modulation doping of ferromagnetism in manganite superlattices · •10 differentially‐pumped...

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


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


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 ?


(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 >


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


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


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


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


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


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)


Presenter

Presentation Notes

Point out transition

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


Metal-like behavior near x=0.5

A‐type AF order• x2‐y2

lie in MnO2

sheets



F state• orbital disorder

• double exchange in 3D


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


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 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

?

Presenter

Presentation Notes

Highly anisotropic conductor In analogy with de gennes idea, highly anistropic conductor, add electrons canting of antiferromagnet to the F side state How does it go to isotropic conductor? Modulation of the exchange interaction

LMOSMOSTO

LMOSMO

LMOSMO

LMOLMOSMO

x = 0.44

x 9

(0 0 ½)

0

x = 0.44 : Canted AF ? Modulated Behavior ?


Compare x=0.44 SL and alloy

Alloy has higher moment than SL, but still < (4 – x) μBCanted ?

SL

SL

alloy alloy


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


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


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


Presenter

Presentation Notes

Locally changing the property of the material in an ordered state

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


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


x = 0.47 superlattice – X-Ray Diffraction


X = 0.47 SL – Magnetization & Resistivity

properties follow trend with other SL compositions

200 Oe


PNR: x = 0.47 SL – Modulation here too!

T = 120 KH = 8150 Oe


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


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


modulation doping of ferromagnetism in manganite superlattices · •10 differentially‐pumped...

Documents