Physical limitations to efficient high-speed spin-torque switching in magnetic tunnel junctions

Heindl, R. A.; Rippard, William H.; Russek, Stephen E.; Kos, Anthony B.

doi:10.1103/physrevb.83.054430

Cited by 26 publications

(18 citation statements)

References 18 publications

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“…A t P -dependent WER slope, similar to our data in Fig. 3, was also reported in previous WER studies [44], [45], although no detailed physical explanation or modeling were given. A similar behavior, resulting in a larger than predicted WER at short t P , was evidenced in the presence of back-hopping and low-probability bifurcated switching (LPBS) [46], [47].…”

Section: Wer Studysupporting

confidence: 90%

A Physics-Based Compact Model of Stochastic Switching in Spin-Transfer Torque Magnetic Memory

Carboni

Vernocchi

Siddik

et al. 2019

IEEE Trans. Electron Devices

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Spin-transfer torque random-access memory (STT-RAM) is gaining momentum as a promising technology for high-density and embedded nonvolatile memory. Owing to random thermal fluctuations, switching transitions generally display statistical variations from cycle to cycle. Stochastic variations are critical to the hindering of memory and computing applications of STT-RAM. To enable the design of STT-RAM circuits for memory and computing, there is a need for accurate compact models capable of predicting the stochastic behavior. Here, we present a detailed model accounting for the anomalous thermal regime of switching deviating from the Néel-Brown thermal model below 200 ns. Anomalous switching is explained by the nonlinear lowering of the energy barrier associated with the perpendicular magnetic anisotropy (PMA). The model is extensively verified against the write-error-rate (WER) data as a function of applied voltage and pulsewidth and experimental switching time-delay distributions. Index Terms-Magnetic tunnel junction (MTJ), spintransfer torque magnetoresistive RAM (STT-MRAM), stochastic switching, switching variability modeling. I. INTRODUCTION S PIN-TRANSFER TORQUE random-access memory (STT-RAM) is attracting a strong interest because of the storage-class memory (SCM) [1]-[5], dynamic RAM (DRAM) replacement [6], and embedded nonvolatile memory [6]-[8] due to its fast switching [9], nonvolatile states, high endurance [10], CMOS compatibility, and low-current operation [11]. STT-RAM and spintronic devices in general find application also in novel non-von Neumann concepts of computing,

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Section: Wer Studysupporting

confidence: 90%

A Physics-Based Compact Model of Stochastic Switching in Spin-Transfer Torque Magnetic Memory

Carboni

Vernocchi

Siddik

et al. 2019

IEEE Trans. Electron Devices

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“…First, in all cases the results indicate a remarkably fast time scale, t c0 ≤ ∼1 ns, considerably less than the ≫1 ns time scale expected 23,24 from the antidamping switching mechanism for an in-plane magnetized free layer in the rigid domain approximation. 9 In addition, the values obtained for t c0 show a clear dependence on the device aspect ratio with the low aspect ratio device having the fastest observed switching speed. Finally, there appears to be a growing asymmetry between the P-to-AP and AP-to-P polarity switching speeds as the aspect ratio is reduced.…”

mentioning

confidence: 90%

Nanosecond-Timescale Low Energy Switching of In-Plane Magnetic Tunnel Junctions through Dynamic Oersted-Field-Assisted Spin Hall Effect

et al. 2016

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Abstract:We investigate fast-pulse switching of in-plane-magnetized magnetic tunnel junctions (MTJs) within 3-terminal devices in which spin-transfer torque is applied to the MTJ by the giant spin Hall effect. We measure reliable switching, with write error rates down to 10 -5 , using current pulses as short as just 2 ns in duration. This represents the fastest reliable switching reported to date for any spin-torque-driven magnetic memory geometry, and corresponds to a characteristic time scale that is significantly shorter than predicted possible within a macrospin model for in-plane MTJs subject to thermal fluctuations at room temperature. Using micromagnetic simulations, we show that in the 3-terminal spin-Hall devices the Oersted magnetic field generated by the pulse current strongly modifies the magnetic dynamics excited by the spin-Hall torque, enabling this unanticipated performance improvement. Our results suggest that in-planeMTJs controlled by Oersted-field-assisted spin-Hall torque are a promising candidate for both cache memory applications requiring high speed and for cryogenic memories requiring low write energies. Keywords:spintronics, spin Hall effect, magnetic tunnel junction, magnetic memory, spin orbit torque,Magnetic random access memory (MRAM) controlled using spin transfer torque (STT) 1-3 , using either in-plane [4][5][6] or perpendicularly magnetized 7,8 magnetic tunnel junctions (MTJs), holds promise for replacing existing best-in-class memory technologies in several application domains because it offers the potential for non-volatility, unlimited read and write endurance, low write energy, and low standby power. For the wide application of STT-MRAM technology it is also crucial to achieve high-speed switching with low write error rates (WERs). Currently, the fastest reliable (< 10 -5 WER) switching times reported for conventional 2-terminal STT-MRAM are 35 ns for in-plane MTJs, 6 and 4 ns for perpendicular MTJs. 8 Theoretical and experimental analyses have led to skepticism about the possibility for significant improvements in switching speed and reliability, particularly for in-plane magnetized devices. 4,9 Consequently, the search for speed has led to more ambitious proposals including orthogonal spin-transfer (OST) MRAM, 10 where sub-ns switching has been demonstrated, but via a precessional non-deterministic mechanism that thus far has not allowed for competitive WERs. Here we investigate the speed and reliability of spin-orbit torque 11-15 switching in three-terminal devices [16][17][18][19][20] that utilize the spin Hall effect (SHE) 21,22 to achieve efficient switching of an in-plane magnetized MTJ. We demonstrate reliable (≤10 -5 WER) switching using current pulses only 2 ns long. This is faster than the best reported value for reliable switching of any previous spin-torque MRAM device 4-9 -in-plane or perpendicular -with a characteristic time scale even faster than the theoretical limit expected for in-plane-magnetized MTJs in the macrospin approximation.9,23,24 Figure 1a sh...

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“…A very large and rapidly growing literature on spin-torque switching, from the perspectives of both theory [1,2,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] and experiment [25][26][27][28][29], now exists. In the following we will focus on the following aspects of the spin-torque switching phenomenon: (1) Additional terms in the LLG equation that arise from Gilbert damping acting on the spin-torque; (2) (5) approximate analytic solution for the switching rate for currents below the critical current for switching; (6) demonstration that, for the purpose of determining switching distributions, the Fokker-Planck equation is equivalent to macrospin simulations that include a random thermal field, with the exception that the Fokker-Planck approach can be applied to determine switching probabilities that are extremely small or very close to unity, thus allowing the investigation of WSER and RSER; and (7) observation that, for the presently considered case with axial symmetry, the equations controlling switching via spin-polarized currents can be mapped onto mathematically equivalent equations for switching via an applied axial magnetic field.…”

Section: Introductionmentioning

confidence: 99%

Switching Distributions for Perpendicular Spin-Torque Devices Within the Macrospin Approximation

Butler¹,

Mewes²,

Mewes³

et al. 2012

IEEE Trans. Magn.

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We model "soft" error rates for writing (WSER) and for reading (RSER) for perpendicular spintorque memory devices by solving the Fokker-Planck equation for the probability distribution of the angle that the free layer magnetization makes with the normal to the plane of the film. We obtain: (1) an exact, closed form, analytical expression for the zero-temperature switching time as a function of initial angle; (2) an approximate analytical expression for the exponential decay of the WSER as a function of the time the current is applied; (3) comparison of the approximate analytical expression for the WSER to numerical solutions of the Fokker-Planck equation; (4) an approximate analytical expression for the linear increase in RSER with current applied for reading; (5) comparison of the approximate analytical formula for the RSER to the numerical solution of the Fokker-Planck equation; and (6) confirmation of the accuracy of the Fokker-Planck solutions by comparison with results of direct simulation using the single-macrospin Landau-Lifshitz-Gilbert (LLG) equations with a random fluctuating field in the short-time regime for which the latter is practical.

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Physical limitations to efficient high-speed spin-torque switching in magnetic tunnel junctions

Cited by 26 publications

References 18 publications

A Physics-Based Compact Model of Stochastic Switching in Spin-Transfer Torque Magnetic Memory

A Physics-Based Compact Model of Stochastic Switching in Spin-Transfer Torque Magnetic Memory

Nanosecond-Timescale Low Energy Switching of In-Plane Magnetic Tunnel Junctions through Dynamic Oersted-Field-Assisted Spin Hall Effect

Switching Distributions for Perpendicular Spin-Torque Devices Within the Macrospin Approximation

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