Thermally assisted spin transfer torque [TAS + STT] is a new switching approach for magnetic tunnel junction [MTJ] nanopillars that represents the best trade-off between data reliability, power efficiency and density. In this paper, we present a compact model for MTJ switched by this approach, which integrates a number of physical models such as temperature evaluation and STT dynamic switching models. Many experimental parameters are included directly to improve the simulation accuracy. It is programmed in the Verilog-A language and compatible with the standard IC CAD tools, providing an easy parameter configuration interface and allowing high-speed co-simulation of hybrid MTJ/CMOS circuits.
Spintronics is a very rapidly emerging area of R&D (Nobel Prize 2007) that has the potential to impact significantly on the future of all aspects of electronics beyond CMOS . Magnetic tunnel junctions [MTJ] is one of the most promising spintronic devices for logic and memory applications, which combines magnetism and electronics and promises high write/read speed, non-volatility, infinite endurance etc. . An MTJ is a nanopillar composed of two ferromagnetic [FM] layers and one oxide thin barrier. The Tunnel MagnetoResistance [TMR] phenomenon exists in MTJs , which describes the different resistance value RP and RAP corresponding to the parallel or anti-parallel configuration of the relative magnetization orientations of the two FM layers, respectively. For practical applications, the magnetization direction of one FM layer is pinned as reference and that of the other FM layer is free to store the binary state (see Figure 1a). Recently, TMR = RAP-RP/RP ratio was found to be more than 604% by using the MgO oxide barrier  and this allows MTJ to present excellent sensing performance.
Today, most of the R&D efforts in MTJ are focused on its switching approaches, which are expected to be scalable, energy efficient, reliable and fast. A number of approaches have been investigated since 2002, such as thermally assisted switching [TAS]  and spin transfer torque [STT] [6, 7]. However all of them suffer from either high power or stability issue and cannot meet the requirements for wide applications. Thermally assisted spin transfer torque [TAS + STT] is an emerging approach [8, 9], which is based on the temperature dependence of exchange bias storage principle , as used in TAS . This switching mechanism involves applying a low current through STT to raise the MTJ temperature above the blocking temperature (Tb) of the antiferromagnetic layer associated to the storage layer, resulting in a hysteresis loop centred about zero (see Figure 1). Tbdepends mainly on the material composition (e.g. ~423K for IrMn and ~573K for PtMn). This method combines the advantages of both TAS and STT technologies, giving the best trade-off among data reliability, power efficiency, speed and density. Unlike other nanodevices , MTJ can be easily integrated with CMOS circuits . Based on hybrid MTJ/CMOS , innovative memory and logic circuits are expected to provide high performance or new functionalities beyond CMOS. A Spice-compatible efficient compact model for MTJ is an essential requirement for the hybrid MTJ/CMOS design and simulation.
Physical model integration
This compact model is based on our previous STT MTJ model, which is composed of two sub-modules representing respectively the sensing and switching operations . For sensing, the MTJ resistance and TMR ratio are calculated to obtain respectively RPand RAP. For switching, the STT critical current, IC, calculation model was implemented to obtain the hysteresis loop margin of storage layer . The present model offers an improvement over the previous work [14–17] as it integrates the temperature evaluation and STT dynamic switching models to describe the TAS + STT switching approach. In order to optimise the simulation speed, one of the most important performances for logic and memory designs, some physical phenomena like the oscillating effects during switching are omitted.
Temperature evaluation model
The temperature evaluation of MTJ depends on the form and duration of switching current according to Equations 1 and 2 [17, 18]. As square current pulses are often used for logic and memory circuit design and simulation, Equation 1 can be then simplified to Equations 3 and 4 to describe respectively the heating and cooling operations driven by a current pulse. This model allows simulating the thermally assisted mechanism of TAS+STT approach. An equivalent electrical circuit corresponding to Equation 1 has been included to monitor the temperature evaluation of MTJ (see Figure 2a), which is based on a simple resistor/capacitor circuit. By adding a multiplier (M0) and an adder (A0), the temperature T can be observed in real time through the voltage node Vtemp(i.e. 1V = 1K). The values of R0and C0are set as constant to obtain τthcalculated by Equation 2,
where V is the voltage across MTJ nanopillar, λ is thermal conductivity of the thermal barrier, CVis heat capacity per unit volume, j is current density, TRis room temperature, thick_b is the thickness of thermal barrier, thick_s is the total thickness of MTJ nanopillar and τthis the characteristic heating/cooling time. This leads to
where Dheatis the heating current pulse duration, Dcoolis the cooling duration, and Theatand Tcoolpresent respectively temperature of MTJ during heating/cooling operations.
Spin Transfer Torque (STT) dynamic switching model
STT dynamic switching behaviours are described by Equations 5 and 6, which are crucial to simulate the power and speed performances of hybrid MTJ/CMOS circuits. Thermal fluctuations induce an initial angle Θ0 between the magnetization of the storage layer and its easy axis , which is approximated by Equation 6. High temperature increases Θ0 and then reduces the STT switching duration, Dswitch. STT state reversal depends on switching current value, Iswitch, which should be higher than the critical current, IC. Dswitchcan be linearly reduced down to according to nanosecond range with high Iswitch. This property is useful for the design and simulation of hybrid MTJ/CMOS circuits dedicated to logic applications, which require very high speed (e.g. approximately gigahertz).
where Haniis in-plane uniaxial magnetic anisotropy field, μ0Msis saturation field in the storage layer, α is Gilbert damping coefficient, γ0is the gyromagnetic constant, Vol is the volume of storage layer and kBis the Boltzmann constant.
Compact model simulation and validation
Co-simulation of Hybrid MTJ/CMOS circuit
This compact model has been developed in Verilog-A language and implemented on Cadence Virtuoso CAD platform . Its default parameters correspond to a MTJ nanopillar BiFe(10)/IrMn(6)/CoFeB(1)/MgO(0.85)/CoFeB(3)/PtMn(6). Thanks to the graphical parameter configuration of Verilog-A, MTJ can be set easily with different material and process parameters. By using CMOS 65 nm design-kit, a simple hybrid circuit (see Figure 2b)  has been successfully simulated (see Figure 3), which validates the functionalities and behaviours of this model. The voltage pulse "Vg1" is activated at 40 ns and Iswitch begins to heat the MTJ from ambient temperature. As its temperature reaches up to Tb2 after ~11.22 ns, the model compares the Iswitch(approx 462.9 uA) with the STT critical current IC (~150 uA) and switches the state of MTJ from parallel [P] to anti-parallel [AP] state in about 6 ns according to the STT dynamic model. As "Vg1" is deactivated, MTJ begins to cool down to ambient temperature. The state can be reversed from AP to P by activating the control signal "Vg2", which generates Iswitch (approx-375.6uA). The Iswitchvalues are asymmetric as a constant voltage supply is used in the simulation (e.g. 1V) and the resistance of MTJ changes between two states (RPand RAP). It is important to note that the voltage pulse width should be longer than Dheat+Dswitchto ensure the reliable switching operation .
Power and die area estimation
The silicon area of this hybrid circuit is ~9.8 um2 as the width of NMOS transistors is set to 1 μm to provide Iswitchmuch higher than ICand reduce the duration down to some nanoseconds. The whole switching operation of TAS + STT between the P and AP states dissipates ~2.7pJ of energy.
In this paper, we present the first compact model for MTJ nanopillar switching using the TAS+STT approach. Transient simulations of a hybrid MTJ/CMOS circuit validate its functionalities and demonstrate that it can be useful to calculate the critical circuit performances like speed, power and die area. The easy parameter interface of the Verilog-A language allows us to analyse the characteristics of MTJ with different materials, area and thin film thickness etc. By using this model, a number of hybrid MTJ/CMOS complex circuits are under investigation in our laboratory.
The authors wish to acknowledge support from the French national projects CILOMAG, ANR-SPIN and NANO2012 project with STMicroelectronics.
IEF, Université Paris-Sud
Chappert C, Fert A, Nguyen Van Dau F: "The emergence of spin electronics in data storage".Nat Mater 2007, 6: 813–823. 10.1038/nmat2024View Article
Zhao WS, Belhaire E, Chappert C, Mazoyer P: "Spin transfer torque (STT)-MRAM-based runtime reconfiguration FPGA circuit".ACM Trans Embedded Computing 2009., 9: article 14 article 14
Julliere M: "Tunnelling between ferromagnetic films".Phys Lett 1975, 54A: 225–226.View Article
Ikeda S, Hayakawa J, Ashizawa Y, Lee YM, Miura K, Hasegawa H, Tsunoda M, Matsukura F, Ohno H: "Tunnel magnetoresistance of 604% at 300 K by suppression of Ta diffusion in CoFeB/MgO/CoFeB pseudo-spin-valves annealed at high temperature".Appl Phys Lett 2008, 93: 082508. 10.1063/1.2976435View Article
Prejbeanu IL, Kerekes M, Sousa RC, Sibuet H, Redon O, Dieny B, Nozière JP: "Thermally assisted MRAM".J Phys Condensed Matter 2007, 19: 165218. 10.1088/0953-8984/19/16/165218View Article
Slonczewski JC: "Current-drivern excitation of magnetic multilayers".J Magnetism Magnetic Mater 1996, 159: L1-L7. 10.1016/0304-8853(96)00062-5View Article
Xi HW, Stricklin J, Li H, Chen YR, Wang XB, Zheng YK, Gao Z, Tang MX: IEEE Trans Magn. 2010, 46: 860–866.View Article
Dieny B, Sousa RC, Herault J, Papusoi C, Prenat G, Ebels U, Houssameddine D, Rodmacq B, Auffret S, Buda-Prejbeanu LD, Cyrille MC, Delaet B, Redon O, Ducruet C, Nozières JP: "Spin-transfer effect and its use in spintronic components".Int J Nanotechnol 2010, 7: 591–614. 10.1504/IJNT.2010.031735View Article
Chen YT: "The Effect of Interface Texture on Exchange Biasing in Ni(80)Fe(20)/Ir(20)Mn(80) System".Nanoscale Res Lett 2009, 4: 90–93. 10.1007/s11671-008-9207-4View Article
Navi K, Rashtian M, Khatir A, Keshavarzian P, Hashemipour O: "High Speed Capacitor-Inverter Based Carbon Nanotube Full Adder".Nanoscale Res Lett 2010, 5: 859–862. 10.1007/s11671-010-9575-4View Article
Matsunaga S, Hayakawa J, Ikeda S, Miura K, Hasegawa H, Endoh T, Ohno H, Hanyu T: "Fabrication of a Nonvolatile Full Adder Based on Logic-in-Memory Architecture Using Magnetic Tunnel Junctions".Appl Phys Exp 2008, 1: 091301.View Article
Zhao WS, Chappert C, Javerliac V, Nozière J-P: "High Speed, High Stability and Low Power Sensing Amplifier for MTJ/CMOS Hybrid Logic Circuits".IEEE Trans Magn 2009, 45: 3784–3787.View Article
Zhao WS, Belhaire E, Mistral Q, Chappert C, Javerliac V, Dieny B, Nicolle E: "Macro-model of Spin-Transfer Torque based Magnetic Tunnel Junction device for hybrid Magneti-CMOS design". In Procedings of the 2006 IEEE International Behavioral Modeling and simulation Workshop. San Jose CA; 2006:40–43.View Article
Elbaraji M, Javerliac V, Guo W, Prenat G, Dieny B: "Dynamic compact model of thermally assisted switching magnetic tunnel junctions".J Appl Phys 2010, 106: 123906.View Article
Das B, Black WC Jr, Pohm AV: "Universal HSPICE macromodel for giant magnetoresistance memory bits".IEEE Trans Magn 2000, 36: 2062–2072. 10.1109/20.875339View Article
Roldan AM, Roldan JB, Reig C, Cubells-Beltran MD, Ramirez D, Cardoso S, Freitas PP: "A DC behavioral electical model for quasi-linear spin-valve devices including thermal effects for circuit simulation".Microelectron J 2011, 42: 365–370. 10.1016/j.mejo.2010.10.005View Article
Sousa RC, Prejbeanu IL, Stanescu D, Rodmacq B, Redon O, Dieny B, Wang JG, Freitas PP: "Tunneling hot spots and heating in magnetic tunnel junctions".J Appl Phys 2004, 95: 6783–6785. 10.1063/1.1667413View Article
Koch RH, Katine JA, Sun JZ: "Time-Resolved Reversal of Spin-Transfer Switching in a Nanomagnet".Phys Rev Lett 2004, 92: 088302.View Article
Devolder T, Hayakawa J, Ito K, Takahashi H, Ikeda S, Crozat P, Zerounian N, Kim JV, Chappert C, Ohno H: "Single-Shot Time-Resolved Measurements of Nanosecond-Scale Spin-Transfer Induced Switching: Stochastic Versus Deterministic Aspects".Phys Rev Lett 2008, 100: 057206.View Article
Virtuoso Spectre Circuit Simulator User Guide 2006.
Zhao WS, Belhaire E, Chappert C, Mazoyer P: "Power and Area Optimization for Run-Time Reconfiguration System On Programmable Chip Based on Magnetic Random Access memory".IEEE Trans Magn 2009, 45: 776–780.View Article
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