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ABSTRACT: Using high bandwidth resistance measurements, we study the single-shot response of tunnel junctions subjected to spin torque pulses. After the pulse onset, the switching proceeds by a ns-scale incubation delay during which the resistance is quiet, followed by a 400 ps transition terminated by a large ringing that is damped progressively. While the incubation delay fluctuates significantly, the resistance traces are reproducible once this delay is passed. After switching, the time-resolved resistance traces indicate micromagnetic configurations that are rather spatially coherent.
Physical Review Letters 03/2008; 100(5):057206. · 7.37 Impact Factor
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T Devolder,
J Hayakawa, K Ito,
H Takahashi,
S Ikeda,
J A Katine,
M J Carey,
P Crozat,
Joo-Von Kim,
C Chappert,
H Ohno
Journal of Applied Physics. 01/2008; 103(7):07A723.
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ABSTRACT: We analyze the statistical distribution of switching durations in spin-transfer switching induced by current steps and discuss biasing strategies to enhance the reproducibility of switching durations in spin valves. We use a macrospin approximation to describe a thin nanomagnet having easy-plane shape anisotropy and uniaxial magnetocrystalline anisotropy. We model the effect of finite temperature as a Boltzmann distribution of initial magnetization states (adiabatic limit). We compare three model spin valves: a spin valve with a free layer whose easy axis is parallel to the pinned-layer magnetization (standard geometry), a pinned layer with magnetization tilted with respect to the free-layer easy axis (pinned-layer biasing), and a free layer whose magnetization is pulled away from the easy axis by a hard-axis bias (free-layer biasing). In the conventional geometry, the switching durations follow a broad regular distribution, with an extended long tail comprising very long switching events. For the two biasing strategies, the switching durations follow a multiply stepped distribution, reflecting the precessional nature of the switching and the statistical number of precession cycles needed for reversal. We derive analytical criteria to avoid switching events lasting much longer than the average switching duration in order to achieve the highest reproducibilities. Depending on the current amplitude and the biasing strength, the width of the switching time distribution can be substantially reduced, the best reproducibility being achieved for free-layer biasing at overdrive current of a few times unity. An even smaller distribution of switching time is expected if the field is applied abruptly and synchronously with the current, following a so-called dynamic free-layer biasing configuration.
Phys. Rev. B. 06/2007; 75(22).
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ABSTRACT: The effects of the current-induced Oersted field and of a hard axis applied field on spin transfer torque (STT) switching have been investigated by performing LLG micromagnetic simulations including a STT term and Gaussian thermal fluctuations. In parallel to anti-parallel switching at large currents, the C-like micromagnetic configuration induced by the Oersted field plays an important role in STT switching. In anti-parallel to parallel switching at large currents, the Oersted field induces a complicated micromagnetic configuration with several vortices. In both cases, the magnetization deviates considerably from macrospin behaviour. However, a macrospin-like behaviour is restored when a hard axis field is present.
Journal of Physics D Applied Physics 02/2007; 40(5):1261. · 2.54 Impact Factor
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ABSTRACT: The authors have performed micromagnetic simulations of spin transfer torque (STT) switching, combined with precessional motion induced by a pulsed or static hard axis field. They have found a significant reduction in the current required for STT switching in the presence of precessional motion. In particular, the switching current can be reduced below the zero-temperature threshold current at dc, when the hard axis field exceeds a fourth of the free layer coercivity. The simulations indicate that the critical current reduction arises from the strong STT present when the magnetization nearly passes through the hard axis during the initial few precessional turns.
Applied Physics Letters 12/2006; 89(25):252509-252509-3. · 3.84 Impact Factor
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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. Comment: submitted to PRB
09/2006;
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ABSTRACT: The spin-transfer switching performance of spin valve nanopillars composed of PtMn17.5/CoFe1.8/Ru8/CoFe2/Cu3.5/CoFe1/NiFe1.8 is measured in this study at current pulse durations of 0.1-10 ns and temperature range of 40-300 K. Results show that the switching speed accelerates marginally when cooling, with a slightly better reproducibility at low temperature. A static field applied along the hard axis however can drastically accelerate the switching, increasing the efficiency of both DC current and pulsed current. This can be attributed to the influence of a noncollinear initial state, and becomes significant when the field is greater than typically a quarter of the anisotropy field.
Magnetics Conference, 2006. INTERMAG 2006. IEEE International; 06/2006
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ABSTRACT: We studied the magnetization dynamics induced by spin transfer torque (STT) in CoFe/Ru/CoFe/Cu/NiFe nanopillars using Landau-Lifshitz-Gilbert micromagnetic simulations including the STT term given by
Slonczewski [J. Magn. Magn. Mater. 159, L1 (1996)]
and a Gaussian thermal random force. Our simulation results clearly show the probabilistic switching behavior. The antiparallel to parallel switching at 77 K requires a smaller overdrive current than at 300 K. The parallel to antiparallel switching is not dependent on temperature. Our results show that C-like bending induced by the Ampere field plays an important role rather than the misalignment which arises from the thermal magnetization fluctuation.
Journal of Applied Physics 04/2006; 99(8):08G519-08G519-3. · 2.17 Impact Factor
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ABSTRACT: We study the current and temperature dependences of the microwave voltage emission of spin-valve nanopillars subjected to an in-plane magnetic field and a perpendicular-to-plane current. Despite the complex multilayer geometry, clear microwave emission is shown to be possible and spectral lines as narrow as 3.8 MHz (at 150 K) are observed. Comment: To appear in Applied Physics Letters
03/2006;
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Applied Physics Letters 01/2006; 88(15):152502. · 3.84 Impact Factor
