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放射光光電子分光によるLa0.6Sr0.4MnO3/SrTiO3�
スピントンネル接合界面の電子状態解析 �
PF研究会「磁性薄膜・多層膜を究める：キャラクタリゼーションから新奇材料の創製へ」2011年10月15（14-15）日＠KEK-PF小林ホール
SR RHEEDMonitoring
Pulsed LaserDeposition
Moving EdgeMask Pattern
Ceramic Targets
組頭 広志 KEK−PF（物構研＆構造物性研究センター）
JSTさきがけ
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Magnetic “dead layer” formation at the STO/LSMO interface
La0.6Sr0.4MnO3�
La0.6Sr0.4MnO3�
SrTiO3 or La0.6Sr0.4FeO3
Motivation
Half Metallic La0.6Sr0.4MnO3
Disappearance of [email protected].
F. Pailloux et al., PRB 66, 014417 (02).
Spin Tunneling Junctions
Tc ~ 370K
Small TMR ratio In actual TMR devices, the performance is far
worse than what is expected from these physical properties.
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Superlattice based on Transition Metal Oxides
Charge Transfer at the interface
? ABO3AʼBʼO3It is indispensable to reveal the electronic
structure
at the heterointerface
Interface EF
t2g
eg
eg
C.T.
"in-situ Photoemission"
-
☆Non-destructive☆Surface (Interface) Sensitive（5~30Å）☆Direct
Determination of Electronic States
High Directionality
High Brilliance
High Resolution
Tunable Photon Energy
Advantage of SR-PES
Chemical Shift DOS Band Structure
SR Analyzerhν e-High-throughput
High-resolution Space Energy Angle
100µm
yx
Elemental Selectivity
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Resonant PES of LSMO (x=0.4)
LSMO
LSFO
I = exp(-d/λ)
IPES
d
LSMO Interface Interface
Probing Electronic Structure at the Interface (Mn 3d PDOS)
hν = 644 eV
Resonant PES Approach
Intensity (arb. units)
660 660
655 655
650 650
645 645
640 640Ph
oton
Ene
rgy
(eV
)
Inte
nsity
(arb
. uni
ts)
Binding Energy (eV)10 515 EF
La0.6Sr0.4MnO3 thin filmT = R. T.
On
Off
-
RHEED Pattern & AFM Images
1 x 1 µm
LSFO x=0.4
Nb-STO substrate
LSMOx=0.55 LSMO x=0.4 (20 ML)
1 ML 2 MLA B C D
A B C D
-
Inte
nsity
(arb
. uni
ts)
Binding Energy (eV)
A B C D
T = R.T.
Mn 2p-3d Resonancehν = 640 eV
EF24 135 -1
STO substrate
LSMOx=0.55
LSMOx=0.4
LSFO x=0.4
1ML2ML
(20ML)
A B C D
LSMO
LSFO Charge Transfer
Spectral Evidence of Charge Transfer at LSMO/LSFO Interface
Mn 2p-3d Resonant PES
Resonant PES of LSMO at Interface
t2geg
K. Horiba, H.K. et al., Phys. Rev. B 71, 155420 (‘05)
H. Kumigashira et al., Appl. Phys. Lett. 84, 5353 (2004).
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LSMO (20ML)
LSFO
LSMO (3ML)
Inte
nsity
(arb
. uni
ts)
730725720715710705700
Photon Energy (eV)
Fe 2p XAS
LSMO/LSFO(1ML)/LSMO
LSMO/LSFO(2ML)/LSMO
Fe-2p XAS spectra
XAS Spectra of Interfacial LSFO Layer
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Comparison of Fe-2p XAS spectra
between Interfacial LSFO layer
and LSFO films
Inte
nsity
(arb
. uni
ts)
730 725 720 715 710 705 Photon Energy (eV)
La1-xSrxFeO3 film
x = 0.67 x = 0.4 x = 0.2 x = 0
Fe 2 p the spin-orbit splitting of the Fe 2 p core hole
Evidence of Charge Transfer from LSMO to LSFO
Fe-2p XAS spectra In
tens
ity (a
rb. u
nits)
725720715710705700
Photon Energy (eV)
Fe 2p XAS
LSMO /LSFO (1ML) /LSMO
LSFO film (x = 0.2)
LSFO film (x = 0.4)
LSMO /LSFO (2ML) /LSMO
H. Wadati, H.K. et al., Phys. Rev. B 71, 035108 (2005).
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LSMO
LSFO
LSMO
Interface in LSMO/LSFO superlattices
Phase diagram of LSMO
A. Urushibara et al., Phys. Rev. B 51, 14103 (1995).
Charge Transfer
AFM or PM dead-layer formation at Interface
Charge-Transfer at Interface
AFM
(La0.6Sr0.4)BO3
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Robust Ti4+ states
Ti3+�
Ti4+�
K. Morikawa et al., Phys. Rev. B 54, 5446 (1996).
C.T.
Mn3+→Mn4+
Ti4+→Ti3+TiO2TiO2SrO
SrOMnO20.6 0.4
MnO2La Sr O0.6 0.4
La Sr O
Charge Transfer
Ti 2p Core Level Spectra at Interfaces
H. Kumigashira et al., Appl. Phys. Lett. 88, 192504 (‘06).
1.5 eV
TiO�2�
TiO�2�SrO�
SrO�
MnO�2�La Sr O�0.6� 0.4�MnO�2�
La Sr O�0.6� 0.4�
STO
LSMO
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Mn3+→Mn4+
Ti4+ TiO�2�TiO�2�SrO�
SrO�MnO�2�
La Sr O�0.6� 0.4�MnO�2�
La Sr O�0.6� 0.4�
STO/LSMO
Difference of 3d levels among transition metals FeO�2�
FeO�2�
MnO�2�La Sr O�0.6� 0.4�MnO�2�
La Sr O�0.6� 0.4�
La Sr O�0.6� 0.4�
La Sr O�0.6� 0.4�
Mn3+→Mn4+
Fe4+→Fe3+
LSFO/LSMO
Origin of Charge Transfer
Mn 3d
t2g
t2g3eg1.6t2g3eg0.6t2g0
Bin
ding
Ene
rgy
Ti 3d Fe 3d
t2gt2g
egeg
e-
Electron-donor layer Charge redistribution at interface
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C. T. from AO Layer to BO2 Layer LSMO/STO interface
(Valence mismatch)LSFO/LSMO interface
(Charge transfer between TM ions)
La0.6Sr0.4O electron-donor layer
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A’B’O3/ABO3 interface: A’B’O3/BO2-ABO3 vs A’B’O3/AO-ABO3
AOB’O2
BO2AO
A’OB’O2
BO2
A’O
…
…
BO2AO
A’OB’O2
BO2AO
A’OB’O2
…
…
…
…
…
…
LSMO(x=0.4)/TiO2-Nb:STO LSMO(x=0.4)/SrO-Nb:STO
+ 0.6- 0.6 + 0.6
- 0.6
How does the band diagram modulates by changing the terminating
layer at the interface?
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Termination� Nb:STO (χi) � LSMO(x=0.4) (φm) �BO2� 4.08 eV � 4.87
eV �
AO � 3.86 eV � 4.73 eV �
0.14 ± 0.05 eV0.22 ± 0.05 eV
Work functions of termination controlled STO and LSMO
TiO2-terminatedNb: STOSrO-terminatedNb: STO
MnO2-terminatedLSMOLa0.6Sr0.4O-terminatedLSMO
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Heterojunctions� ΦB [exp.] � φm – χi�[exp.] �LSMO/TiO2-Nb:STO�
1.2 ± 0.1 eV � 0.65 eV �
LSMO/SrO/Nb:STO� 0.6 ± 0.1 eV� 1.01 eV �
Built-in potential of termination controlled LSMO
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LSMO(x=0.4)/TiO2-Nb:STO LSMO(x=0.4)/SrO/Nb:STO
…………
Direction reversing
The direction of interface dipole changes depending on the
terminating layer.
Vacuum level
+ -
+ -
Band diagrams of termination controlled LSMO
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Origin of the interface dipole(a) n-type interface (b) p-type
interface
ρ -V ρ -V
Robust Ti4+ states H. Kumigashira et al., Appl. Phys. Lett. 88,
192504 (‘06).
Ti 2p core level of LSMO/STO/LSMO sandwiched structures
0.3e-
0.3e-
-
H. Kumigashira et al., Appl. Phys. Lett. 88, 192504 (2006).
Origin of the interface dipole
0.6e-
0.3e-
0.3e-
M. Minohara, H.K. et al., Phys. Rev. B 81, 235322 (2010).
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€
V = Qdεrε0S
εr = 30 [Ref.1]
V = +0.5 eV (n-type)V = -0.2 eV (p-type)
Experimental results
V = +0.5 eV (n-type)V = -0.4 eV (p-type)
[1] R. D. Shannon, J. Appl. Phys. 73, 348 (1993).
Origin of the interface dipole
M. Minohara, H.K. et al., Phys. Rev. B 81, 235322 (2010).
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まとめ�ABO3/LSMO接合界面の電子状態について調べるために放射光電子分光を行った。
ヘテロ界面の化学状態
ヘテロ界面のバンドダイアグラム
LSFO/LSMO: FeとMnイオン間の電荷移動
STO/LSMO: TiとMnイオン間の電荷移動は起こらず、Ti4+状態を維持（Mn側が収納）
La0.6Sr0.4O electron-donor layer 遷移金属イオンの3d準位の相対位置、
およびFillingが重要。
LSMO/TiO2-STO: 0.5 eVの界面ダイポール形成 LSMO/SrO-STO: -0.4
eVの界面ダイポール形成（界面ダイポールの反転）
静電ポテンシャルの発散を抑制するために、Mn側の価数が変調
遷移金属イオンの3d準位位置、AO層の電荷量、終端面を考慮した界面設計
