A Generalized HSPICE* Macro-Model for Pseudo-Spin-Valve GMR Memory Bits

Transcription

1 A Generalized HSPICE* Macro-Model for Pseudo-Spin-Valve GMR Memory Bits by Bodhisattva Das William C. Black, Jr. Department of Electrical and Computer Engineering, Iowa State University (Work sponsored in part by Honeywell SSEC; and DARPA) *HSPICE is an authorized trademark of Avant! Corporation IEEE ISCAS 98 Monterey, CA 1

2 Outline Introduction to Giant Magneto Resistance (GMR) Devices GMR Characteristics and Applications Pseudo-Spin-Valve Characteristics Typical Generalized HSPICE Macro-Model Schematic Attributes Simulation Current and Future Work Summary and Conclusion B. Das, W. C. Black ECpE - ISU 2

3 Introduction to GMR Devices Two ferromagnetic layers separated by non-magnetic spacer layer Spin-Valve : one layer pinned by anti-ferromagnetic layer Pseudo-Spin-Valve : both the layers are free to rotate (not pinned) Permalloy 1 R R max k.cosφ M 2 φ M 1 H Permalloy 2 Nonmagnetic Inner Layer Resistance is function of difference between magnetizations Variations of 6%+ have been demonstrated for practical bits B. Das, W. C. Black ECpE - ISU 3

4 Typical Characteristics and Applications of GMR Characteristics: Stable magnetic states (non-volatile) Resistant to ionizing radiation Significant signal improvement relative to older singlelayered Anisotropic Magneto Resistance (AMR) material Applications: Non-volatile memory (MRAM) Magnetic sensors Disk read-heads Isolation devices B. Das, W. C. Black ECpE - ISU 4

5 Basic GMR Bit Configuration sn1 sense current wd1 wd0 word current sn0 Read with sense current (may stack bits) Write design options: word current alone (small memories) coincident sense and word current (large memories) B. Das, W. C. Black ECpE - ISU 5

6 Spin-Valve Characteristics N1 N2 R H RN3 P6 P5 Graph N N4 I N Graph P P4 P3 C R L P2 I P N5 N6 P1 I W (word) Hysteretic in nature Combination of graphs P and N. I W >I P : Graph P; I W <I N : Graph N Two distinct states : Graph P : St. 1 ; Graph N : St. 0 (or vice versa) I W >I P : write to St. 1 ; I W <I N : write to St. 0 ; I N <I W <I P : read I N and I P are weak functions of sense current B. Das, W. C. Black ECpE - ISU 6

7 Typical Pseudo-Spin-Valve P6 P5 P9 N9 N5 N6 N1 N2 N7 N8 P8 P7 N10 N3 P3 P10 Bottom (harder) layer magnetization curve: higher coercive force P2 P1 P6 P5 P9 N9 N5 N6 N1 N2 N7 N8 P8 P7 N10 N3 P3 P10 Top (softer) layer magnetization curve: lower coercive force P2 P1 B. Das, W. C. Black ECpE - ISU 7

8 Typical Pseudo-Spin-Valve (contd.) P6 R H R P5 P9 N9 N5 N6 N1 P8 N2 P7 R L N10 P4 N4 N7 N3 P3 P10 Combination of two curves : one for actual top layer and the other for bottom layer The graph is the absolute difference of the two Symmetric Major and Minor loops C P2 I W N8 P1 B. Das, W. C. Black ECpE - ISU 8

9 Macro-model Four terminal sub-circuit : Two word line terminals, two sense line terminals sn1 wd1 GMR wd0 Four simple parts : Input (word) circuit sn0 Bistable multivibrator or Schmitt Trigger Decision circuit Output (sense) circuit B. Das, W. C. Black ECpE - ISU 9

10 Macro-model (contd.) V TL L+ V TH = -L (R1/R2) V TL = -L + (R1/R2) Eopamp : VCVS Gd : VCR [1:1] cntl by V(7,wd0) Gr : VCCS (+/-1) = V(4,wd0)/L [ L = L + = -L ] Gb : PWL VCR cntl by V(3,wd0) Vc wd1 7 R1 5 wd1 Rw wd0 Input (word) Circuit 3 Gd wd0 wd0 Decision Circuit Gr Bistable Multivibrator wd0 R2 Gb Vmax= L + Vmin= L sn1 sn0 B. Das, W. C. Black ECpE - ISU 10 L 4 Eopamp wd0 V TH Output (sense) Circuit

11 Generalized PSV Characteristics R P6 N9 P5 N5 P9 N6 R H N1 P8 N2 P7 N10 R L P4 N3 P3 N4 P10 N7 P2 Hysteretic in nature: combination of systems P and N I W > I P2 : System P ; I W < I N2 : System N Each system can be divided into two graphs Four distinct states can be utilized: two bit memory C I W N8 P1 B. Das, W. C. Black ECpE - ISU 11

12 Generalized PSV Charac. (contd.) As two-state memory : I W >I P2, I W <I N2 : write ; I N2 <I W <I P2 : read I N2 and I P2 (write thresholds) are weak functions of sense current N9 N5 N6 Graph Nb N4 N1 N2 Graph Na N7 N10 N3 N8 System N can be divided into two graphs : Na and Nb B. Das, W. C. Black ECpE - ISU 12

13 Generalized PSV Charac. (contd.) P6 P5 P9 Graph Pb P8 P7 P4 Graph Pa P3 P10 System P can be divided into two graphs : Pa and Pb Non-symmetric Major and Minor loops Major loop : Na and Pa Minor loop : Nb and Pb P2 P1 B. Das, W. C. Black ECpE - ISU 13

14 Versatility Attributes of the Model Usable with any kind of PSV GMR elements Generalized and simple : can be modified easily to represent other kinds of hysteretic characteristics Flexibility All variables parameterized :.PARAM statements No component needing power supply : non-volatile Portability subcircuit (.inc) can be included in any HSPICE netlist: great help for GMR memory testing No semiconductor device : no variation with different device models B. Das, W. C. Black ECpE - ISU 14

15 Simulation Results Sub-circuit simulated with HSPICE : did converge! Simulations give proper results R for the GMR bit : 5% with R L =100Ω, R H =105Ω DC analysis done for a wide range of word currents Transient analysis shows proper major and minor loop characteristics Simulation of non-volatile latch structures with this GMR model Simulation of novel Pseudo-Spin-Valve GMR memory structures with this model (the structure can detect all four states but destructive read) B. Das, W. C. Black ECpE - ISU 15

16 Current and Future Work Already done: Thermal behavior (transient and dc) incorporated Asteroid curve incorporated: write thresholds depend on sense current value In the pipeline: Noise analysis (transient?), esp. near the write thresholds A universal GMR model usable for both memory and magnetic sensors (including read heads) B. Das, W. C. Black ECpE - ISU 16

17 B. Das, W. C. Black ECpE - ISU 17

18 Summary and Conclusion First HSPICE circuit model for Pseudo-Spin- Valve GMR element Successfully modeled the resistance vs. word current characteristic of a Pseudo-Spin-Valve GMR bit The model is simple, flexible and versatile Simulations show proper results The model is very general in nature, can be applied to other hysteretic characteristics Extended to include thermal effects and sense-currentdependency of write thresholds B. Das, W. C. Black ECpE - ISU 18

19 Questions? B. Das, W. C. Black ECpE - ISU 19