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A snapshot review of double magnetic junctions for STT-MRAM

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Abstract

The development of double magnetic junctions for spin-transfer torque magnetoresistive random access memory (STT-MRAM) is reviewed, with an emphasis on work from IBM. A brief overview of the theory of spin-transfer torque in double magnetic tunnel junctions is given, showing that for high spin-polarization, up to a factor of 10 improvement in switching efficiency is theoretically possible. Experimental results on double magnetic tunnel junctions, using two tunnel barriers, show a factor of two improvement in switching efficiency. Experimental results on double spin-torque magnetic tunnel junctions, using one tunnel barrier and one low resistance spacer, show close to a factor of two improvement in switching efficiency, and enable reliable switching down to 250 ps.

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Fig. 1
Fig. 2

© 2017 IEEE. Review of results for the single magnetic tunnel junction, for four different values of the spin polarization, P. (a) Normalized spin torque for constant voltage applied across the junction. Note the spin torque is symmetric about θ = π/2, causing the switching voltage for both switching polarities to be equal. R⊥ is the resistance when θ = π/2. (b) Normalized resistance. The magnetoresistance (MR) makes the antiparallel resistance larger than the parallel resistance. MR values are listed corresponding to each of the four values of P used. (c) Combining (a) and (b) with Ohm’s law gives the normalized spin torque for constant current applied through the junction. Because, at constant current, the spin torque is larger near θ = π than near θ = 0, the switching current is lower for antiparallel to parallel switching than for parallel to antiparallel switching

Fig. 3

© 2015 IEEE. TEM of a 31 nm double magnetic tunnel junction. The two white lines are the two tunnel barriers

Fig. 4
Fig. 5

© 2015 IEEE. Spin-torque switching efficiency of double magnetic tunnel junction (DMTJ) arrays versus reference single magnetic tunnel junction (SMTJ) arrays as a function of MTJ size

Fig. 6

© 2017 IEEE. Dependence of spin-torque switching efficiency on resistance-area product (RA), for two different materials designs of single magnetic tunnel junctions. Each data point is the median of ~ 100 devices (35 nm diameter)

Fig. 7

© 2021 IEEE. Write-error-rate curve comparison for a 42 nm diameter double spin-torque magnetic tunnel junction (black) versus a reference single magnetic tunnel junction (red), for 10 ns write pulses

Fig. 8

© 2022 IEEE. Write-error-rate (WER) curves of 194 double spin-torque magnetic tunnel junctions (55 nm diameter) using 500 ps write pulses, down to 10−6 write-error-rate

Fig. 9

© 2022 IEEE. Write-error-rate (WER) curves of 194 double spin-torque magnetic tunnel junctions (55 nm diameter) using 250 ps write pulses, down to 10−6 write-error-rate

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Acknowledgments

The authors gratefully acknowledge contributions from the IBM MRAM team in Yorktown Heights.

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  1. IBM T. J. Watson Research Center, Yorktown Heights, NY, 10598, USA

    D. C. Worledge & G. Hu

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Correspondence to D. C. Worledge.

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Worledge, D.C., Hu, G. A snapshot review of double magnetic junctions for STT-MRAM. MRS Advances 8, 131–137 (2023). https://doi.org/10.1557/s43580-023-00538-w

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