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

SPIN 08 October 11, 2008 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

SPIN 08 October 11, 2008 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

SPIN 08 October 11, 2008 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         Neville Mott (1934)

SPIN 08 October 11, 2008 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

SPIN 08 October 11, 2008 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. Current in a metallic conductor

SPIN 08 October 11, 2008 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. Current in a ferromagnetic conductor

SPIN 08 October 11, 2008 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. Spin-Dependent Scattering 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

SPIN 08 October 11, 2008 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. Spin-valve Antiferromagnet

SPIN 08 October 11, 2008 spin-valve multi-layer Co 95 Fe 5 /Cu [110]  R/R~110% at RT Field ~10,000 Oe Py/Co/Cu/Co/Py  R/R~8-17% at RT Field ~1 Oe NiFe + Co nanolayer NiFe Co nanolayer Cu Co nanolayer NiFe FeMn H(Oe) H(kOe) [011] 10 MR(%) Giant Magnetoresistance (GMR) NOBEL PRIZE ! Fert and Gruenberg

SPIN 08 October 11, 2008 Magnetic engineering at the atomic scale Spin Valve GMR sensor + interface engineering Spin Valve Magnetic Tunnel Junction Ferromagnet Spacer layer Metal or insulator Anti- Ferromagnet + interface layer + Artificial Antiferromagnet  

SPIN 08 October 11, 2008 Hard Disk Drive

SPIN 08 October 11, 2008 Year 25% CAGR ~200% 60% IBM Disk products Lab demos 1 st Thin Film Head 1 st MR Head 1 st GMR Head IBM RAMAC (1 st Hard Disk Drive)  100 Gb/in 2 ~30% Hard Disk Drive areal density evolution

SPIN 08 October 11, 2008

Seagate 2006

SPIN 08 October 11, 2008 Hard Disk Drive capacity shipped per year  100 Exabytes in ~2005 Year Bytes Shipped / Year ~100% CGR

SPIN 08 October 11, 2008 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

SPIN 08 October 11, 2008 Spin Polarized Electron Tunneling: FM-I-FM Pinned FM Free FM Juliere (1975)

SPIN 08 October 11, 2008 CORRECTION: The legend in Figure 1(d) should read T = 295 K. T=295 K

SPIN 08 October 11, 2008

Exchange-Biased Magnetic Tunnel Junction (MTJ) Non-Volatile Memory! Field H=0  R/R Ti, Ti/Pd or Ta/ Pt Si, quartz, N58 Underlayer Antiferromagnet CoFe or NiFe/CoFe Al 2 O 3 CoFe/NiFe Top lead Substrate Free ferromagnet Pinned ferromagnet Tunnel barrier Antiferromagnet Bottom electrode MR (%) Field (Oe) “”“” “”“” “”“” “”“” Magnetization

SPIN 08 October 11, 2008 History of development of MTJs Record TMR –500%

SPIN 08 October 11, 2008 Conventional MRAM (1T-1MTJ)  Thermal Stability Factor SCALING PROBLEM Beyond 65 nm node!!! Freescale

SPIN 08 October 11, 2008 Net change inper e  Spin Torque Transfer Switching  Absorbed Angular Momentum  Torque Polarizing “fixed” layer (thick) Active “free” layer (thin) Spin polarized current generates torque on magnetization of free layer Torque  MR ratio 0.5-5% J c ~10 7 A/cm 2 Katine et al, Phys. Rev. Lett. 84, (2000) 3149.

SPIN 08 October 11, 2008 Switching current scales down with cell size  ~ 6mA  ~ 0.5mA Albert et al, Appl Phys. Lett., (2000). Grandis Inc

SPIN 08 October 11, 2008 “State of the Art” in STT-MTJ’s Reductions in J c ~ 9×10 5 A/cm 2 and TMR ~ 73% MgO  increases  The improvement is over amorphous AlO x tunnel barriers that were initially studied and gave J c ~ 8×10 6 A/cm 2 and TMR ~ 42% J. Hayakawa, JJAP 41 (2005) L1267 Thermal Stability Factor Not Satisfied!!

SPIN 08 October 11, 2008 Current Scaling – MRAM vs STTRAM

SPIN 08 October 11, 2008 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 Need to reduce current density required to switch cell while achieving high MR% J c needs to be lowered to ~10 5 A/cm 2

SPIN 08 October 11, 2008 New Materials  Lower Switching Current Density Spin Transfer Model M s Saturation Magnetization  Decrease  Gilbert damping parameter  Decrease  Spin Transfer Efficiency  Increase [J.C. Slonczewski J. Magn Mater. 159 (1996) L1] Also require: Anisotropy Energy / kT > 60 for 10 year retention

SPIN 08 October 11, 2008 New Materials  Can we do better? Co 70 Fe 20 B 10  P ~ 53 %  ~ P measured using Superconducting Tunneling Spectroscopy (STS) with an AlO x tunnel barrier and  was determined with FMR characterization  post anneal CrO 2  P ~ 94 %  ~ [C. Bilzer et al, JAP 100 (2006) ] [P. Lubitz et al, JAP 89 (2001) 6695] [Parker et al, PRL 88 (2002) ] [P.V. Paluskar et al JAP 99 (2006) 08E503] M 1-x Cr x O 2  Newly Discovered RT Ferromagnetic Oxides! M=V and Ru VO 2   ×10 with charge injection J c ~ 10 4 A/cm 2

SPIN 08 October 11, 2008 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 Key Advantages and Potential of STTRAM

SPIN 08 October 11, 2008 Spin-Current Switched MRAM Spin Transfer Nano-Oscillators 50 nm 1  m Spin Torque Nano-Oscillators Simulations: OOMMF math.nist.gov/oommf/ I Tunnel junction High-speed switching Tunable High Q oscillator (2 GHz – 100 GHz) Au Cu 0.7 T,  = 10 o CoFe NiFe I simulation data

SPIN 08 October 11, 2008 f = GHz  f = 3.00 MHz 0.5 GHz/mA Summary of Present Status Field Tunable Current Tunable 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 28 GHz/T

SPIN 08 October 11, 2008 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:  SMT oscillators could fill the “THZ gap.” “THz gap”

SPIN 08 October 11, 2008 I dc = 7.4 mA I dc = 7.6 mA I dc = 7.8 mA I dc = 7.85 mA locked Electronic Phase Control of Oscillations The relative phase can be varied using the DC current! W. H. Rippard et al, Phys. Rev. Lett. 95, (2005).

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

SPIN 08 October 11, 2008 A biased at 11.5 mA; B swept 0 – 15 mA  nV) 2 /Hz A B Locked A B A B Phase Locking 500 nm Spaced Contacts Kaka et al, Nature, Sept Spin valve A B 500 nm IBIB IAIA When phase locked power increases & linewidth decreases

SPIN 08 October 11, 2008 Applications of Spin Transfer Nano-Oscillators Signal S(t) in Induced spin waves Component signals out Point contact STNOs 200 nm i1i1 i2i2 i3i3 STNO I2I2 sin(  t+  2 ) STNO I3I3 sin(  t+  3 ) STNO I4I4 sin(  t+  4 ) STNO I1I1 sin(  t+  1 ) SMT device/GMR sensor Spin-transfer oscillator Near-field antenna Chip-to-chip microwireless Nanoscale Phased Array High-speed parallel signal processing Reference oscillator Not going to replace existing VCOs! Target new applications requiring nanoscale high frequency components!