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A New Spin on Electronics -Spintronics-

A New Spin on Electronics -Spintronics-. Stuart Wolf University of Virginia Presented at SPIN 08 October 11, 2008 Charlottesville, VA. Beyond Conventional Electronics: Spintronics. Conventional Electronics Charge Based on number of charges and their energy

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A New Spin on Electronics -Spintronics-

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  1. A New Spin on Electronics-Spintronics- Stuart Wolf University of Virginia Presented at SPIN 08 October 11, 2008 Charlottesville, VA

  2. Beyond Conventional Electronics: Spintronics • Conventional Electronics Charge • Based on number of charges and their energy • Performance limited in speed and dissipation • Spintronics Spin • Based on direction of spin and spin coupling • Capable of much higher speed at very low power

  3. Outline of talk • Spin Transport • Spintronic sensors for Magnetic Recording • Magnetic Random Access Memory (MRAM) • Spin Transfer Torque Random Access Memory (STTRAM) • Spin Torque Nano-Oscillators

  4. Spin Dependent Transport • in all ferro and ferri-magnetic systems current is carried independently in two spin-channels • conductivity in two channels can be very different • can be described by spin-dependent mean free paths or scattering times • current is spin-polarized • manipulate flow of spin polarized current  useful sensors and memories s = s+ s s>> s Neville Mott (1934)

  5. Two main types of digital data storage • Random access memory • Hierarchy of memories • SRAM- fast but expensive • DRAM- less fast and less expensive • Highly reliable but volatile • Flash: non-volatile, less expensive, very slow, limited endurance • Hard disk drives • Massive storage • Non-volatile • Very cheap • Very slow • Less reliable! Digital data storage

  6. Current in a metallic conductor In a non-magnetic conductor, electrons scatter the same amount regardless of spin as current flows. How much they scatter determines the resistance of the device.

  7. Current in a ferromagnetic conductor In a Ferromagnetic conductor, however, electrons scatter differently depending on whether they are spin up or spin down. In this case, the spin up electrons are scattered strongly while the spin down electrons are scattered only weakly.

  8. Spin-Dependent Scattering If a non-magnetic conductor is sandwiched between two oppositely magnetized ferromagnetic layers, a number of electrons will scatter strongly when they try to cross between layers.  this gives higher resistance. If the ferromagnetic layers are magnetized in the same direction, far fewer electrons are strongly scattered and more current flows This is measured as lower resistance  Useful for sensing magnetic fields or as a magnetic memory element

  9. Antiferromagnet Spin-valve To make a technologically useful device, a “pinning” layer is added to make it harder to change the magnetization of one layer than the other. The pinning layer can be a simple layer of an antiferromagnetic material.

  10. Giant Magnetoresistance (GMR) MR(%) Co95Fe5/Cu [110] NOBEL PRIZE ! Fert and Gruenberg multi-layer H(kOe) [011] DR/R~110% at RT Field ~10,000 Oe Py/Co/Cu/Co/Py 10 NiFe Co nanolayer Cu Co nanolayer NiFe FeMn spin-valve H(Oe) DR/R~8-17% at RT Field ~1 Oe NiFe + Co nanolayer

  11. Magnetic engineering at the atomic scale Spin Valve GMR sensor + interface engineering + Artificial Antiferromagnet Ferromagnet + interface layer Spacer layer Metal or insulator Spin Valve Magnetic Tunnel Junction  + interface layer Ferromagnet  Anti- Ferromagnet

  12. Hard Disk Drive

  13. Hard Disk Drive areal density evolution ~30% 100 Gb/in2 Lab demos 1st GMR Head ~200% 1st MR Head 60% 1st Thin Film Head 25% CAGR IBM Disk products IBM RAMAC (1st Hard Disk Drive) Year

  14. Seagate 2006

  15. Hard Disk Drive capacity shipped per year 100 Exabytesin ~2005 ~100% CGR Bytes Shipped / Year Year

  16. Spintronics  Spin valve sensor  Major impact on hard disk drive storage  enabled >400x increase in storage capacity since 1998  makes possible minaturization of hard disk drives  cell phones, PDA, MPEG players  makes possible access to all information Spintronics  Magnetic Tunnel Junction  Major impact on random access memory?  Just introduced to hard disk drive storage

  17. Spin Polarized Electron Tunneling: FM-I-FM Juliere (1975) Free FM Pinned FM

  18. 1975 1982 1995 1995 T=295 K T=295 K

  19. 80 60 40 20 0 -100 -80 -60 -40 -20 0 20 40 60 80 100 Exchange-Biased Magnetic Tunnel Junction (MTJ) Top lead Free ferromagnet Tunnel barrier Pinned ferromagnet Antiferromagnet Bottom electrode Substrate DR/R Ti, Ti/Pd or Ta/ Pt CoFe/NiFe Al2O3 CoFe or NiFe/CoFe Antiferromagnet Magnetization Underlayer Si, quartz, N58 “1” Field MR (%) H=0 “0” Non-Volatile Memory! Field (Oe)

  20. History of development of MTJs Record TMR –500%

  21. Conventional MRAM (1T-1MTJ) Freescale  Thermal Stability Factor SCALING PROBLEM Beyond 65 nm node!!!

  22. Spin Torque Transfer Switching Absorbed Angular Momentum Torque Torque  Active “free” layer (thin) Polarizing “fixed” layer (thick) Net change in per e- Spin polarized current generates torque on magnetization of free layer  MR ratio 0.5-5% Jc~107A/cm2 Katine et al, Phys. Rev. Lett. 84, (2000) 3149 .

  23. Switching current scales down with cell size DI~ 6mA Albert et al, Appl Phys. Lett.,77 3809 (2000). DI ~ 0.5mA Grandis Inc

  24. “State of the Art” in STT-MTJ’s Reductions in Jc~ 9×105 A/cm2 and TMR ~ 73% MgO  increases h Thermal Stability Factor Not Satisfied!! The improvement is over amorphous AlOx tunnel barriers that were initially studied and gave Jc ~ 8×106A/cm2 and TMR ~ 42% J. Hayakawa, JJAP 41 (2005) L1267

  25. Current Scaling – MRAM vs STTRAM

  26. Challenges for STTRAM • Switching Current Density  Too High! • Small current needed to decrease size of MOSFET in series with MTJ cell (1T-1MTJ) • Small voltage across device needed to reduce probability of tunneling barrier breakdown Jc needs to be lowered to ~105 A/cm2 Need to reduce current density required to switch cell while achieving high MR%

  27. Spin Transfer Model New Materials  Lower Switching Current Density • MsSaturation Magnetization  Decrease • aGilbert damping parameter  Decrease • h Spin Transfer Efficiency  Increase Also require: Anisotropy Energy / kT > 60 for 10 year retention [J.C. Slonczewski J. Magn Mater. 159 (1996) L1]

  28. New Materials  Can we do better? P ~ 53 % a ~ 0.032 [P.V. Paluskar et al JAP 99 (2006) 08E503] Co70Fe20B10 [C. Bilzer et al, JAP 100 (2006) 053903] P measured using Superconducting Tunneling Spectroscopy (STS) with an AlOx tunnel barrier and a was determined with FMR characterization  post anneal P ~ 94 % a ~ 0.0023 CrO2 [Parker et al, PRL 88 (2002) 196601] [P. Lubitz et al, JAP 89 (2001) 6695] M1-xCrxO2 Newly Discovered RT Ferromagnetic Oxides! M=V and Ru VO2  Dr×10 with charge injection Jc~ 104 A/cm2

  29. Key Advantages and Potential of STTRAM • Excellent write selectivity ~ localized spin-injection within cell • Highly Scalable ~ write current scales down with cell size • Low power ~ low write current • Simpler Architecture ~ no write lines, no bypass line and no cladding • High Speed ~ Few nanoseconds

  30. Spin Torque Nano-Oscillators Spin-Current Switched MRAM High-speed switching I Tunnel junction 50 nm simulation Spin Transfer Nano-Oscillators 0.7 T, q = 10o I Tunable High Q oscillator (2 GHz – 100 GHz) data Au NiFe CoFe Cu 1 mm Simulations: OOMMF math.nist.gov/oommf/

  31. Summary of Present Status Current Tunable Field Tunable 0.5 GHz/mA 28 GHz/T Narrow Band • Oscillators are tunable over a • wide range of frequencies via • applied field or current • Output is narrow band with Q • values > 10,000 • Voltage outputs in the mV regime f = 17.052 GHz Df = 3.00 MHz

  32. Fundamental Frequency Limits The gyromagnetic precession frequency of spins has no upper bound! For ultra-small contacts of diameter 3 nm < d < 8 nm, intralayer exchange dominates the energetics: “THz gap” SMT oscillators could fill the “THZ gap.”

  33. locked Electronic Phase Control of Oscillations Idc= 7.4 mA Idc= 7.6 mA Idc= 7.8 mA Idc= 7.85 mA The relative phase can be varied using the DC current! W. H. Rippard et al, Phys. Rev. Lett. 95, 067203 (2005).

  34. Phase Locking in Closely Spaced Spin Transfer Nano-Oscillators A Locked B

  35. Phase Locking 500 nm Spaced Contacts (nV)2/Hz A 3.7 Locked 13.3 IB IA B Spin valve A 500 nm B When phase locked power increases & linewidth decreases A biased at 11.5 mA; B swept 0 – 15 mA B B Locked A A Kaka et al, Nature, Sept. 2005

  36. sin(wt+q4) sin(wt+q1) sin(wt+q3) sin(wt+q2) STNO STNO STNO STNO I1 I4 I3 I2 Applications of Spin Transfer Nano-Oscillators SMT device/GMR sensor Signal S(t) in 200 nm Point contact STNOs Induced spin waves i1 i3 i2 High-speed parallel signal processing Component signals out Near-field antenna Spin-transfer oscillator Chip-to-chip microwireless Not going to replace existing VCOs! Target new applications requiring nanoscale high frequency components! Nanoscale Phased Array Reference oscillator

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