年月日マスタタイトルの書式設定 - SPring-8...XAS using TEY is an useful technique to investigate barrier interface quality in MTJs Experiment 1 CMS‐based MTJs マスタ

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09.4.9 1

-  Applications of half-metals to spintronics devices and utilizations of soft X-ray magnetic circular dichroism -

Graduate School of Engineering,Tohoku university

Ins:tute for Materials Research (IMR), Tohoku university Y. Sakuraba, T. Iwase, K. Takanashi

T. Hiratsuka, T. Kubota, M. Oogane, Y. Ando

第1回先端磁性材料研究会「次世代HDDヘッド,および,MRAM材料の進展と評価技術」　　　2009年3月17日　　

ハーフメタルのスピントロニクスデバイス応用と

軟X線MCD解析

桜庭裕弥，岩瀬拓, 高梨弘毅　　　東北大学金属材料研究所

平塚喬士，窪田崇秀, 大兼幹彦, 安藤康夫　東北大学工学研究科

児玉謙司, 中村哲也, 　　　高輝度光科学研究センター/SPring-8 K. Kodama, T. Nakamura JASRI/Spring‐8





09.4.9 2

Outline

● Introduction : Half-metallic materials 　Half-metallic Heusler alloys

● Experiments

Experiment 1 Co2MnSi-based magnetic tunnel junctions

Experiment 2 MTJs with L10/L21-ordered alloy hybrid electrodes

Experiment 3 Half-metallic Ferri-magnets : Mn2VAl

Experiment 4 Co2MnSi-based CPP-GMR devices

● Summary





09.4.9 3

Introduc-on Half‐metallic materials

Oxides ： CrO2, Fe3O4, La2/3Sr1/3MnO3 etc. Zinc-blend type compounds: CrAs, MnAs etc. Half-Heusler alloys: NiMnSn, PtMnSb etc. Full-Heulser alloys : Co2MnSi, Co2MnGe, etc.

Candidates of half-metallic materials

E

EF

P = 1

Half-metal

EF

P < 0.6

3d Ferromagnet

Spin-polarization

€

P =D↑(EF ) −D↓(EF )D↑(EF ) + D↓(EF )





09.4.9 4

Introduc-on Applica0ons of half‐metallic materials

● MTJs & CPP-GMR devices

● Spin-Transistor

€

TMR ratio =2P1P21− P1P2

×100 (%)

€

JC ∝αg(θ)

MS2t

Spin-torque induced magnetization switching

Spin-injection efficiency :

€

g(θ) = P1 /(1+ P1P2 cosθ)

Schmidt et al. PRB (2000)

FM/SC/FM junction

Sugahara et al. APL (2004)

Spin-MOSFET based on Half-metallic source and drain





09.4.9 5

Introduc-on

Co2MnSi (CMS) High Curie temp.: TC ~ 985 K

High degree of L21 chemical ordering is easily obtained

Half-metallic energy gap EG : 0.4 ~ 0.6 eV

Half‐metallic full‐Heusler alloy Co2MnSi

CMS is a promising candidate in the half-metal candidates.





09.4.9 6

Experiment 1 Fabrica0on of CMS‐based MTJs

UHV magnetron spuBering system (P < 1 x 10‐7Pa)

MTJ Al-O barrier : Plasma oxidation of Al layer Micro-fabrication : PL & Ar ion milling (element size : 5 x 5 - 100 x 100µm2) Annealing : 200 -500℃ with applying H MR, I-V curve : DC 4-probe method

Cr buffer layer Deposited@RT→Post-annealed@700℃

MgO(001) - subs

Cr (001) 40 nm

Co2MnSi(001)30 nm

Co75Fe25 5nm/Co2MnSi 10nm

IrMn 10 nm

Ta 5 nm

Al-O barrier

Co2MnSi bottom epitaxial electrode Composition :Target Co2.00Mn1.28Si1.30 → Film : Co2.00Mn1.00Si1.08 Deposited@RT → Post-annealed@Ta = 250 - 650℃ Crystal structure, the degree of ordering : XRD Magnetization : SQUID,VSM Surface image : AFM

Epitaxial relation : MgO(001)<100>/Cr(001)<110>/CMS(001)<110>

Lattice misfit < 5%





09.4.9 7

Experiment 1 CMS‐based MTJ

Stoichiometry

Highly L21-ordering Co2.00Mn1.00Si1.08 (ICP)

SL21 > 0.8 (XRD)

Extremely flat and sharp interfacial morphology

Magnetic tunnel junction (MTJs) with epitaxial CMS bottom electrode

Flat surface Ra < 0.2 nm

Conventional Al-O amorphous barrier

HR-TEM image

High-quality epitaxial CMS electrode





09.4.9 8

Experiment 1 CMS‐based MTJs

MgO(001) - subs

Cr (001) 40 nm

Co2MnSi(001)30 nm

Co75Fe25 5nm

IrMn 10 nm

Ta 5 nm

Al-O barrier

Co2MnSi : Mn has large affinity to oxygen compared to conventional 3d FM.

Magnetic impurities easily create between CMS/Al-O interface.

Optimization of interfacial chemical conditions is necessary.

1. Plasma oxidation time for Al-O : tox = 10 - 240 s

2. Insert thin Mg layer between CMS and Al-O.

Thickenss of Mg layer : tMg = 0 -1.3 nm

(Formation energy of oxide : Mg < Al, Co, Mn, Si)





09.4.9 9

XAS :Mn L-absorption edges

A1, B1, C1: large amound of Mn-O at interface

D1, E1: few Mn-O at interface

Additional multiplet structure indicates the presence of Mn-O impurities at interface.

Experiment 1 CMS‐based MTJs Investigation of interfacial Mn impurities by XAS using TEY (ALS beamline 6.3.1)





09.4.9 10


Mg thickness dependence of TMR ratio





09.4.9 11

Mg 1.0 nm0.8 nm

0.5 nm0 nm

Bias V = 1 mV

H (Oe)

203%@2K

93%@RT

Co2MnSi/Mg(t)/Al(1.3)-O(tox = 50 s)/CoFe

Temperature dependence of TMR ratio

CMS/Mg/Al-O/CoFe TMR = 203% (T = 2K)

0.97 < PCo2MnSi < 1.00 Nearly ideal half-metallic spin-polarization

Sakuraba et al. J. Mag. Soc. Jpn. (2007)

0.50 < PCoFe < 0.52

XAS using TEY is an useful technique to investigate barrier interface quality in MTJs






09.4.9 12

Co2MnSi based-MTJs

Co2MnSi/Al-O/Co2MnSi-MTJ TMR = 67%(RT), 570%(2K) Sakuraba et al. APL 88, 192508 (2006)

Co2MnSi/MgO/CoFe-MTJ TMR = 217%(RT), 753%(10K) Tsunegi et al. APL 93, 112506 (2008)

800

600

400

200

0

TMR

ratio

(%)

30025020015010050 Temperature (K)


CMS/MgO/CoFe

CMS/Al-O/CMS

CoFe/Al-O/CoFe

Co2MnSi/MgO/Co2MnSi-MTJ TMR = 182%(RT), 705%(4.2K) Ishikawai et al. JAP (2009)

CMS/Al-O/CoFe

Disappearance of half-metallicity at RT is the largest problem in CMS-based MTJs.

cf. CoFeB/MgO/CoFeB-MTJ TMR = 604%(RT), 1144%(4.2K)





09.4.9 13

Origin for disappearance of half‐metallcity at RT

Parallel Anti-parallel

Spin-flip at interface due to magnon excitation.

Co2MnSi/MgO

0.3 meV

1.3 -0.5

0.4

17 3.1

3.5

8

JCo = 93 meV JMn = 58 meV JMn = 158 meV

JCo = 19meV

Sakuma et al. J. Appl. Phys. 105, 07C910 (2009)

Weak exchange energy of Co at the interface.

Large magnon excitation by thermal energy is the most possible origin for disappearance of half-metallicity at RT in CMS-based MTJs.





09.4.9 14

Experiment 2 MTJs using L10/L21‐ordered alloy hybrid electrodes

MTJs with perpendicular magnetized electrodes are recent trend.

For developing Spin-RAM

・High thermal stability : KuV/kBT > 60 ・Small critical current density of spin-transfer torque switching : JC0 < 106 A/cm2

FePt/Fe/MgO/FePt – MTJ (MMM 2008, TOSHIBA)

→TMR ratio > 100% at R. T.





09.4.9 15

MgO（001）- sub.

Cr 40 nm

Pt 3 nm

CoPt 20 nm

MgO

CoPt

Ta

CMS 0.5 - 5 nm

CMS 0.5 - 5 nm

(001)-CoPt/CMS/MgO/CMS/CoPt fully-epitaxial MTJ can be fabricated. Lattice misfit between CMS(001) and CoPt(001) ~ 4%

High thermal stablity due to large magnetic anisotropy

Suppression of magnon excitation at CMS/barrier interface.

Large spin-polarization (TMR ratio) at RT






09.4.9 16

MgO（001）- sub.

Cr 40 nm

Pt 3 nm

CoPt 20 nm CMS 1 - 5 nm

35x10 3

30

25

20

15

10

5

0

Inte

nsity

(cps

)

80 70 60 50 40 30 20

2θχ (deg.)

CMS 5nm

CMS 4nm

CMS 2nm

CMS 3nm

CMS 1nm

CM

S (2

00)

CM

S (4

00)

CM

S (2

20)

CoP

t (11

0)

MgO

(200

)

CoP

t (20

0)

MgO

(220

)

CoP

t (22

0)

Structural analysis : XRD in-plane geometry (X-ray incident angle 2θ < 0.4deg.)

・B2-ordering even in 1nm-thick. ・fully-epitaxial growth

CMS: Deposited@RT →annealed@450℃






09.4.9 17

Surface flatness : AFM

0.5

0.4

0.3

0.2

0.1

0.0R

a (n

m)

54321CMS Thickness (nm)

4

3

2

1

0

P - V

(nm)

Surface roughness is enough small to deposit MgO barrier.






09.4.9 18

M-H curves : SQUID, VSM

1.0x10-3

0.5

0.0

-0.5

-1.0

M(e

mu/

cm2 )

-20 -10 0 10 20Field (kOe)

CMS 1 nm 2 nm 3 nm 4 nm 5 nm

H // out-of-plane

Magnetic behavior of CMS layer can not be identified because of… (1) too small MCMS compared with MCoPt

(2) the error in subtraction procedure of MgO-subs contribution

Cr 40 nm

MgO（001）- sub.

Pt 3 nm CoPt 20 nm

CMS 1 - 5 nm ?

Other experimental techniques to observe surface moment is needed.

XMCD






09.4.9 19

e- e-

e-

e-

λx : penetration length of X-ray λxcosθ : penetration depth of X-ray λe : escape depth of photo electron

~ 2-4 nm

XMCD : SPring-8 BL25SU

Co2MnSi Co2MnSi

CoPt

Elements specific (Co and Mn) M-H curves can be measured.






09.4.9 20

5

4

3

2

1

0 In

tens

ity (a

rb. u

nits

) 720 700 680 660 640

Photon energy (eV)

d CMS = 5 nm

4 nm

3 nm

2 nm

1 nm

Mn L2,3 absorption edges

-1.0 -0.5 0.0

0.5

1.0 Normalized intensity

Mn Co

Mn Co

-1.0 -0.5 0.0

0.5

1.0

Normalized intensity

1 0 -1 Magnetic field (T)

Mn Co

-1.0 -0.5

0.0

0.5

1.0

Normalized intensity

Element specific M-H (H//out-of-plane) XMCD spectra (H//out-of-plane)

・dCMS < 3nm : CMS has perpendicular magnetic anisotropy, but magnetic moment is very small.

dCMS = 1 nm

dCMS = 3 nm

dCMS = 5 nm

L3

L2

L3 -edge






09.4.9 21

What is the next genera0on reading head? Requirements : Large MR ratio for high S/N ratio, Small RA for high speed reading

Reported MR ratio in small RA region 120

100

80

60

40

20

0 M

R ra

tio (%

)

0.01 2 3 4 5 6 7 0.1 2 3 4 5 6 7

1 2 3 4 5 RA (Ω•µm2)

Ove

r 1Tb

it/in

ch2

CPP-GMR with NOL (Toshiba)

Co2MnSi/Cu (TDK) Co2MnGe/Cu (ALPS)

Co2FeAl/Cu (HGST)

CoFe/Cu (Toshiba)

MgO-MTJs Large MR ratio, but difficult to achieve small RA. CoFeB/MgO/CoFeB : RA = 0.4 Ω•µm2 ,MR = 50%

RA = 2.1 Ω•µm2 ,MR = 206% Nagamine et al. APL(2006)

Isogami et al. MSJ conf.(2008)

CPP-GMR Small RA, but difficult to achieve large MR ratio.

CoFe/Cu/CoFe : RA = 0.12 Ω•µm2 ,MR = 1.3% Yuasa et al. JAP(2002)

(ii)Using half-metallic full-Heusler alloy

(i)Using Nano-oxide layer(NOL) CoFe/NOL/CoFe : RA = 0.58 Ω•µm2 ,MR = 8.2%

Saito et al. Intermag(2004)

Fukuzawa et al. JPD(2007)

Co2MnGe/Cu/Co2MnGe : RA = 0.079 Ω•µm2 ,MR = 11.5%

Experiment 4 CMS‐based CPP‐GMR devices





09.4.9 22

CMS/Ag/CMS fully‐epitaxial CPP‐GMR device

◆ The highest MR ratio (28.8%@RT) is the best record in all of the reports to date.

cf. CoFe/Cu/CoFe : MR ~ 2.3%@RT

Experiment 4 CMS‐based fully‐epitaxial CPP‐GMR devices

MgO

Cr

Ag

Cr CMS Ag

CMS Ag/Au

20 nm 5 nm CMS

Ag

CMS

HR-TEM image

42

40

38

36

34

32

30

RA

[mΩ

•µm

2 ]

-200 -100 0 100 200Field [Oe]

30

20

10

0

MR

[%]MR ratio = 28.8% @RT

RA = 30.9 mΩ•µm2

ΔRA = 8.92 mΩ•µm2





09.4.9 23

Summary

● XAS based on TEY is an useful technique to observe interfacial chemical state between barrier and FM electrodes in MTJs.

● Element specific magnetization measurement based on XMCD is an helpful technique to fabricate new types of half-metals such as, perpendicular magnetized half-metals and ferri-magnetic half-metals.

Documents

年 月 日 マスタ タイトルの書式設定 - SPring-8...XAS using TEY is an useful technique to investigate barrier interface quality in MTJs Experiment 1 CMS‐based MTJs マスタ

年月日マスタタイトルの書式設定 - SPring-8...XAS using TEY is an useful technique to investigate barrier interface quality in MTJs Experiment 1 CMS‐based MTJs マスタ