Tunnel Magnetoresistance Effect and Its Applications S. Yuasa, R. - PowerPoint PPT Presentation

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 - e Charge Spin S Electron Electronics Magnetics ・ diode ・ magnetic recording Magneto- ・ transistor ・ permanent magnet resistance Since 1988 LSI Spintronics Hard Disk Drive (HDD) Both charge and spin of 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 Lord Kelvin MR = 1 ~ 2 % 1985 GMR effect A. Fert, P. Grünberg MR = 5 ~ 15 % ( Nobel Prize 2007 ) 1990 TMR effect T. Miyazaki, J. Moodera 1995 MR = 20 ~ 70 % 2000 2005 2010

Tunnel magnetoresistance (TMR) effect FM electrode Tunnel barrier FM electrode Parallel (P) state Antiparallel (AP) state Tunnel Resistance R P : low Tunnel Resistance R AP : high Magnetic tunnel junction (MTJ) MR ratio ≡ ( R AP – R P ) / R P (performance index)

Room-temperature TMR in 1995 T. Miyazaki J. S. Moodera (Tohoku Univ.) (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 N S N S S N S N N S N S S N S N N S N S Head Medium S N S N Read head Rotation Recording density （ Gbit / inch 2 ） GMR TMR AMR 1000 Next-generation read GMR ヘッドの出現 head is indespensable ■ ■ 100 ■ ■ ■ ■ ■ ■ ■ for > 200 Gbit / inch 2 . ■ ■ ■ ■ ■ ■ 10 ■ ■ ■ ■ ■ ■ ■ ■ TMR head ■ ■ ■ ■ ■ ■ 1 ■ ■ ■ ■ GMR head 0.1 ■ ■ 0.01 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 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 p n + n + CMOS Freescale’s 4 Mbit-MRAM based on Al-O MTJs Cross-section structure 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 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 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 FM 1 FM 2 FM 2 barrier barrier e e Energy Energy Energy Energy E F E F E F E F D 2 ↓ D 2 ↑ D 2 ↓ D 1 ↑ D 2 ↑ D 1 ↓ D 1 ↑ D 1 ↓ Parallel (P) state Antiparallel (AP) state Tunnel resistnce: R P Tunnel resistnce: R AP ) ( − D ( E ) D ( E ) α ↑ α ↓ = F F α = 1, 2. MR ≡ ( R AP – R P ) / R P = 2 P 1 P 2 / (1 – P 1 P 2 ), , P α + ( ) ( ) ( ) D E D E α ↑ α ↓ F F Spin polarization P

Tunneling process in MTJs Amorphous Al-O barrier Crystalline MgO(001) barrier No symmetry 4-fold symmetry Δ 2’ Δ 5 Δ 1 Δ 2’ Δ 5 Δ 1 Fe(001) Fe(001) Δ 1 Al-O MgO(001) Fe(001) Δ 1 Only the Bloch states with Δ 1 Various Bloch states tunnel incoherently. symmetry tunnel dominantly. MR ratio < 100% at RT

Fully spin-polarized Δ 1 band in bcc Fe(001) 1.5 Δ 1 ↓ majority spin 1.0 minority spin E - E F ( eV ) Δ 1 ↑ 0.5 E F 0.0 -0.5 Γ H (001) direction Fully spin-polarized Δ 1 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 Δ 1 band. ( e.g . bcc Fe 1- x Co x , 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 t MgO = 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) tunnel barrier 160 140 120 100 Amorphous Al-O AIST [1] 80 tunnel barrier 60 Nancy 40 CNRS-CSIC 20 0 2005 1995 2000 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) Amorphous 260 CoFeB 240 Anelva - AIST [4] MgO(001) 220 IBM [3] MR ratio (%) at RT FeCo(001) 200 AIST [2] Crystal MgO(001) Textured MTJ 180 tunnel barrier 160 140 MgO(001) 120 Fe(001) 100 Amorphous Al-O AIST [1] Fully epitaxial 80 MTJ tunnel barrier 60 Nancy 40 CNRS-CSIC 20 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 or Free layer 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 Tunnel barrier 4-fold 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 bcc CoFeB(001) above 250 ºC Amorphous CoFeB Crystal- Textured MgO(001) Textured MgO(001) lization Amorphous CoFeB Amorphous CoFeB bcc CoFeB(001) As-grown MTJ Crystallization of CoFeB MgO(001) layer acts as a template to crystallize amorphous CoFeB. “Solid Phase Epitaxy” Because the Δ 1 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 Si wafer (8 or 12 inch) Standard sputtering machine in HDD industry 100 wafers a day !

Industrial applications MR effects 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 devices MgO-TMR head 2010 Spin-torque Microwave, etc. MRAM

Technologies for HDD read head Write head Recording medium N S N S S N S N N S N S S N S N N S N S Head Medium S N S N Read head Rotation Recording density （ Gbit / inch 2 ） GMR TMR AMR 1000 Next-generation read GMR ヘッドの出現 head is indespensable ■ ■ 100 ■ ■ ■ ■ ■ ■ ■ for > 200 Gbit / inch 2 . ■ ■ ■ ■ ■ ■ 10 ■ ■ ■ ■ ■ ■ ■ ■ TMR head ■ ■ ■ ■ ■ ■ 1 ■ ■ ■ ■ GMR head 0.1 ■ ■ 0.01 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 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 . Magnetic shield 20 nm (bottom lead) ◆ Density > 250 Gbit / inch 2 achieved. TEM image ◆ Applicable up to 1 Tbit / inch 2 .