Magnetic Race- Track Memory: Current Induced Domain Wall Motion!

Transcription

1 Magnetic Race- Track Memory: Current Induced Domain Wall Motion! Stuart Parkin IBM Fellow IBM Almaden Research Center San Jose, California

2 Digital data storage 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!

3 MRAM (Magnetic Random Access Memory) non-volatile high performance cheap compared to other solid state memories expensive compared to hard disk drives Magnetic tunnel junction MT storage elements JA MA VA M2 M1 PC V1 CA IBM-IFX 16 Mbit MRAM chip Challenge: can we build a solid state device with the same cost as a hard disk drive but the performance and reliability of solid state memory?

4 Emerging memory technologies Quantity FRAM PFRAM SiC Bipolar Molecular NanoX tal Flow PCRAM PMC Polymer Perovskite 3DROM Spin MRAM

5 Storage-Class Memory: Domain-Wall Magnetic Shift Register Philosophy Want a solid-state memory with no moving parts which is very cheap and of moderate to high performance Main approaches Make extremely small cells Requires significant engineering developments Current roadmaps suggest that f<30nm will be possible within 5 years, thus making this approach extremely challenging Access multiple bits from one set of logic Similar philosophy used in conventional storage drives and in millipede However we want a solid state memory with no moving parts Recent developments in magnetic materials makes this approach viable and attractive by storing information in domain walls (spatially varying order parameter in homogeneous material) Lots of new science: Spin currents and torque, domain wall fringing fields

6 Vortex and transverse domain wall structures R.D. McMichael and M.J. Donahue, IEEE Trans. Magn. 33, 4167 (1997) Micromagnetic simulations LLG Micromagnetics Simulator Mike Scheinfein Cell size 5 x 5 x 5 nm 3 Vortex wall Increasing width, thickness width 250nm thickness 10 nm Transverse wall width 140nm thickness 5 nm Magnetostatic energy dominates: (shape anisotropy) magnetic moments along the wire head-to-head domain walls Larger structures : more complex DW structures (double vortex, )

7 Domain wall motion θ = 0, φ Current torque on DW t t 0 Massless motion!! θ (Magnetic field pressure on DW, ) t 0, φ t 0 From Sadamichi Maekawa

8 Theory of current-driven domain wall motion Adiabatic vs non adiabatic spin torque DW velocity vs current 1000 electron spins follows local magnetization (wide wall limit) electron spins lags behind magnetization (narrow wall limit) Spin torque amplitude : u (1 m/s J=10 6 A/cm 2 ) Non-adiabatic contribution ~ β u Velocity (m/s) damping α=0.01 DW width 50 nm Hp=1000 Oe u (m/s) β=5α β=α β=α/5 β=0 The adiabatic spin-torque is like a damping term (dissipative) the critical current in intrinsic (related to DW properties) no motion occurs below the critical current turbulent motion occurs above the critical current high velocity The non-adiabatic spin-torque is like a magnetic field (precessional) DW velocity is non-zero for ideal wires the critical current is related to defects (roughness)

9 DW motion under field and current 1280 x 140 x 15 nm 3 Strong damping a=1 Field-driven dynamics Current-driven dynamics time time Magnetic field H=-200 Oe Positive current H= 0 Negative current Field-driven dynamics : neighboring walls move in opposite directions Current driven dynamics : neighboring walls move in the same direction

10 Magnetic Race-track Memory A novel three-dimensional spintronic storage class memory The capacity of a hard disk drive but the reliability and performance of solid state memory - a disruptive technology based on recent developments in spintronic materials and physics Parkin, US patents , , Current pulses move domains along racetrack shift register TMR sensor to read bit pattern Special current pulse-driven element to re-write a bit

11 Magnetic Shift Register Memory Writing a bit current pulse on special write element Parkin, US patents , ,

12 Magnetic Race-Track Memory: Domain-Wall Magnetic Shift Register Alternating layers of two ferromagnetic materials to pin domain walls domain wall Information stored as domain walls in vertical race track Reading and writing carried out along bottom of race track Electronics built under race track using conventional CMOS Domains moved around track using nano second long pulses of current - Data stored in the third dimension in tall columns of magnetic material - Domains race around track for reading and writing - 10 to 100 times the storage capacity of conventional solid state memory - Could displace flash memory and hard disk drives for many applications

13 Magnetic Shift Register Concept Domain wall positions in race-track pinned by notches in walls of magnetic columns

14 Magnetic Shift Register Memory Nanosecond long current pulses push domain walls around race-track due to a spin torque from transfer of spin angular momentum Shift current pushes domains through stack Writing device Write Element Read Element Reading device

15 Magnetic Shift Register Memory Magnetic race-tracks can be connected in series Many other configurations possible

16 DRAM trench IBM Research Top Mid Bottom DRAM trench: ~10 μm tall 0.09 μm wide

17 Fabrication of race-track - prototype race-tracks under development - trench with notches demonstrated - plating with magnetic material a major challenge

18 Writing bits into race-track Reading bits in race-track Current moves domain wall in nearby wire Magnetic fringing field from moving domain wall writes bits Domains in racetrack form part of magnetic tunneling junction

19 Writing with Domain Wall Fringing Fields: Simulation 10 nm 10 nm (a) B x (mt) 100 nm nm (b) (c) (d) Scheme of the experiment: (a) Small ferromagnetic (Py) element (10 nm thick ellipse) is placed 10 nm above Co stripe (500 nm long, 100 nm wide, and 10 nm thick); (b) B x component of the domain wall fringing field as a function of x 10 nm above the stripe; (c,d) Domain color maps (top view) illustrate that sweeping a domain wall across the stripe results in the reversal of the Py magnetization: (b) before, and (c) after the reversal. The magnetization direction is color coded to the color wheel.

20 Micromagnetic Simulations of a Racetrack 4 domain walls, located next to one another bipolar pulses, 2.8 ns long I (ma) ns pulse 10.8 μm x 210 nm x 10nm 20mA = 10 9 A/cm t (ns) +I Spacing between notches ~ 1.1 μm Wire is smooth between the notches (no roughness) Notches are made from SEM images of real wires (slightly different from one another)

21 Current induced domain wall motion in magnetic nanowires Domain Wall (DW) race track memory pin and depin DWs controllably Current driven DW motion Current Critical current to depin DW - vs pinning strength - vs DW structure

22 Fabrication of magnetic nano-wires Focused Ion Beam (FIB) Electron beam lithography 1) Sputter deposition (shadow mask) 2) Coarse FIB Py Py SiOx 3) Fine FIB Py 4 μm Py SiOx Pt Au SiOx Py

23 E-beam patterned magnetic nanowire to explore motion of DWs gold contacts Pointed end to prevent DW nucleation Notches for DW trapping Pad for DW injection Wedged injector L-shaped structure 4 μm 4 μm

24 Probing DW structure using MFM Topography AFM Vortex Domain Wall Micromagnetic simulations 2.0µm 300nm Magnetic Tranverse Domain Wall MFM 2.0µm Domain Wall AFM MFM M profile div(m)

25 Structure of domain wall at a single notch Structure of DWs trapped at a notch Vortex wall Metastable states: Seven different structures at a given notch 300nm Transverse wall AFM MFM M profile div(m) Micromagnetic simulations MFM imaging micromag simulations (divm) The energetics of a domain wall trapped at a notch is complex Many metastable states are observed depending upon the history Magnetic states must be well controlled to ensure reproducibility

26 Energy of metastable DW structures transverse transverse transverse transverse Energy (erg) DW Position (μm) vortex counter-clockwise vortex counter-clockwise vortex clockwise vortex clockwise

27 Patterning permalloy nanowires -Ni 81 Fe 19 (thickness 10 nm) blank film deposited on Si substrates - E-beam lithography - Gold/Rhodium contact pads Notches for DW trapping (pinning center) Pointed end to assist DW annihilation 4um t: 10-40nm w: nm Au pads

28 Injecting DW into nanowire 1. Saturate magnetization of nanowire M H sat

29 Injecting DW into nanowire 2. Apply local magnetic field - Local field created by passing current through contact wire I M H I Total field Local field (H I ) Position (um)

30 Injecting DW into nanowire 2. Apply local and global magnetic field - If the total field is larger than the nucleation field reversed domain created below the contact I H I M H bias Required nucleation field (~100 to 500 Oe) DW formed! Position (um)

31 Injecting DW into nanowire 3. Apply global magnetic field - global field (H bias ) applied by Helmholtz coil M H bias reversed domain expands and DW propagates under H bias

32 Injecting DW into nanowire 3. Apply global magnetic field M if H bias is moderately small (i.e. smaller than the pinning field of the notch) DW pinned at the notch Controlled nucleation and pinning of DW

33 Probing the existence of DW in the nanowire - Anisotropic Magneto-Resistance (AMR) R High resistance ΔR Low resistance - Resistance difference is proportional to the volume of transverse magnetizatoin

34 Successive resistance measurement of DW states H sat I H bias H bias ΔR (Ohms) Resistance (Ohms) Number Experiment - meta-stable states at the notch Counts ΔR (Ohms)

35 Meta-stable states at the notch H bias t=10nm, w=200nm, notch depth=0.4w Vortex wall Transverse wall Counts (a. u.) DW at notch ~ ~ R(DW)-R(Sat) calculated from simulation Can create different DW states DW in wire R(DW)-R(sat)

36 Depinning DW from a notch with magnetic field M Notch pinning potential DW particle in a well

37 Depinning from a notch with magnetic field 1.0 H= H (Oe) Depinning Probability

38 Depinning from a notch with magnetic field 1.0 H= H (Oe) Depinning Probability

39 Depinning from a notch with magnetic field 1.0 H= H (Oe) Depinning Probability

40 Depinning from a notch with magnetic field 1.0 H= H (Oe) Depinning Probability

41 Depinning from a notch with magnetic field 1.0 H= H (Oe) Hdepin=45 Oe Depinning Probability

42 Depinning from a notch with magnetic field 1.0 Vortex wall Depinning Probability 45 Oe H (Oe)

43 Depinning from a notch with magnetic field 1.0 Vortex wall Transverse wall Depinning Probability 45 Oe 62 Oe H (Oe) Same notch different pinning potential for different wall structures

44 Depinning DW from a notch with current R Trap DW at the notch (Apply magnetic field) Pass voltage pulse Study the presence of DW by AMR

45 Depinning DW from a notch with current H Trap DW at the notch (Apply magnetic field) Pass voltage pulse Study the presence of DW by AMR

46 Depinning DW from a notch with current H Voltage pulse Trap DW at the notch (Apply magnetic field) Pass voltage pulse (~nanoseconds) Study the presence of DW by AMR

47 Depinning DW from a notch with current R Trap DW at the notch (Apply magnetic field) Pass voltage pulse Study the presence of DW by AMR

48 Depinning DW from a notch with current 8 S2245-w3c2 J83, , 4ns, L2 t pulse =4ns - Fit to an analytical model (LLG equation based) J C (A/cm 2 ) x Vortex wall Field (Oe)

49 Depinning DW from a notch with current 8 S2245-w3c2 J83, , 4ns, L2 t pulse =4ns 5.2 x 10 8 (A/cm 2 ) J C (A/cm 2 ) x Vortex wall Transverse wall Field (Oe)

50 Joule heating Time-resolved resistance measurement Vout Vin oscillo R R(t)/R(0) S2245-w1c5 J41 short GSGB Time (ns) 3.54V 3.15V 2.51V 1.99V 1.58V 1.00V 0.50V R(V)/R(0) S2245-w3 300nm/w4c3 300nm SiOx sub Si 10ns Voltage (V)

51 Depinning DW from a notch with current Vortex wall Transverse wall Without heating considered (Resistance at static level, low R high Jc) 5.2 x 10 8 (A/cm 2 ) J C (A/cm 2 ) x S2245-w3c2 J83, , 4ns, L2 t pulse =4ns Field (Oe)

52 Depinning DW from a notch with current Vortex wall Transverse wall With heating considered (Resistance during pulse, high R low Jc) 3.1 x 10 8 (A/cm 2 ) J C (A/cm 2 ) x S2245-w3c2 J83, , 4ns, L2 t pulse =4ns Field (Oe)

53 Depinning DW from a notch with current Vortex wall Transverse wall J C (A/cm 2 ) x Before heating (t<1ns) S2245-w3c2 J83, , 4ns, L2 5.2 x 10 8 J C (A/cm 2 ) x During heating S2245-w3c2 J83, , 4ns, L2 3.1 x Field (Oe) J C for DW depinning before heating starts Field (Oe) J C for DW depinning during or after heating Same J C for the two states at low field

54 Critical current vs pinning potential Theory 1D model: Luc Thomas, Yaroslaw Bazaliy G. Tatara et. al., Phys. Rev. Lett., 92, (2004) J C is independent of pinning potential depth as long as V PIN < 10 7 erg/cm 3 Atomic point contact type constraint Depinning Field (Oe) wire width (nm) Notch depth/wire width Potential (erg/cm3) x10 4 J C (A/cm 2 ) x nm 7 200nm 300nm pulse duration 4ns Pinning Field (Oe)

55 Depinning of a domain wall : theoretical description One dimensional model for DW motion: DW is described by two dynamic variables -position - momentum (tilt away from equilibrium direction) Critical current vs magnetic field Analytical: current-like regime z Ψ y u c (m/s) Analytical: Field-like regime Calculated α= q x H (Oe) Discovery of two regimes for current-driven de-pinning - field-like behavior: critical current depends upon field and pinning strength - current-driven depinning: critical current essentially independent on field and pinning strength Excellent agreement with experiments Spintronics Domain wall motion

56 Current induced change in DW structure Seven consecutive identical current pulses in the MFM -Pulse: I=7mA J ~ A/cm 2 / 5 to 10 ms e - Domain Wall structure is modified by current pulses - both transverse and vortex walls are observed

57 Current induced Domain wall motion - summary Oscillatory depinning DW motion driven by current pulses Complex dependence upon pulse length and amplitude Oscillatory dependence upon pulse length Period between 3.5 and 7 ns Model Oscillatory motion of the DW within a pinning potential Oscillatory depinning reproduced - both with 1D model and micromagnetics for both vortex and transverse walls Depinning occurs when the pulse length in in sync with the DW oscillations DW inertia : motion after the end of the pulse Notch potential dependence Field driven DW depinning depends on DW states Current driven DW depinning weak dependence on DW states weak dependence on pinning potential important for applications

58 Spintronics new sensor and memory devices based on manipulating spin polarized current spin-valve and MTJ devices Hard disk drives >400 fold increase in storage capacity Magnetic Random Access Memory promises a solid state memory which is non-volatile, high performance and cheap Magnetic race-track memory promises a novel data storage device with the capacity and cost of a hard disk drive but with the performance and reliability of solid state memory

59 Hype cycle for emerging technologies! Source: Gerstner, August 2005

60 SpinAps Tea m Masamitsu Hayashi, Luc Thomas, Rai Moriya, Charles Rettner