Cr酸化物系垂直交換バイアス薄膜の界面磁性と機能性

大阪大学大学院工学研究科 白土 優

共同研究者

大阪大学 中谷亮一

高輝度光科学研究センター(SPring-8） 中村哲也，木下豊彦，鈴木基寛

東北大学 鳴海康雄，野尻浩之

物質・材料研究機構 三俣千春

SPring-8利用促進協議会・先端磁性材料研究会（第6回） 「スピントロニクス材料におけるX線磁気観察の新展開」

平成25年3月11日、於 連合会館

Contents

研究背景，目的

• 磁気デバイスとスピンバルブ薄膜 • 交換バイアスと反強磁性材料 • 交換バイアスの新しい機能性と新規デバイス

a-Cr2O3(0001)薄膜による垂直交換バイアスとその等温反転 Y. Shiratsuchi et al., Physical Review Letters, vol.109, pp.077202(4pp) (2012).

Y. Shiratsuchi et al., Applied Physics Letters, vol. 100, pp.262413(4pp) (2012).

まとめ，今後の展望

Exchange bias and antiferromagnet

Exchange bias

Unidirectional magnetic anisotropy at FM/AFM interface

Exchange-biased magnetization of FM is fixed to the particular direction.

Conventional exchange bias by Mn3Ir alloy

• High JK, high TB, low critical thickness, high oxidative resistance …

• Rare-metal (both Mn and Ir)

• In-plane directed effect

• Static effect

Exchange bias for future spin-electronics

Exchange bias for the future spin-electronics devices

• Rare metal free

• Out-of-plane (perpendicularly) directed effect

Especially for the perpendicularly magnetized devices, both interfacial FM spins and interfacial

AFM spins have to be perpendicular to film.

• Isothermal switching; Make the exchange bias a dynamic effect.

2int aJJ AFMFM

SS

Exchange coupling energy

Exchange bias can be reversed by reversing the antiferromagnetic spin direction.

a-Cr2O3 as an antiferromagnet

Magnetoelectric effect P = aH, M = a’E

Crystal structure : corundum (a = 4.961 Å, c = 13.599 Å)

Néel temperature : ~307 K

Cr spin direction ： parallel to c axis ferromagnetic alignment in c plane

V. J. Folen et al., Phys. Rev. Lett. 6, 607 (1961). Crystal structure and spin configuration of a-Cr2O3

Perpendicular exchange bias using a-Cr2O3 layer

80 100 120 140 160 180 200 220 240 260 280 300 3200.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Unid

ire

ction

al m

ag

netic

an

iso

tro

py e

nerg

y / e

rg・

cm

-2

Temperature / K

tCr2O3

= 50 nm

tCr2O3

= 70 nm

tCr2O3

= 120 nm

tCr2O3

= 150 nm

tCo

= 0.8 nm

Y. Shiratsuchi et al., APEX, 3 113001 (2010).

a-Cr2O3

buffer

substrate

Co(0.8 nm)

Pt(5.0 nm)

tCr2O3

Temperature dependence of unidirectional magnetic anisotropy energy

for the films with different a-Cr2O3 layer thickness

High perpendicular exchange bias about 0.3 erg/cm2

The abrupt onset of exchange bias.

Perpendicular exchange bias using a-Cr2O3 layer

50 100 150 200 250 3000.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

FM layer: Co1-x

Nix

tCo

1-xNi

x

= 1.0 nm

x = 0.1

x = 0.2

x = 0.5

Excha

ng

e a

nis

otr

op

y e

nerg

y / e

rg/c

m2

Temperature / K

Y. Shiratsuchi et al., IEEE Trans. Magn., 48, 2885 (2012).

a-Cr2O3(50 nm)

buffer

substrate

Co1-xNix(1.0 nm)

Pt(5.0 nm)

Temperature dependence of exchange magnetic anisotropy energy for

the Pt/Co1-xNix/a-Cr2O3/Pt thin films with different Ni composition.

Exchange anisotropy energy hardly changes by the Ni composition

when the Ni composition x < 0.5.

Structural characterizations

(c) FFT from Grain 1 (d) FFT from Grain 2

(a) RHEED of a-Cr2O3 layer (b) HRTEM

(0006)

(0006)

(0006)

(00012)

(0006)

(0006)

Well-ordered a-Cr2O3 layer

Sharp interface

Oriented growth

Pt(111) Co(111)

a-Cr2O3(0001) Pt(111)

Interfacial Cr spins film

Y. Shiratsuchi et al., PRL 109, 077202 (2012).

Spin orientation after field cooling

570 580 590 600 610 760 780 800 820 840-5

-4

-3

-2

-1

0

1

2

3

4

5

6

7

8

9

10

Abso

rptio

n /

a.u

.

Photon energy / eV

-4

-2

0

2

4

6

8

10

12

14

MC

D /

a.u

.

x20

XAS : R : L

XMCD

Cr Co

TEY mode

Temperature: 180 K

Magnetic field, 1.9 T Film

Film normal k

Fiel

d

10 M.

Clear MCD signal of Co and Cr

XAS of at Cr L2,3 edges are similar to that of bulk a-Cr2O3.

XAS and XMCD spectra at Co L2,3 edges and Cr L2,3 edges.

h-

h+

Y. Shiratsuchi et al., PRL 109, 077202 (2012).

a-Cr2O3

buffer

substrate

Co(0.8 nm)

Pt(5.0 nm)

Element-specific magnetization curve

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

6

7

Cr

@180K

MC

D / a

.u.

Magnetic Field / T

Co

-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

MC

D / a

.u.

XMCD hysteresis loops of Co and Cr.

Antiferromagnetic interfacial coupling Between Co and Cr.

Cr spin reverses together with Co spins.

Vertical shift seems to be visible in XMCD loop of Cr. However, verification is needed.

Y. Shiratsuchi et al., PRL 109, 077202 (2012).

Element-specific magnetization curves

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

0.020

cooling field = +0.4 T

cooling field = -0.4 T@235 K

XM

CD

/ a

.u.

Magnetic field / T

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

0.020 cooling field = +0.4 T

cooling field = -0.4 T@180 K

XM

CD

/ a

.u.

Magnetic field / T

Element-specific magnetization cures at (a) 235 K and (b) 180 K

after field-cooling of +0.4 T (red) or -0.4 T(blue).

The vertical shift of the Cr M-H curve is accompanied with the exchange bias.

Both vertical shift and exchange bias field are reversed by the cooling-field direction.

Y. Shiratsuchi et al., PRL 109, 077202 (2012).

Shape of XMCD spectrum

XMCD spectrum at Cr L2,3 edges at 180 K after field-cooling of +0.4 T (red) or -0.4 T(blue).

570 575 580 585 590 595 600 605-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

Cooling field -0.4T

@+1.0T

@-1.0T

XM

CD

/ a

.u.

Photon energy / eV

570 575 580 585 590 595 600 605-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

Cooling field +0.4 T

@+1.0T

@-1.0T

Photon energy / eV

XM

CD

/ a

.u.

A B

A B

(a) (b)

The relative peak intensity of A & B is changed by the applied field direction.

The shape of the XMCD spectrum is reversed by the cooling-field direction.

Y. Shiratsuchi et al., PRL 109, 077202 (2012).

Is the pinned spin model available in our case?

570 575 580 585 590 595 600 605-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

Cooling field +0.4 T

@+1.0T

@-1.0T

Photon energy / eV

XM

CD

/ a

.u.

A B

@180 K

XMCD spectrum for Cr L2,3 edges at 180 K

after field-cooling under +0.4 T.

Y. Shiratsuchi et al., PRL 109, 077202 (2012).

Isothermal switching of exchange bias

Detection of switching of exchange bias

-20-10 0 10 20 30 40 50 60 70 80 90100

0

5

10

15

20

25

30

35

Ma

gn

etic F

ield

/ T

Time / ms

-20-10 0 10 20 30 40 50 60 70 80 90100

0

5

10

15

20

25

30

35

Ma

gn

etic F

ield

/ T

Time / ms

HEX

HC

H

M

Pulse Magnetic Field Magnetization Curve

Sample: Pt(1.0 nm)/Co(0.5 nm)/a-Cr2O3(50 nm)/Pt(20 nm)/subs.

Switching of exchange bias, experiment@BL25SU, SPring-8

• Temperature: 77 K after field-cooling at -5 kOe from 320 K • Detection method: XMCD measurements under pulsed magnetic field • XMCD spectrum at remament state

570 575 580 585 590 595 770 780 790 800 810-1.20

-1.00

-0.80

-0.60

-0.40

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

Co

(b) XMCD spectrum

after field-cooling

after switching by positive field

MC

D /

a.u

.Photon energy / eV

Cr (x20)

Switching of exchange bias 1: positive magnetic field

(a) ESMCs of Co and (b) XMCD spectrum

before and after the switching of exchange bias by positive magnetic field.

XMCD signal starts to decrease at 80 kOe and becomes constant above 90 kOe.

XMCD spectrum reversed. Y. Shiratsuchi et al., APL 100, 262413 (2012).

Switching of exchange bias 2: negative magnetic field

570 575 580 585 590 595 770 780 790 800 810-1.20

-1.00

-0.80

-0.60

-0.40

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

Co

(b) XMCD spectrum

after field-cooling

after switching by positive field

after switching by negative field

MC

D /

a.u

.Photon energy / eV

Cr (x20)

(a) ESMCs of Co and (b) XMCD spectrum

before and after the switching of exchange bias by negative magnetic field.

Exchange bias in the negative direction

Decrease of XMCD signal at -80 kOe, and constant above -90 kOe.

XMCD spectrum was reversed． Y. Shiratsuchi et al., APL 100, 262413 (2012).

-150 -100 -50 0 50 100 150 200 250 300-1.0

-0.5

0.0

0.5

1.0

after positive pulsed field

after negative pulsed fieldR

em

an

en

t X

MC

D /

a.u

.

Maximum Magnetic Field Strength / kOe

Reversibility of switching of exchange bias

Dependence of remanent XMCD signal after applying the pulsed magnetic field.

Switching of exchange bias is reversible.

Switching field is same for the positive- and the negative-exchange bias.

Y. Shiratsuchi et al., APL 100, 262413 (2012).

Summary and future…

Single device accessibility

Low power consumption process

High speed switching

Working temperature etc

should be achieved.

For the Co/a-Cr2O3 perpendicular exchange-biased system,

Interfacial uncompensated (UC) Cr spins

Un-reversed UC Cr spins

were detected, and

Isothermal switching of PEB by pulsed high magnetic

field

was demonstrated.

Problem and future

Innovative spin-electronics devices