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
Electronics Electron Magnetics
・diode ・magnetic recording
Magneto-
・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 NN
S NN S
NN S
Head Medium S
Rotation Read head
Recording density （Gbit / inch2）

AMR GMR TMR

1000
GMRヘッドの出現 Next-generation read
100 ■
■ ■
■
head is indespensable
■ ■
■■ for > 200 Gbit / inch2.
■
10 ■ ■
■
■

■
■ TMR head
■
1
■ ■

0.1 ■
GMR head

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

MTJ
Bit Line
“0”
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.

<Advantages>
Non-volatile, high speed, infinite write endurance, etc.
<Disadvantage>
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

EF EF EF EF

D1↑ D1↓ D2↑ D2↓ D1↑ D1↓ D2↑ D2↓

Parallel (P) state Antiparallel (AP) state

Tunnel resistnce: RP Tunnel resistnce: RAP

( Dα ↑ ( EF ) − Dα ↓ ( EF ) )
MR ≡ (RAP – RP) / RP = 2P1P2 / (1 – P1P2), Pα = , α = 1, 2.
+
( Dα ↑ ( EF ) Dα ↓ ( EF ) )
Spin polarization P
 Tunneling process in MTJs
Amorphous Al-O barrier Crystalline MgO(001) barrier
No symmetry 4-fold symmetry
Δ2’ Δ5 Δ2’ Δ5
Fe(001) Δ1 Fe(001) 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

Δ1↓ majority spin

1.0
minority spin

E - EF ( eV )
0.5
Δ1↑

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
MR ratio ( % ) 200 MR = 247%

T = 293 K
MR = 180%
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]
MR ratio (%) at RT 200
AIST [2]
180 Crystal MgO(001)
160 tunnel barrier
140
120
100
Amorphous Al-O AIST [1]
80 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)
MR ratio (%) at RT FeCo(001)
200
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 tunnel barrier MTJ
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 bcc CoFeB(001)
Amorphous CoFeB
Crystal-
Textured MgO(001) Textured MgO(001) lization

Amorphous CoFeB Amorphous

bcc 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

S
S NN
S NN S
NN S
Head Medium S
Rotation Read head
Recording density （Gbit / inch2）

AMR GMR TMR

1000
GMRヘッドの出現 Next-generation read
100 ■
■ ■
■
head is indespensable
■ ■
■■ for > 200 Gbit / inch2.
■
10 ■ ■
■
■

■
■ TMR head
■
1
■ ■

0.1 ■
GMR head

0.01
1990 1992 1994 1996 1998 2000 2002 2004 2006 2008
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

(bottom lead)
◆Density > 250 Gbit / inch2 achieved.
◆Applicable up to 1 Tbit / 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

Ru
1 nm CoFe
CMOS

Write current density, JC0 ～ 2 x 106 A/cm2

JC0 of 5 x 105 A/cm2 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

JC0 < 106 A/cm2 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
TMR effect
perspectives
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.