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Optimization and Integration of Performances of pSTT-MRAM by Numerical Simulations

김동천 (Kim, Dong Cheon, 경희대학교 일반대학원)

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초록 moremore
Perpendicular STT-MRAM(pSTT-MRAM) is attracting next-generation memory due to fast writing speed, high scaling, high cycling capability, low power consumption and non-volatility. This study focuses on various simulations about pSTT-MRAM. The simulations can be helpful to provide an understanding of STT-MRAM behavior. The magnetization switching inside ferromagnetic layer driven by an external field and STT is studied with two simulators based on the macrospin approximation. One is in-house simulator developed by Spintec, and the other is commercial simulator Sentaurus Device. We also study the influence of various factors such as external field, damping factor and thermal fluctuation in magnetization dynamics. The results from both simulators are compared. Furthermore, MTJ models are simulated by Sentaurus Device, and the results are compared with experimental data.
Perpendicular STT-MRAM(pSTT-MRAM) is attracting next-generation memory due to fast writing speed, high scaling, high cycling capability, low power consumption and non-volatility. This study focuses on various simulations about pSTT-MRAM. The simulations can be helpful to provide an understanding of STT-MRAM behavior. The magnetization switching inside ferromagnetic layer driven by an external field and STT is studied with two simulators based on the macrospin approximation. One is in-house simulator developed by Spintec, and the other is commercial simulator Sentaurus Device. We also study the influence of various factors such as external field, damping factor and thermal fluctuation in magnetization dynamics. The results from both simulators are compared. Furthermore, MTJ models are simulated by Sentaurus Device, and the results are compared with experimental data.
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1 Overview of thesis 1
1.1 Background 1
1.2 Motivation 2
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1 Overview of thesis 1
1.1 Background 1
1.2 Motivation 2
1.3 Objectives 3
2 Introduction to STT-MRAM 5
2.1 Basic concepts of micromagnetic theory 5
2.1.1 The basics of ferromagnetism 5
2.1.2 Free energy for a ferromagnetic system 6
2.1.3 Length scales in micromagnetism 7
2.1.4 Magnetically equilibrium state 7
2.1.5 Equation of magnetization dynamic motion 8
2.1.6 Modification of spin transfer torque phenomena 9
2.2 Magnetic tunnel junction (MTJ) 10
2.2.1 Structure of MTJ 10
2.2.2 TMR effect 11
2.2.3 In-plane and out-of plane magnetization 13
2.3 Spin-transfer torque magnetic random access memory (STT-MRAM) 15
2.3.1 The structures of STT-MRAM cells 15
2.3.2 Writing process by STT in STT-MRAM 17
2.3.3 The switching current of STT-MRAM 17
2.3.4 The milestones of STT-MRAM development 18
2.3.4.1 STT effect : Efficient write approach 18
2.3.4.2 TMR enhances 19
2.3.4.3 Perpendicular magnetic anisotropy 20
2.4 Numerical simulation of STT 20
2.4.1 Macrospin approximation 20
2.4.2 Numerical micromagnetism solvers 21
3 STT-MRAM simulation and modeling 23
3.1 Macrospin simulation with in-house simulator. 23
3.1.1 Model of the free layer with macrospin approximation 23
3.1.2 The parameters used for simulation 24
3.1.3 Switching magnetization by external field in the absence of STT 25
3.1.3.1 Coercive field dynamics as a function of the damping factor 27
3.1.3.2 Coercive field dynamics as a function of the Temperature 28
3.1.4 Switching magnetization by STT without external field 29
3.1.4.1 The critical voltage as a function of pulse widths with different STT components 30
3.2 Macrospin simulation with Synopsys simulator 32
3.2.1 Introduction of Sentaurus Device 32
3.2.2 Free layer modeling by Sentaurus Device 33
3.3 Comparison between Spintec and Synopsys STT simulators 34
3.3.1 Comparison between two simulations with in-plane external field 34
3.3.2 Comparison between two simulations with out of plane external field 35
3.4 Comparison between simulation and experiment 36
3.4.1 Model of MTJ by using Sentaurus simulator 36
3.4.2 Various models of MTJ in shape 37
3.4.3 The parameters in MTJ model 38
3.4.4 I-V characteristics and TMR in P and AP state 39
3.4.5 Comparison with experiment in TMR 40
3.4.6 Switching magnetization by STT in MTJ 42
4 Conclusions and perspectives 45
4.1 Conclusions 45
4.2 Perspectives 46
Appendix A Coding in Sentarus Device 59
A.1 Modeling of a single ferromagnetic layer 59
A.2 Modeling of a 2D MTJ structure 61
A.3 Calculating by using STT modules 68
A.4 Visualizing the magnetization dynamics 71