Time-Resolved Spin-Torque Switching and Enhanced Damping in Permalloy/Cu/Permalloy Spin-Valve Nanopillars

Emley, N. C.; Krivorotov, I. N.; Ozatay, O.; Garcia, A. G. F.; Sankey, J. C.; Ralph, Daniel; Buhrman, R. A.

doi:10.1103/physrevlett.96.247204

Cited by 55 publications

(47 citation statements)

References 28 publications

(43 reference statements)

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“…Note that our procedure, in contrast to time-domain averaging of the magnetization response [14,15], is able to detect rare events. We shall see that this is important since some specific initial conditions lead to a quasi-divergence of the switching time.…”

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confidence: 99%

Distribution of the magnetization reversal duration in subnanosecond spin-transfer switching

et al. 2007

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confidence: 99%

Distribution of the magnetization reversal duration in subnanosecond spin-transfer switching

et al. 2007

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“…As a test of our method we repeated the simulations of current-induced magnetization reversal in Ref. 17 and found good agreement.…”

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confidence: 98%

Feedback control of noise in spin valves by the spin-transfer torque

Bandopadhyay

Brataas

Bauer

2011

Applied Physics Letters

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The miniaturization of magnetic read heads and random access memory elements makes them vulnerable to thermal fluctuations. We demonstrate how current-induced spin-transfer torques can be used to suppress the effects of thermal fluctuations. This enhances the fidelity of perpendicular magnetic spin valves. The simplest realization is a dc to stabilize the free magnetic layers. The power can be significantly reduced without losing fidelity by simple control schemes, in which the stabilizing current-induced spin-transfer torque is controlled by the instantaneous resistance. © 2011 American Institute of Physics. ͓doi:10.1063/1.3556270͔Magnetic spin valves with metal or insulator spacers are used for hard disk read heads as well as for magnetic random access memory elements. The physical principle of operation is the giant magnetoresistance ͑GMR͒ or tunnel magnetoresistance ͑TMR͒, which varies with the relative magnetization between a layer with fixed magnetization ͑e.g., by exchange biasing͒ and a free layer with a magnetization that is allowed to ͑re͒orient the magnetization direction. Further miniaturization is thwarted by the increased thermal fluctuations associated with smaller magnets, however, because the magnetic anisotropy energies scale with the total magnetic moment. 1 GMR read heads in the form of nanometer-scale pillars are currently considered as possible replacements for TMR read heads.2 Their higher sensitivity comes at the cost of noise which is enhanced by the spin-transfer-torque-induced coupling of electronic and magnetic fluctuations.3,4 Here, we demonstrate that the spin-transfer torque can also be used to suppress the noise in spin valves. The necessary power to achieve a given fidelity can significantly be reduced by feedback control of the electric current-induced torque by the instantaneous resistance.An electric current passed through a spin valve interacts with the magnetic order parameter when the magnetizations are noncollinear. This is caused by the spin current component polarized normal to the magnetization of the free layer, which is absorbed at the interface. The loss of angular momentum in the spin current is transferred to the magnetization as a spin-transfer torque, 5-10 which effectively leads to magnetic damping or antidamping of the magnetization dynamics, depending on the current direction. Low frequency noise can be suppressed by increasing the magnetic anisotropy of the free magnetic layer at the cost of reduced sensitivity. Covington 3 proposed to reduce noise by applying a spin-transfer torque which opposes the intrinsic damping so that the damping seems to be reduced. According to the fluctuation-dissipation theorem the rms random field that perturbs the magnetization at thermal equilibrium is proportional to the square root of the Gilbert damping constant ͓as Eq. ͑3͒ below͔. The average spin-transfer torque dynamically changes the energy of the ferromagnet ͑FM͒ but does not change the Brownian fluctuating fields the ferromagnet experiences, except at very low temperat...

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“…Measurements in the time domain [5][6][7][8][9][10][11] can provide the most direct information about spin-torque-driven magnetic dynamics. However, most previous timeresolved studies [5][6][7][8][9] employed stroboscopic approaches, which average over many events so that they reveal only average behavior and hide individual variations. Magnetic tunnel junctions (MTJs) can now provide sufficiently large resistance signals (relative to metal spin valves [5][6][7][8]) for single-shot measurements.…”

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Single-Shot Time-Domain Studies of Spin-Torque-Driven Switching in Magnetic Tunnel Junctions

et al. 2010

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We report single-shot measurements of resistance versus time for thermally assisted spintorque-driven switching in magnetic tunnel junctions. We achieve sufficient sensitivity to resolve the resistance signals leading up to switching, including the variations between individual switching events. Analyses of pre-switching thermal fluctuations allow detailed measurements of coherence times and variations in magnetization precession amplitude. We find that with a small in-plane hard-axis magnetic field the magnetization dynamics are more spatially coherent than for the case of zero field. Page 2 of 13Magnetization switching induced by spin transfer torque [1,2] is of interest both for probing the fundamental physics of magnetic dynamics and for applications in storage technologies [3,4]. Measurements in the time domain [5][6][7][8][9][10][11] can provide the most direct information about spin-torque-driven magnetic dynamics. However, most previous timeresolved studies [5][6][7][8][9] employed stroboscopic approaches, which average over many events so that they reveal only average behavior and hide individual variations. Magnetic tunnel junctions (MTJs) can now provide sufficiently large resistance signals (relative to metal spin valves [5][6][7][8]) for single-shot measurements. Two initial experiments have measured distributions of spin-torque switching times in MTJs using single-shot techniques [10,11] but they did not resolve the dynamics leading to switching. Here we report single-shot measurements of spin-torque switching in MTJs with sufficiently improved sensitivity to study the pre-switching resistance signals in detail. We observe the variations between switching processes caused by thermal fluctuations and can perform comprehensive analyses of the fluctuations prior to switching. We find that switching is more spatially coherent when the magnetic moments of the electrodes are initially offset (at an angle different than 0º or 180º) than when the moments are collinear.The MTJ samples that we study have the layer structure (in nm): bottom contact . Our discussion of currentdriven reversal will focus on switching from the anti-parallel (AP) state to parallel (P) state because this required approximately 30% lower voltages than P-to-AP switching.Page 3 of 13However, measurements of P-to-AP dynamics are qualitatively similar. We will examine switching both in the case of zero total field on the free layer (in which an applied field cancels H d ) and the case that a small (100 Oe) field is applied along the in-plane hard axis, to rotate the free-layer magnetization approximately 15º from the strictly AP configuration (see inset, Fig. 1(a)). We will present data from a single sample, but have studied two other devices in zero field and one other in the hard-axis field, with all showing the same differences depending on field geometry.We perform single-shot measurements using the circuit shown in Fig. 1(b). After initializing the sample in the AP state we use a pulse generator to produce an 100 ns long, 300 ps ri...

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Time-Resolved Spin-Torque Switching and Enhanced Damping in Permalloy/Cu/Permalloy Spin-Valve Nanopillars

Cited by 55 publications

References 28 publications

Distribution of the magnetization reversal duration in subnanosecond spin-transfer switching

Distribution of the magnetization reversal duration in subnanosecond spin-transfer switching

Feedback control of noise in spin valves by the spin-transfer torque

Single-Shot Time-Domain Studies of Spin-Torque-Driven Switching in Magnetic Tunnel Junctions

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