical and numerical calculations. An
analysis of the influence of the static and dynamic dipolar coupling between
the nano-pillar magnetic layers on the mode spectrum is performed
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