Tunnel Magnetoresistance Effect and Its Applications

S. Yuasa, R. Matsumoto, A. Fukushima, H. Kubota, K. Yakushiji, T. Nakamura, Y. Suzuki and K. Ando Collaborators

Osaka University (High-frequency experiment)

Canon Anelva Corp. (R & D of manufacturing technology)

Toshiba Corp. (R & D of Spin-MRAM)

Funding agencies Outline

(1) Introduction

(2) Epitaxial MTJs with a crystalline MgO(001) barrier

(3) CoFeB / MgO / CoFeB MTJs for device applications Spintronics N

Charge -e Spin S Electron Electronics Magnetics ・diode ・ Magneto- magnetic recording ・transistor ・permanent magnet resistance

Since 1988 LSI Spintronics Hard Disk Drive Both charge and spin of (HDD) the electron is utilized for novel functionalities. What is “magnetoresistance” ? A change in resistance by an application of H. Magneto-Resistance ; MR

Resistance (R)

Magnetoresistance ratio (MR ratio)

Magnetic field H required to induce MR change

0 Magnetic field (H)

MR ratio at RT & a low H (~1 mT) is important for practical applications. Magnetoresistance MR ratio (RT & low H) Year AMR effect 1857 MR = 1 ~ 2 % Lord Kelvin

1985 GMR effect A. Fert, P. Grünberg MR = 5 ~ 15 % (Nobel Prize 2007) 1990

TMR effect 1995 MR = 20 ~ 70 % T. Miyazaki, J. Moodera

2000

2005

2010 Tunnel magnetoresistance (TMR) effect

FM electrode

Tunnel barrier

FM electrode

Parallel (P) state Antiparallel (AP) state

Tunnel Resistance RP : low Tunnel Resistance RAP : high

Magnetic tunnel junction (MTJ)

MR ratio ≡ (RAP – RP)/RP (performance index) Room-temperature TMR in 1995

T. Miyazaki (Tohoku Univ.) J. S. Moodera (MIT) Ferromag. electrode Amorphous Al-O MR ratios of 20 – 70% at RT Ferromag. electrode

Al-O – based MTJ Technologies for HDD read head

Write head

Recording medium

S S N N N N S N S S Head Medium S N Rotation Read head ） 2 AMR GMR TMR 1000 GMRヘッドの出現 Next-generation read ■ ■ 100 ■ head is indespensable ■ ■ ■ 2 Gbit / inch ■■ for > 200 Gbit / inch . （ ■■ 10 ■■ ■ ■ TMR head ■ 1 ■ ■ ■ GMR head 0.1 ■

0.01 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 Recording density Recording density Year Magnetoresistive Random Access Memory (MRAM)

MTJ “0” Bit Line Ward Line “1”

Non-volatile memory Magnetoresistive Random Access Memory (MRAM)

Bit Line MTJ

Write Line Word Line

n+ p n+ CMOS Freescale’s 4 Mbit-MRAM Cross-section structure based on Al-O MTJs Volume production since 2006.

Non-volatile, high speed, infinite write endurance, etc. High-density MRAM is difficult to develop.

MR ratios > 150% at RT are required for developing Gbit-MRAM. MR effects Device applications MR ratio (RT & low H) Year AMR effect HDD head 1857 MR = 1 ~ 2 % Inductive head 1985 GMR effect MR = 5 ~ 15 % 1990

MR head TMR effect 1995 MR = 20 ~ 70 %

GMR head 2000 Memory

TMR head 2005 MRAM

Much higher MR ratios were required 2010 for next-generation devices. Outline

(1) Introduction

(2) Epitaxial MTJs with a crystalline MgO(001) barrier

(3) CoFeB / MgO / CoFeB MTJs for device applications Theoretical prediction of giant TMR effect in Fe/MgO/Fe

Fe(001)

MgO(001) Fe(001)

Fully epitaxial MTJ

< First-principle calculations > ・Butler et al., Phys. Rev. B 63, 056614 (2001). ・Mathon & Umerski, Phys. Rev. B 63, 220403 (2001).

MR ratio > 1000% Spin polarization P

Tunnel Tunnel FM 1 barrier FM 2 FM 1 barrier FM 2

e e

Energy Energy Energy Energy

E E F EF F EF

D D D D1↑ D1↓ 2↑ 2↓ D1↑ D1↓ D2↑ 2↓ Parallel (P) state Antiparallel (AP) state

Tunnel resistnce: RP Tunnel resistnce: RAP α α

( Dα (E ) − Dα (E ) ) = ↑ F ↓ F = 1, 2. MR ≡ (RAP – RP) / RP = 2P1P2 / (1 – P1P2), P , (D ↑(EF ) + Dα↓(EF ) ) α Spin polarization P Tunneling process in MTJs

Amorphous Al-O barrier Crystalline MgO(001) barrier No symmetry 4-fold symmetry Δ 2’ Δ5 Δ2’ Δ5 Fe(001) Fe(001) Δ1 Δ1

Al-O MgO(001) Δ1

Fe(001) Δ1

Various Bloch states Only the Bloch states with Δ1 tunnel incoherently. symmetry tunnel dominantly. MR ratio < 100% at RT Fully spin-polarized Δ1 band in bcc Fe(001) 1.5 majority spin Δ1↓ 1.0 minority spin Δ ( eV ) 0.5 1↑ F E -

E 0.0 EF

-0.5 Γ (001) direction H

Fully spin-polarized Δ1 band ⇒ Giant MR ratio is theoretically expected. Not only bcc Fe but also many other bcc alloys based

on Fe or Co have fully spin-polarized Δ1 band. (e.g. bcc Fe1-xCox , Heusler alloys) Fully epitaxial Fe/MgO/Fe MTJ grown by MBE

Fe(001) (Pinned layer)

MgO(001)

Fe(001) (Free layer)

2 nm TEM image

S. Yuasa et al., Nature Materials 3, 868 (2004). Magnetoresistance of epitaxial Fe/MgO/Fe MTJ

300

tMgO = 2.3 nm

T = 20 K 200 MR = 247%

T = 293 K MR = 180%

MR ratio ( % ) 100

0 -200 -100 0 100 200 H ( Oe ) MTJs with a single-crystal MgO(001) barrier

S. Yuasa et al., Nature Materials 3, 868 (2004). Magnetoresistance of textured MgO-based MTJ

MTJs with a (001)-oriented poly-crystal (textured) MgO barrier

S. S. P. Parkin et al., Nature Materials 3, 862 (2004). Up to 600% at RT 260 240 “Giant TMR effect” 220 IBM [3] 200 AIST [2] 180 Crystal MgO(001) 160 tunnel barrier 140 120 100 Amorphous Al-O AIST [1] 80

MR ratio (%) at RT tunnel barrier 60 Nancy 40 20 CNRS-CSIC 0 1995 2000 2005 Year

[1] Yuasa, Jpn. J. Appl. Phys. 43, L558 (2004). [2] Parkin, Nature Mater. 3, 862 (2004). [3] Yuasa, Nature Mater. 3, 868 (2004). Outline

(1) Introduction

(2) Epitaxial MTJs with a crystalline MgO(001) barrier

(3) CoFeB / MgO / CoFeB MTJs for device applications MgO(001) 260 Amorphous CoFeB 240 Anelva - AIST [4] 220 IBM [3] MgO(001) 200 FeCo(001) Crystal MgO(001) AIST [2] 180 Textured MTJ tunnel barrier 160 140 MgO(001) 120 Fe(001) 100 Amorphous Al-O AIST [1] Fully epitaxial 80 MTJ MR ratio (%) at RT tunnel barrier 60 Nancy 40 20 CNRS-CSIC 0 1995 2000 2005 Year

[1] Yuasa, Jpn. J. Appl. Phys. 43, L558 (2004). [2] Parkin, Nature Mater. 3, 862 (2004). [3] Yuasa, Nature Mater. 3, 868 (2004). [4] Djayaprawira, SY, APL 86, 092502 (2005). MTJ structure for practical applications

For MRAM & HDD read head

Free layer or Tunnel barrier Pinned layer Ru This structure is FM (Co-Fe) based on fcc (111). AF layer (Pt-Mn or Ir-Mn) for exchange biasing

MgO(001) cannot be grown on fcc (111). 4-fold symmetry 3-fold symmetry MTJ structure in as-grown state Collaboration with Canon-Anelva

Amorphous CoFeB

Textured MgO(001)

Amorphous CoFeB

TEM image Djayaprawira, SY, Appl. Phys. Lett. 86, 092502 (2005).

◆Ideal for device applications This structure can be grown on any kind of underlayers by sputtering deposition at RT + post - annealing. CoFeB / MgO / CoFeB - MTJ with practical structure

Crystalline symmetry Free layer 4-fold Tunnel barrier Pinned layer Amorphous SyF structure

AF layer for 3-fold exchange- biasing

Standard bottom structure for MRAM and HDD head Crystallization of CoFeB by post - annealing

S. Yuasa et al., Appl. Phys. Lett. 87, 242503 (2005).

Annealing Amorphous CoFeB above 250 ºC Amorphousbcc CoFeB(001) CoFeB Crystal- Textured MgO(001) Textured MgO(001) lization Amorphous CoFeB Amorphousbcc CoFeB(001) CoFeB

As-grown MTJ Crystallization of CoFeB

MgO(001) layer acts as a template to crystallize amorphous CoFeB. “Solid Phase Epitaxy”

Because the Δ1 band in bcc CoFeB(001) is fully spin-polarized, CoFeB/MgO/CoFeB MTJs show the giant TMR effect. Sputtering deposition Canon-ANELVA C-7100 system

φ 8 inch

Thermally oxidized Standard sputtering machine Si wafer (8 or 12 inch) in HDD industry 100 wafers a day ! MR effects Industrial applications MR ratio (RT & low H) Year HDD head AMR effect 1857 MR = 1 ~ 2 % Inductive head 1985 GMR effect MR = 5 ~ 15 % 1990

MR head TMR effect 1995 MR = 20 ~ 70 %

GMR head 2000 Memory

Giant TMR effect TMR head 2005 MR = 200 ~ 600 % MRAM Novel MgO-TMR head devices 2010 Spin-torque MRAM Microwave, etc. Technologies for HDD read head

Write head

Recording medium

0.01 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 Recording density Recording density Year MgO-TMR head for ultrahigh-density HDD

Wafer of MgO-TMR head MgO-TMR head Cut

Inte- gration

Inte- gration Magnetic shield (top lead)

Permanent Permanent magnet magnet

MgO–MTJ ◆Commercialized in 2007. 20 nm Magnetic shield ◆Density > 250 Gbit / inch2 achieved. (bottom lead) ◆Applicable up to 1Tbit/inch2. TEM image MR effects Industrial applications MR ratio (RT & low H) Year HDD head AMR effect 1857 MR = 1 ~ 2 % Inductive head 1985 GMR effect MR = 5 ~ 15 % 1990

MR head TMR effect 1995 MR = 20 ~ 70 %

GMR head 2000 Memory

Giant TMR effect TMR head 2005 MR = 200 ~ 600 % MRAM Novel MgO-TMR head devices 2010 Spin-torque MRAM Microwave, etc. Spin-torque MRAM （SpinRAM）

M. Hosomi et al.(Sony), Technical Digest of IEDM 2005, 19.1.

MTJ CoFeB

MgO

CoFeB

1 nm CoFe CMOS

6 2 Write current density, JC0 ～ 2x10 A/cm

5 2 JC0 of 5x10 A/cm is required for Gbit-scale SpinRAM. SpinRAM having perpendicular magnetization

T. Kishi (Toshiba), SY et al., IEDM (2008) 12.6.

Upper metal or

50 nm Upper electrode MgO(001) Storage layer MTJ MgO Referenc Bottom elctrode e layer 50nm A TEM image of 50 nm-sized MTJ Perpendicularly-magnetized electrodes

6 2 JC0 <10 A/cm achieved !

Perpendicularly magnetized MTJ A CMOS integrated MTJ array is a promising technology for Gbit-scale Spin-RAM. MR effects Industrial applications MR ratio (RT & low H) Year HDD head AMR effect 1857 MR = 1 ~ 2 % Inductive head 1985 GMR effect MR = 5 ~ 15 % 1990 :: commercialized commercialized

MR head :: perspectives perspectives TMR effect 1995 MR = 20 ~ 70 %

GMR head 2000 Memory

Giant TMR effect TMR head 2005 MR = 200 ~ 600 % MRAM Novel MgO-TMR head devices 2010 Spin-RAM Microwave, etc.