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Modulation Doping of Ferromagnetism in Manganite Superlattices
Tiffany S. SantosCenter for Nanoscale MaterialsArgonne National Laboratory
NIST Summer SchoolMay 14, 2010
Support: DOE‐BES Contract # DE‐AC02‐06CH11357
Ozone-MBE Team
LaMnO3
/SrMnO3
superlattice
Tiffany SantosAnand BhattacharyaBrittany Nelson‐CheesemanSteve May (now at Drexel U.)
STEM: Amish Shah, J.‐M. Zuo, UIUC
nano.anl.gov
CNM
Tiffany Santos, 2010 NIST Summer School
Talk Outline
Disordered La/SrRandom Alloy La1‐x
Srx
MnO3
Ordered La/SrSMO1
/ LMO1
O. Chmaissem, et al., PRB 2003
Tuning across the F/AF phase transition at x=0.5 by digital synthesis
Delta‐doping of ferromagnetism into the antiferromagnetic state
Manganites (La1-x Srx MnO3 ): Magnetic Ordering
Exhibit wide variety of ordering phenomena (magnetic, orbital)
Magnetic ordering related to Mn valence state
SMO, d3
band
insulator
Mn 3d
states
LaMnO3SrMnO3
t2g
eg
LMO, d4
Mott
insulator
La1/3
Sr2/3
MnO3
: insulating AFM
La2/3
Sr1/3
MnO3
: half‐metal, CMR effect,TC
> RT
Tiffany Santos, 2010 NIST Summer School
Competition between double exchange and superexchange
Double exchange
t ~ cos(θ/2)θ
Mn3+
Superexchange
Ferromagnetic, metallic
Mn4+
Antiferromagnetic, insulating
Mn4+ Mn4+
C. Zener, PRB (1951)
θ(tpd
)2
~
cos2(θ)
J. B. Goodenough, Phys. Rev. (1955)
Determines the magnetic properties across the phase diagram
Tiffany Santos, 2010 NIST Summer School
F-AF Phase Transition at x ~ 0.5
Previous x=0.5 studies: Koida et al, PRB (2002); P. K. Muduli et
al, JPCM (2007);M. Izumi et al, PRB (2000); Y. Konishi et al, J. Phys. Soc. Jpn.
(1999)
A‐type AF order• x2‐y2
lie in MnO2
sheets
• F double exchange in plane
• AF superexchange along c
F state• orbital disorder
Has TN
and TC
Order vs Disorder of Sr/La on A-sites
Disordered La/SrRandom Alloy La1‐x
Srx
MnO3
Ordered La/SrSMO1
/ LMO1
vs
Why study order ?
Tiffany Santos, 2010 NIST Summer School
(R0.7 M0.3 )MnO3 : Suppressed TC with A-site Disorder
avg. cation radius <rA
> = 1.23 Ådoping level
x
= 0.30
σ2
(rA
) = <rA2> ‐
<rA
>2
Rodriguez-Martinez and Attfield, PRB (1996)
Cation disorder decreases TC
Variance
in local tolerance factor t =< A − O >
2 < Mn − O >
Tiffany Santos, 2010 NIST Summer School
Ln0.5 Ba0.5 MnO3 : New Ground States with Ordered Analogs
D. Akahoshi et
al., PRL (2003)
A‐site disorder drastically changes the ground stateTiffany Santos, 2010 NIST Summer School
Ozone-Assisted Molecular Beam Epitaxy
• Pure ozone
• 10 differentially‐pumped K‐cells
• 3 e‐beam sources
• in situ RHEED
• Rate measured by QCM
• Composition calibrated by RBS
• Atomic absorption in progress
This is a user facility !nano.anl.gov
Tiffany Santos, 2010 NIST Summer School
Ozone-Assisted Molecular Beam Epitaxy
• Pure ozone
• 10 differentially‐pumped K‐cells
• 3 e‐beam sources
• in situ RHEED
• Rate measured by QCM
• Composition calibrated by RBS
• Atomic absorption in progress
Ozone
Ca Sr Ni Ba Mn Co Ti LaCr V
RHEED
AA lampAA PMT
This is a user facility !nano.anl.gov
Tiffany Santos, 2010 NIST Summer School
101
102
103
104
105
106
44 45 46 47 48
Cou
nts
2Θ (deg)
STO (0 0 2)
LMO (0 0 2)
LaMnO3 , SrMnO3 on SrTiO3
10-1
100
101
102
103
104
100 150 200 250 300
LMO 20 uc / STO
ρ(Ω
-cm
)
T (K)
101
102
103
104
105
106
107
46 47 48 49 50
Cou
nts
2Θ (deg)
STO (0 0 2)
SMO (0 0 2)
SrMnO3LaMnO3
10-1
100
101
102
103
104
105
50 100 150 200 250 300 350
ρ (Ω
-cm
)
T (K)
cLMO
= 3.96 Å cSMO
= 3.78 Å
cSTO
= 3.91 Å
LMOSMO
STO
LMOSMO
LMOSMO
LMOSMOSMO
LMOSMOSTO
LMOSMO
LMOSMO
LMOLMOSMO
LMOSMOSTO
x = 0.50
x = 0.44 x = 0.55
Digital Synthesis
SMO1
/ LMO1
superlattice
x 9 x 9
x 40
total thickness ~ 30 nm
compare with alloys of equivalent composition…Tiffany Santos, 2010 NIST Summer School
Reflection High Energy Electron Diffraction
‐ oscillations of specular peak‐ layer‐by‐layer growth
SrTiO3
substrate LaMnO3
/SrMnO3
superlatticeLMOSMOSTO
LMOSMO
LMOSMO
LMOLMOSMO
x = 0.44
5 supercells
Tiffany Santos, 2010 NIST Summer School
Smooth Surfaces: AFM and STM of x=0.50 SL
875 nm x 875 nm
350 nm x 350 nm
STM
In collaboration with Matthias Bode, CNM
AFM
X-Ray Reflectivity: Simulation
LMO
LMO
LMO
LMO
SMO
SMO
SMO
SMOSMO
STO
x 9 LMOSMO
STO
x 40
x=0.50
x=0.55
Tiffany Santos, 2010 NIST Summer School
Single Layer Control: X-Ray Reflectivity
SL peaks not possible with layer roughness of a single unit cell
2θ=14°
LMOSMO
STO
x 40
x = 0.50
LMO
LMO
LMO
LMOLMO
SMO
SMO
SMO
STO
x 9
SMO
x = 0.44
x = 0.55 SL
x = 0.55 alloy
Tiffany Santos, 2010 NIST Summer School
X-ray Diffraction(002) peak of the SrMnO3
/LaMnO3
superlattices
(002) RC width:
~ 0.049°
for the SLs
~ 0.039°
for the alloys
(~0.030°
for STO)
Tiffany Santos, 2010 NIST Summer School
Enhanced Metallicity with A-site Order
• metal‐like behaviorρ
< 3 mΩ‐cm
x = 0.55
x = 0.50
x = 0.44
x = 0.44
x = 0.50, 0.55
• disorder raises resistivity
Tiffany Santos, 2010 NIST Summer School
Metal-like behavior near x=0.5
A‐type AF order• x2‐y2
lie in MnO2
sheets
• F double exchange in plane
• AF superexchange along c
F state• orbital disorder
• double exchange in 3D
Tiffany Santos, 2010 NIST Summer School
A-type AF order: Neutron Diffraction, x = 0.55
Structurally forbidden (0 0 ½) diffraction peak below TN
coherence length ξ ~ film thickness
TN enhanced from bulk TN=220 K
Measured at HFIR, ORNLCollaboration with Lee Robertson, Jerel Zarestky
LMO
LMO
LMO
LMO
SMO
SMO
SMO
SMO
STO
x 9
MnO2
sheets
bulk T
N
0
0
A-type AF order, x=0.50 LMOSMO
STO
x 40
bulk T
N
Tiffany Santos, 2010 NIST Summer School
0
LMOSMO
STO
x 40
bulk T
N
TN greatly increased over bulk TNNo F phase at high T
Strain, rather than A‐site order, leads to enhanced TN:
compression of c‐axis stronger superexchange
Enhanced Néel Temperature
?
La1-x Srx MnO3 on the Verge of Ferromagnetism: x=0.44, 0.47
A‐type AF metal• x2‐y2
lie in MnO2
sheets
• F double exchange in plane
• AF superexchange along c
F state• double exchange 3D
• orbital disorder
Adding mobile carriers to AF
Canting due to double exch.
(single orbital model by de Gennes, Phys. Rev. 1960;degenerate orbital model byvan der Brink & Khomskii, PRL ’99)
This study:The transition from an
anisotropic AF
to an
isotropic F
?
LMOSMOSTO
LMOSMO
LMOSMO
LMOLMOSMO
x = 0.44
x 9
(0 0 ½)
0
x = 0.44 : Canted AF ? Modulated Behavior ?
Tiffany Santos, 2010 NIST Summer School
Compare x=0.44 SL and alloy
Alloy has higher moment than SL, but still < (4 – x) μBCanted ?
SL
SL
alloy alloy
Tiffany Santos, 2010 NIST Summer School
R ++(qz
)
R – – (qz
)
z
x
H,M
H,M
Polarized Neutron ReflectometryStudy magnetic depth profiles with sub‐nm resolution
Sensitive to the in‐plane component of magnetization
Measurement at NIST Center for Neutron Research
–
NG1 and AND/R, Brian Kirby and Brian Maranville
–
Suzanne te Velthuis, Argonne
Tiffany Santos, 2010 NIST Summer School
x=0.44 Alloy: Polarized Neutron Reflectivity
H = 6750 OeMsat
II HT = 120 K
Alloy has no Bragg peak no modulation
PNR: Modulated Moment in x=0.44 SL
Bragg peak at q = 0.18 Å‐1 modulated momentcommensurate with SL structure
H = 6750 OeMsat
II HT = 120 K
LMOSMOSTO
LMOSMO
LMOSMO
LMOLMOSMO
x = 0.44
34.6 Å
LMOSMOSTO
LMOSMO
LMOSMO
LMOLMOSMO
LMOSMO
LMOSMO
LMOSMO
LMOLMOSMO
LMOSMO
LMOSMO
LMOSMO
LMOLMOSMO
low M
high M
low M
high M
high M
low M
x 7
H
x=0.44: PNR fitting model
Tiffany Santos, 2010 NIST Summer School
Butterfly spin structure‐
canted, modulated
High M layer: 2.2 µB
, 6 u.c., θ
= 102°
Low M layer: 0.7 µB, 3 u.c., θ
= 157°
θ H
Delta-Doping of FerromagnetismInserting a single layer of electrons enhances double‐exchange along the c‐axis canting higher moment
Tuning the strength of the ferromagnetic exchange interactions
Locally change the property of the material, without introducing disorder or frustration
LMOSMOSTO
LMOSMO
LMOSMO
LMOLMOSMO
LMOSMO
LMOSMO
LMOSMO
LMOLMOSMO
LMOSMO
LMOSMO
LMOSMO
LMOLMOSMO
low M
high M
low M
high M
high M
low M
H
La0.60
Sr0.40
MnO3
La0.6
Sr0.4
FeO3
(I)La0.45
Sr0.55
MnO3
(M)Compare with:
What is the magnitude of the induced moment?
How far does the delta‐doped charge extend?
Can we design a modulated profile with a different periodicity?
AF
F
Izumi, Tokura, et al PRB ‘99,‘00
Tiffany Santos, 2010 NIST Summer School
x=0.47 superlattice
LMOSMOSTO
LMOSMO
LMOSMO
LMOLMOSMO
x = 0.47
x 4LMOSMO
LMOSMO
LMOSMO
SMOLMOSMOLMO
R+ +
R ‐ ‐
2 Bragg peaks expected
Double LaMnO3
in middle
On the verge of ferromagnetism
PNR simulation
Tiffany Santos, 2010 NIST Summer School
x = 0.47 superlattice: SL x 4 / L / SL x 5
LMOSMOSTO
LMOSMO
LMOSMO
LMOLMOSMO
x = 0.47
x 4LMOSMO
LMOSMO
LMOSMO
SMOLMOSMOLMO
Thickness = 287 ÅSurface roughness < 2 Å
X‐ray reflectivity
Tiffany Santos, 2010 NIST Summer School
x = 0.47 superlattice – X-Ray Diffraction
Tiffany Santos, 2010 NIST Summer School
X = 0.47 SL – Magnetization & Resistivity
properties follow trend with other SL compositions
200 Oe
Tiffany Santos, 2010 NIST Summer School
PNR: x = 0.47 SL – Modulation here too!
T = 120 KH = 8150 Oe
Tiffany Santos, 2010 NIST Summer School
PNR fitting model: x=0.47 SL
low M
high M
LMOSMO
LMOSMO
LMOSMO
LMOLMOSMO
LMO
��x 3
SMO
LMO
SMO
LMOSMO
SMOLMOSMOLMO
LMOSMO
SMO
LMOSMO
low M�
LMOLMO
LMO
LMOSMO
SMOLMOSMOLMO
LMOSMO
SMO
LMOSMO
high M
low M�
Induced moment Length scale for carrier confinement
High M layer: 2.2 µB
, 6 u.c., θ
= 102°
Low M layer: 0.7 µB, 13 u.c., θ
= 158°
Salvador
et al. APL ‘99; C. Adamo et al APL ’08; many
othersA. Bhattacharya et al
PRL ‘08S. May et al, Nat. Mat. ‘09
Koida
et al, PRB
(2002);
T. Santos et al PRB
‘09
La1-x Srx MnO3 Ordered Analogs
Tiffany Santos, 2010 NIST Summer School
Conclusions
Tuning across F‐AF phase boundary using digital synthesis
Enhanced TN over bulk La1‐xSrxMnO3, measured by neutron diffraction
Canted, modulated magnetic profile revealed by PNR
–
Delta‐doping of ferromagnetism without disorder
x=0.47 superlattice− Solved the magnetic structure:
Magnitude of the induced moment
Length scale for spreading of charge
Tiffany Santos, 2010 NIST Summer School