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SCIENTIA SINICA Physica, Mechanica & Astronomica, Volume 46 , Issue 10 : 107306(2016) https://doi.org/10.1360/SSPMA2016-00185

Recent progresses in spin transfer torque-based magnetoresistive random access memory (STT-MRAM)

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  • ReceivedApr 1, 2016
  • AcceptedMay 16, 2016
  • PublishedAug 15, 2016
PACS numbers

Abstract

Spin transfer torque-based magnetoresistive random access memory (STT-MRAM) promises to be the next-generation low-power universal memory thanks to the non-volatility, infinite endurance and fast switching speed. In particular, recently the research and application of the STT-MRAM have been greatly advanced by the launch of the commercial chip. In this paper, we firstly present the basic principle and development history of the STT-MRAM, highlighting the improvement in the write technology and magnetic anisotropy. Then the recent progresses are reviewed including the following three aspects: first, much research effort has been made to explore the influence of the fabrication process and device structure on the interfacial perpendicular magnetic anisotropy; second, CoFeB-MgO double-interface structure has been proposed to enhance the thermal stability of the magnetic tunnel junction (MTJ) without the increase in the write current; third, emerging spin-orbit torque (SOT) has attracted massive research interest since it promises to overcome the speed bottleneck and high-risk barrier breakdown suffered by the conventional STT. Finally, the recent development of the STT-MRAM chips is summarized.


Funded by

国家自然科学基金(61571023,61501013,61471015)

国家科技部国际科技合作与交流项目项目(2015DFE12880)

中国博士后科学基金(2015M570024)

北京市科委项目(D15110300320000)


Acknowledgment

赵巍胜衷心感谢与法国南巴黎大学Albert Fert教授、美国加州大学洛杉矶分校Wang教授以及匹兹堡大学Chen教授的讨论与合作. 感谢高钰茜同学对参考文献的核对. 本文中所有图的再版已经获得原出版单位的授权.


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  • Figure 1

    (Color online) Mainstream memory architecture of the computing systems.

  • Figure 2

    (Color online) (a) Core structure of the magnetic tunnel junction; (b) tunnel magnetoresistance effect.

  • Figure 3

    (Color online) Principle of spin-dependent tunneling.

  • Figure 4

    (Color online) MRAM switched by the magnetic field [22].

  • Figure 5

    (Color online) (a), (b) Structure of the MTJ switched by Toggle and corresponding cell layout [23]; (c) schematic of the thermal assisted switching [24].

  • Figure 6

    (Color online) (a) Schematic of the spin transfer torque; (b) influence of the spin transfer torque on the magnetization dynamics.

  • Figure 7

    (Color online) (a) MTJ with perpendicular magnetic anisotropy; (b) the magnetization versus magnetic field along the in-plane and perpendicular orientation, indicating that the easy-axis is perpendicularly aligned.

  • Figure 8

    (Color online) (a) CoFeB-MgO MTJ with perpendicular magnetic anisotropy; (b) and (c) magnetization hysteresis loop when the thickness of the CoFeB free layer is 2.0 and 1.3 nm, respectively. Inset: the dependence of KtCoFeB on tCoFeB [38].

  • Figure 9

    (Color online) (a) Spin-orbit coupling effects on wave function character at the Γ point of interfacial Fe d and neighbor oxygen pz orbitals for the pure Fe/MgO interface. The three subcolumns in each column show the band levels for out-of-plane (left) and in-plane (right) orientations of magnetization as well as for the case without spin-orbit interaction included (middle). Numbers are the percentage of orbital character components within Wigner-Seitz spheres around interfacial atoms [40]; (b) onsite projected magnetic anisotropy energy (MAE) for different “interface 1/Fe7/interface 2” structures. Interface 1: Fe7/vacuum, pure Fe7 [MgO]11, underoxidized Fe7 [MgO]11 (O vacancy), Mg vacancy Fe7/MgO, and over-oxidized Fe7 [MgO]11. Interface 2: Fe7/vacuum and pure Fe7/MgO11 [43].

  • Figure 10

    (Color online) (a) Magnetic anisotropy energy of blanket Ta/CoFeB/MgO and Ru/CoFeB/MgO layers [51]; (b) dependence of KtCoFeB on tCoFeB for Hf (5) or Ta (5)/Co40Fe40B20/ MgO (2)/Ta(5) (in nm) [52].

  • Figure 11

    (Color online) Schematics of crystalline structures for (a) MgO/CoFe/X; (b) MgO/CoFe; (c) CoFe/X; and (d) CoFe thin film. A 15 Å vacuum layer is included on top of all the structures [58].

  • Figure 12

    (Color online) Layer-resolved magnetic anisotropy energy of CoFe/X systems with different capping materials. Nine CoFe monolayers, five X monolayers and a vacuum layer are included in the structures (as shown in Figure 11(c)) [58].

  • Figure 13

    (Color online) (a) Schematic of CoFeB-MgO double-interface MTJ stack [76]; (b) structure of double-interface MTJ with a Co/Pt mutilayer SyF [80].

  • Figure 14

    (Color online) Thermal stability barrier (a) and critical switching current of double-interface or single-interface MTJs (b) as a function of junction size [81], where Δ=E/kBT.

  • Figure 15

    (Color online) Simulation results of magnetization switching driven by STT in an in-plane anisotropy MTJ.

  • Figure 16

    (Color online) Three-terminal MTJ switched by spin orbit torque. (a) Induced by Rashba effect; (b) induced by Spin Hall effect.

  • Figure 17

    (Color online) To achieve the deterministic switching of perpendicular-magnetic-anisotropy MTJ, the spin orbit torque must be combined with an external magnetic field.

  • Figure 18

    (Color online) The structure asymmetry along y-axis induces an equivalent magnetic field along z-axis, which can achieve the deterministic switching of the perpendicular magnetization [101].

  • Figure 19

    (Color online) Schematic of the spin-Hall-assisted STT [105,106].

  • Figure 20

    (Color online) Simulation results of magnetization switching driven by spin-Hall-assisted STT [106]. (a), (b) Given a fixed STT write current density, the profiles of magnetization switching are different if the SHE write current density is larger or smaller than a threshold; (c) simulation results of magnetization switching driven by the conventional STT; (d) simulation results of magnetization switching driven by an SHE write current of 0.5 ns combined with the conventional STT, where it can be seen that the incubation delay of the STT is eliminated.

  • Figure 21

    (Color online) Schematic of the device in which the magnetization switching is driven by the spin Hall effect and exchange bias from antiferromagnetic layer [107,108]. For (a) and (b), the spin Hall effect is generated by heavy-metal Pt and antiferromagnetic layer PtMn, respectively.

  • Table 1   Performance comparison of various non-volatile memories

    Metrics

    eFlash (NOR)

    Flash (NAND)

    RRAM

    PCRAM

    STT-MRAM

    Endurance (cycles)

    105

    105

    109 109

    unlimited

    Read/write speed (ns)

    10/103

    100/106

    1–100

    10/100

    2–30

    Density

    medium

    higha)

    higha)

    higha)

    medium

    Write power

    highb)

    highb)

    medium

    medium

    medium

    thanks to the multibit storage per cell;

    due to high write voltage

  • Table 2   Performance of recent STT-MRAM

    Year

    Group

    Capacity

    CMOS (nm)

    Cell (μm2), die/chip (mm2)

    Speed (ns)

    Power or current

    2010

    Univ Toronto/Fujitsu Lab[111]

    16 Kbit

    130

    cell: 5.525

    R: 9, W: 9–10a)

    W: 0.4–0.87 mA

    2010

    Toshiba[112]

    64 Mbit

    65

    cell: 0.3584, Die: 47.124

    30

    R: 10 μA, W: 49 μA

    2010

    Hynix/Grandis[113]

    64 Mbit

    54

    Cell: 0.041

    R: <20

    W: 140 μA

    2010

    Hitachi/Univ Tohoku[114]

    32 Mbit

    150

    cell: 1, chip: 94.83

    R: 32, W: 40

    W: 300 μA

    2010

    IBM[115]

    4 Kbit

    array

    W: 50

    W: ~200 μA

    2011

    Qualcomm[116]

    1 Mbit

    45

    cell: 0.1026, chip: 0.27

    R: 10

    2012

    Everspin[32,117]

    64 Mbit

    90

    10~50

    2013

    TSMC[118]

    1 Mbit

    40

    macro: 0.56 mm2

    R: 10

    W: 281–283 μA

    2013

    NEC/Univ Tohoku[119]

    1 Mbit

    90

    cell: 3.46

    R: 1.5, W: 2.1

    W: 50–100 mA

    2013

    Univ Tohoku/NEC[120]

    1 Mbit

    90

    cell: 2.19

    R: 8, W: 40

    R: 10.7 mW, W: 4.3 mW

    2013

    Toshiba[121]

    512 Kbit

    65

    cell: 0.504

    8

    R: 4 mW, W: 15 mW

    2013

    Toshiba[122]

    1 Mbit

    65

    cell: 0.45

    R: 4, W: 4

    R: 0.142 nJ, W: 0.372 nJ

    2013

    Infineon/TUM[123]

    8 Mbit

    40

    R: 23

    2014

    TDK-Headway[124]

    8 Mbit

    90

    cell: 0.4

    W: <5, R: 4

    2015

    IBM[125]

    4 Kbit

    array

    W: 20–50

    2015

    Samsung/Univ Sungkyunkwan[126]

    8 Mbit

    die: 6.3

    R: 9.1

    2015

    Qualcomm/TDK-Headway[11]

    1 Mbit

    40

    cell: 0.065

    R: 20, W: 20–100

    3.2 μW/Mbps

    2015

    Toshiba[127]

    1 Mbit

    65

    cell: 0.11

    R: 3.3, W: 3.0

    R: 71.2 μJ/MHz, W: 166.2 μJ/MHz

    2016

    Toshiba/Univ Tokyo[128]

    4 Mbit

    65

    cell: 0.11

    R: 3.3

    W: 92.4 μJ/MHz

    R: read, W: write

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