Intrinsic properties of phase-pure Co- and Mn-doped ZnO ...info.ifpan.edu.pl/spintech5/presentations/chambers...Mn-doped ZnO/ -Al 2O 3: 1. Epitaxial growth (PLD) & properties 2. Indirect

Intrinsic properties of phaseIntrinsic properties of phase--pure Copure Co-- and and MnMn--doped doped ZnOZnO epitaxial filmsepitaxial films

S.A. ChambersFundamental and Computational Sciences DirectoratePacific Northwest National Laboratory, Richland, WA

Work funded by US Department of Energy, Office of Basic Energy Sciences,Division of Materials Science & Engineering

SPINTECH V – July 7-11, 2009 -- Krakow, Poland

CollaboratorsCollaboratorsCollaborators

T.C. Kaspar, T. Droubay, C.M. Wang, V. Shutthanandan, P. Nachimuthu, M.

Engelhard, Z. Zhu – PNNL

C.A. Johnson, K.M. Whittaker, K.R. Kittilstved, D.R. Gamelin –

U. Washington

S.M. Heald, D.E Keavney – APS/ANL

A. Ney, K. Ollefs, S. Ye, T. Kammermeier, V. Ney – U. Duisburg-Essen

F. Wilhelm, A. Rogalev – ESRF

OutlineOutline

Introduction – spin-spin and spin-lattice interactions in dilute magnetic oxides

Co-doped ZnO/-Al2O3:1. Previous work -- theoretical predictions & experiments2. Epitaxial growth by PLD3. Structural, electronic and magnetic properties

Mn-doped ZnO/-Al2O3:1. Epitaxial growth (PLD) & properties2. Indirect evidence for spinoidal decomposition3. Review of traditional assumptions on dopant distributions –a new model for nanomaterials

Summary

M+n M+n

carrier mediated exchange

M+n M+(n-1)

double exchange

e-

Dopant spin interactions in epitaxial oxide filmsDopantDopant spin interactions in epitaxial oxide filmsspin interactions in epitaxial oxide films

mag. dopant

host cation

lattice defect

defect mediated exchange

super exchange

oxygen host cation

magnetic dopant (M)

New physics -- Coey & Chambers, “Oxide Dilute Magnetic Semiconductors – Fact or Fiction?”, MRS Bulletin 33, 1053 (2008).

typically AF

“How do we control materials processes at the level of electrons?”

Why ZnO?Why Why ZnOZnO??

Reasonable electron mobilities (in epitaxial films).

Direct gap semiconductor – strong emission in the UV.

Suitable for transparent high-T applications.

Spin coherence up to (at least) 280K.

Long spin lifetimes (~200 ps) at 280K.

n-ZnO/-Al2O3(001)Ghosh et al., APL 96, 232507 (2005)

H:ZnO/-Al2O3(001)Li et al., APL 92 ,152105 (2008)

40

35

30

25

20

15

Mob

ility

(cm

2 /V

s)

300250200150100500Temperature (K)

PL

inte

nsity

(arb

. uni

ts)

385380375370365360wavelength (nm)

T=4K

H2

O2

bulk ZnO

(a)

H:ZnO/-Al2O3(001)Li et al., JAP 104 ,053711 (2008)

Theoretical predictions Theoretical predictions –– Mn:ZnOMn:ZnO

Dietl, Ohno, Matsukura, Cibert, Ferrand, Science 287, 1019 (2000)

300K

Mn2+-doped (5%), p-type - 3.5x1020 cm-3)

Sato, Katayama-Yoshida, Physica E10, 251 (2001)

80 40 0 40 80Ene

rgy

diffe

renc

e (m

Ry)

0

30

15

-15

N concentration (%) Ga concentration (%)

Ferro stable

Para Stable

Mn:ZnO

CPA + KKR

p-type n-type

Theoretical predictions Theoretical predictions Co:ZnOCo:ZnO

LDA

LDA-SIC

Toyoda et al.Physica B (2006)

half metallicity!

e↓ t2↓

e↑ t2↑

Co(II) + e-CB ↔ Co(I)

Sato et al.SST (2002)

CPA + KKR

10 5 0 -5binding energy relative to Fermi level (eV)

x10

ZnO(001)Co (x=0.05):ZnO(001)

Co 3d derived

10 5 0 -5binding energy relative to Fermi level (eV)

x10

ZnO(001)Co (x=0.05):ZnO(001)

Co 3d derived

Spin coupling in CoSpin coupling in CoxxZnZn11--xxOO

n P(x=0.10,n) 0 0.281 0.382 0.233 0.084 0.025 0.00

Co2+

t2

e

?E = 10Dq

Co2+

t2

e

E = 10Dq

Co2+

t2

e

?E = 10Dq

Co2+

t2

e

E = 0.6 eV

Co(II) – 3.0 B(orbital moment

quenched)

Ms ≤ ~(1/3)*3= ~1 B/Co

Co

O

m surroundingcation sites(Zn or Co)

For a given x, what is the probability of finding n dopants in m sites? 1.0

0.8

0.6

0.4

0.2

0.0P

(x,n

)0.300.200.100.00

x

P(x,n=0)P(x,n=1)P(x,n=2)P(x,n=3)P(x,n=4)P(x,n=5)

m = 12

?

AF – Dietl et al.PRB 2007

nmn xxnmn

mnxP

)1()!(!

!),(

Abundant & widely divergent results for Co: Abundant & widely divergent results for Co: ZnOZnO!!More than 400 papers!!Room-temperature ferromagnetism with “giant” moments

of ~3.36 B/Co polycrystalline n-AlxCo0.05Zn0.95O grown by sputtering -- Liu, J. Phys:Cond. Mat. (2007)

Paramagnetism only in OPAMBE-grown n-CoxZn1-x /-Al2O3(001) (x = 0 0.15) -- Pacuski et al., PRB (2006)

Intermediate ferromagnetism (≤ 0.2 – 0.4 B/Co) in OPAMBE-grown n-CoxZn1-x /-Al2O3(001) (x = 0 0.12) that didn’t scale with conductivity – Liu et al., APL (2007)

Questions about material quality & adequacy of characterization

OffOff--axis PLD of Coaxis PLD of Co-- & & MnMn--doped doped ZnOZnO on on --AlAl22OO33(001) (001) & (012) from & (012) from nanoparticlenanoparticle & conventional targets& conventional targets

Gamelin Group - UWSchwartz et al, JACS (2003)

increasing OH-

(1-x)Zn(OAc)2 + xCo(OAc)2 + 2NMe4OH Co2+:ZnO + H2O + 2NMe4OAc

Possible structures Possible structures -- CoCo0.10.1ZnZn0.90.9O (001)/O (001)/--AlAl22OO33(001) (001)

lattice mismatch = -32%

OZnOOAl2O3

aZnO || asub aZnO || asubcos30

OZnO

OAl2O3

Alignment of the O sublattices gives greater stability

lattice mismatch = +18%

Structure Structure –– CoCo0.10.1ZnZn0.90.9O(001)/O(001)/--AlAl22OO33(001) (001)

Al2O3 (202)Al2O3 (202)Al2O3 (202)Al2O3 (202)Al2O3 (202)Al2O3 (202)

ZnO (101)ZnO (101)

c-Al2O3

c-Co:ZnO

c-Al2O3

c-Co:ZnO

Inte

nsity

(arb

. uni

ts)

191817161514 (degrees)

ZnO(002)

FWHM = 0.061°

Inte

nsity

100806040202 (degrees)

ZnO(002)

ZnO(004)Al

2O3

Al2O 3

Al2O

3

Inte

nsity

(arb

. uni

ts)

191817161514 (degrees)

ZnO(002)

FWHM = 0.061°

Inte

nsity

100806040202 (degrees)

ZnO(002)

ZnO(004)Al

2O3

Al2O 3

Al2O

3

out of plane

Inte

nsity

(arb

. uni

ts)

6766656463 (degrees)

ZnO(104)

FWHM = 0.073°

Inte

nsity

-100 0 100Phi (degrees)

ZnO(104)

6766656463 (degrees)

ZnO(104)

FWHM = 0.073°

6766656463 (degrees)

ZnO(104)

FWHM = 0.073°

Inte

nsity


ZnO(104)

Inte

nsity


ZnO(104)

in plane

(101) pole figure

epitaxial relationship:(110)ZnO || (012)Al2O3 & [001]ZnO || [011]Al2O3 a-Co:ZnO on r-sapphire

a/a = 1.53% along [001]ZnO= 18.3% along [110]ZnO

Structure – Co0.1Zn0.9O/-Al2O3(012) Structure Structure –– CoCo0.10.1ZnZn0.90.9O/O/--AlAl22OO33(012) (012) In

tens

ity (

arb

. uni

ts)

313029282726 (degrees)

ZnO(110)

Al 2O

3

FWHM =0.088°

Inte

nsity

706560555045402 (degrees)

Al2O

3 ZnO(110)

Inte

nsity

(ar

b. u

nits

)

313029282726 (degrees)

ZnO(110)

Al 2O

3

FWHM =0.088°

Inte

nsity

706560555045402 (degrees)

Al2O

3 ZnO(110)

out of plane

Inte

nsity

(ar

b.un

its)

33.032.532.031.531.030.5 (degrees)

ZnO(211)

FWHM =0.127 °

Inte

nsity


ZnO(211)

Inte

nsity

(ar

b.un

its)

33.032.532.031.531.030.5 (degrees)

ZnO(211)

FWHM =0.127

Inte

nsity


ZnO(211)

in plane

r-Al2O3 a-Co:ZnO r-Al2O3

a-Co:ZnO

1 nm a-Co:ZnO1 nm a-Co:ZnO

Zeroing in on the dopant –x-ray absorption spectroscopy (XAS)

Zeroing in on the Zeroing in on the dopantdopant ––xx--ray absorption spectroscopy (XAS)ray absorption spectroscopy (XAS)

Co

O

Zn

E

K

L2

L3

VB (full) CB (empty)

binding energy

dens

ity o

f sta

tes K

L2

L3

VB (full) CB (empty)

binding energy

dens

ity o

f sta

tes

Heald et al., Phys Rev. B 79, 075202 (2009) Chambers, Adv. Mat., in press (2009)Ney et al. submitted (2009)

XX--ray absorption nearray absorption near--edge spectroscopy (XANES) &edge spectroscopy (XANES) &extended xextended x--ray absorption fine structure (EXAFS)ray absorption fine structure (EXAFS)

SrCo0.01Ti0.99O3

x-ray energy (eV)

norm

aliz

ed a

bsor

ptio

n,

Co K-edge EXAFS

810080007900780077007600

Co K-shell XAS

CoTiO3CoOCo

-20 -10 0 10 20 30 40 50E – Eo (eV)

Eo = 7708.8 eVCoTiO3CoOCo

-20 -10 0 10 20 30 40 50E – Eo (eV)

Eo = 7708.8 eV

Nor

mal

ized

abs

orpt

ion,

E

o

Co0.04Ti0.96O2

-0.4

-0.2

0

0.2

0.4

0.6

0 2 4 6 8 10 12

k (

k)

k (Å-1)

s polarizationp polarization

0 1 2 3 4 5 6

F(R

)

R (Å)

(k) = [(k) - o(k)]/o(k)= (fk/kR2)sin[2kR + o(k)]

x exp(-R – 22k2)

F(R) = ∫(k)exp(ikR)dkk1

k2

XX--ray magnetic circular ray magnetic circular dichroismdichroism (XMCD)(XMCD)

= IL3l – IL3r

= IL2l – IL2r

Circularly polarized x-rays & magnetic field.Angular momentum of x-ray is transferred (in part)

to the spin of the bound electron via spin-orbit coupling.Right circular photons transfer opposite

momentum as left circular photons.Spin states are opposite in 2p3/2 (L3) & 2p1/2 (L2)No spin flip during photoexcitation.2p↑ 3d↑ and 2p↓ 3d↓Circularly polarized excitation senses spin

imbalance in the unoccupied d states.Dichroism = AL-R = measure of spin imbalance in

empty statesAtom specific magnetometry

Co speciation Co speciation –– Co:ZnO/Co:ZnO/--AlAl22OO33(001)(001)X

-ray

abs

orpt

ion

(arb

. uni

ts)

50403020100-10X-ray energy, E-Eo (eV)

10% Co:ZnO/Al2O3 conv

Co metal

CoTiO3CoO

Co K-edge XANES - APS2% Co:ZnO/Al2O3 nano

Co is Co(II), but not CoOStrong hybridization to O

Co 1s 3ddipole forbiddenweakly allowed for

Co 3d and O 2p mixing

Kaspar et al., PRB 77, 201303(R) (2008)

Co local structure Co local structure –– Co:ZnO/Co:ZnO/--AlAl22OO33(001)(001)

2.0

1.5

1.0

0.5

0.0

(R

) (Å

-3)

6543210R (Å)

10% Co:ZnO conv (Co K-edge)2% Co:ZnO nano (Co K-edge)

pure ZnO (Zn K-edge)Co & Zn K-edge EXAFS - APS

Co(II) substitutes for Zn

XX--ray linear ray linear dichroismdichroism (XLD) (XLD) ––Co:ZnO/Co:ZnO/--AlAl22OO33(001)(001)

Ney et al., PRL 100, 157201 (2008)

> 95% of Co at lattice sites

Co LCo L--edge XMCD edge XMCD –– Co:ZnO/Co:ZnO/--AlAl22OO33(001)(001)

0.010

0.005

0.000

-0.005

-0.010

XM

CD

(arb

. uni

ts)

-4 -2 0 2 4H (T)

T=50K0.010

0.005

0.000

-0.005

-0.010

XM

CD

(arb

. uni

ts)

-4 -2 0 2 4H (T)

T=50K

Co(II) is paramagnetic

Photon Energy (eV)Photon Energy (eV)

0.26

0.24

0.22

0.20

0.18

0.16

XA

S (

I R+

I L)/2

10

5

0

XM

CD

(IR-IL )

T=50K H=4T

-5

-10

800795790785780775770-15 x 10-3

0.26

0.24

0.22

0.20

0.18

0.16

XA

S (

I+

I)/2

10

5

0

XM

CD

(I-I

T=50K H=4T

-5

-10

800795790785780775770-15 x 10-3

40x10-3

30

20

10

XM

CD

(a.

u.)

30025020015010050Temperature (K)

H=5T40x10-3

30

20

10

XM

CD

(a.

u.)

30025020015010050Temperature (K)

H=5T

40x10-3

30

20

10

0

XMD

C (I

L–

I R)

782780778776774Photon Energy ( eV)

0.8

0.4

0.0

-0.4

XMC

D (a

rb. u

nits

)

782780778776774Photon Energy (eV)

5.18.06 H=5T4.3K10K20K30K40K50K60K80K100K125K150K200K250K300K

40x10-3

30

20

10

0

XMD

C (I

L–

I R)


0.8

0.4

0.0

-0.4

XMC

D (a

rb. u

nits

)



40x10-3

30

20

10

0

XMD

C (I

L–

I R)


40x10-3

30

20

10

0

XMD

C (I

L–

I R)


0.8

0.4

0.0

-0.4

XMC

D (a

rb. u

nits

)


0.8

0.4

0.0

-0.4

XMC

D (a

rb. u

nits

)



e-h

ITEY

H

MIFY e-h

ITEY

H

MIFY

At At remanenceremanence –– Co:ZnO/Co:ZnO/--AlAl22OO33(001)(001)0.65

0.60

0.55

0.50

0.45

0.40

0.35

XAS

(arb

. uni

ts)

800795790785780775770Photon Energy (eV)

10x10-3

8

6

4

2

0

XMC

D (arb. units)

T=50K H=0, TEYXAS -ve saturationXAS +ve saturation

XMCD diff

50x10-3

40

30

20

10

XA

S (a

rb. u

nits

)

800795790785780775770Photon Energy (eV)

10x10-3

8

6

4

2

0

XM

CD

(arb. units)

T=50K H=0, TFYXAS -ve saturationXAS +ve saturationXMCD diff

No hint of ferromagnetism associated with Co(II), even at 50K, in as-grown films

(insulating)

e-h

ITEY

H

MIFY

Can we make Can we make nn--Co:ZnOCo:ZnO without reducing without reducing structural structural Co(IICo(II)?)?

norm

aliz

ed a

bsor

ptio

n

773077207710x-ray energy (eV)

0.025Co + 0.975Co:ZnO0.050Co + 0.950Co:ZnO0.075Co + 0.925Co:ZnO0.100Co + 0.900Co:ZnO

norm

aliz

ed a

bsor

ptio

n


0.025Co + 0.975Co:ZnO0.050Co + 0.950Co:ZnO0.075Co + 0.925Co:ZnO0.100Co + 0.900Co:ZnO0.150Co + 0.850Co:ZnO

Co K-edge XANES

norm

aliz

ed a

bsor

ptio

n

77807760774077207700x-ray energy (eV)

n-Co:ZnO (=1 m-cm)semi-insulating Co:ZnO

77167710

0.05 B/Co ~3% of Co is Co(0)

norm

aliz

ed a

bsor

ptio

n


semi-insulating Co:ZnOCo metal

Our n-Co:ZnO has no detectable Co(0)

Does making the films n-type activate room temperature ferromagnetism?

Does making the films Does making the films nn--type activate room type activate room temperature ferromagnetism?temperature ferromagnetism?

Resistivity (-cm)Convntl. target Nanopart. targetProcessing condition

O2pressure

Substrate temp

Additional dopant

c-Al2O3 r-Al2O3 c-Al2O3 r-Al2O3

Deposition 10 mTorr 550°C --- 70000 95000 15000 1000

Anneal Vacuum 700°C --- 0.13 0.11 2.4 0.52

Deposition 10 mTorr 550°C 1% Al 0.4 0.18

Anneal Vacuum 700°C 1% Al 0.004

Deposition 2x10-5 Torr

(plasma) 550°C --- 0.15 0.54

Deposition Vacuum 400°C --- 7.9 0.025 0.053 0.011

Deposition Vacuum 400°C 1% Al 0.0014 0.0007

Kaspar et al., New J. Phys 10, 055010 (2008)

Structurally excellent n-type Co:ZnO is not made ferromagnetic at RT by adding carriers

Structurally excellent Structurally excellent nn--type type Co:ZnOCo:ZnO is is notnot made made ferromagnetic at RT by adding carriersferromagnetic at RT by adding carriers

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

Mom

ent (

µ B/C

o)

-10000 -5000 0 5000 10000Field (Gauss)

as-grownCo0.1Zn0.9O

pure ZnO

n-type (Al)Co0.1Zn0.9Oas grown

annealn-type

Co0.1Zn0.9O

anneal again

anneal again!anneal again!!

(-cm)

>10,000

>10,000

0.4

0.1

0.02

0.01

0.004

H in planeT = 300K

Dependence of magnetization on conductivityDependence of magnetization on conductivity

0.06

0.05

0.04

0.03

0.02

0.01

0.00

Sat

urat

ion

mom

ent (

µ B/C

o)

10-3 10 -1 101 103 105

Resistivity ( -cm)

c - Co:ZnO

a - Co:ZnO

beforeanneal

afteranneal

]

]

Is Is nn--Co:ZnOCo:ZnO a a lowlow--TTcc DMS?DMS?

60

40

20

0

-20

-40

-60

Mom

ent ( e

mu)

-40 -20 0 20 40Magnetic Field (kOe)

5K300K

n-Co0.06Zn0.94O( = 1 m-cm)

-2

-1

0

1

2

Mom

ent ( e

mu)

-1000 -500 0 500 1000Magnetic Field (Oe)

n-Co0.06Zn0.94O( = 1 m-cm)2.5x10

-6

2.0

1.5

1.0

0.5

Mom

ent ( e

mu)

30025020015010050Temperature (K)

field cooled(in 4 T @ 10 mT)zero field cooled(in 0 T @ 4 mT)

nn--Co:ZnOCo:ZnO is paramagnetic at all temperaturesis paramagnetic at all temperatures

So much for Co....So much for Co....

Cobalt: Kobald (German), goblin or evil spirit, from cobalos (Greek)

Manganese: Magnes (Latin), magnet

what about what about MnMn??

Inte

nsity

(arb

. uni

ts)

191817161514 (degrees)

ZnO(002)

FWHM = 0.061°

-Al2O3(001)

Mn:ZnO(001)

Mn:ZnO(001)

Structure Structure –– MnMnxxZnZn11--xxO(001)/O(001)/--AlAl22OO33(001) (001)

Droubay et al., PRB 79, 155203 (2009)

Mn K-edge XANES

norm

aliz

ed a

bsor

ptio

n

658065706560655065406530

Photon Energy (eV)

MnMn charge state in Mn:ZnO/charge state in Mn:ZnO/--AlAl22OO33(001)(001)XX--ray absorption nearray absorption near--edge spectroscopy edge spectroscopy

(XANES)(XANES)

Mn metal

Mn(II)

Mn:ZnO film

nanoparticles

Mn3O4

Mn(IV)

Mn(III)

Mn(II)Mn(III)

Mn+???

1s Mn 3d (t2) + Mn 4p (t2) + O 2p

tetrahedral coordination

Pre-edge feature

e↑

Mn 3d + 4p + N 2pmajority spin 10Dq

t2↑

e↓

t2↓Mn 3d + 4p + N 2pminority spin 10Dq

Eexc

Mn 1s

First-principles XANES calculationsTitov et al. PRB 72, 115209 (2005)

XANES theoryXANES theory–– Mn(III)Mn(III)xxGa(III)Ga(III)11--xxNN

Two-peak pre-edge Mn(III)One-peak pre-edge Mn(II)

Mn K-edge XANES

norm

aliz

ed a

bsor

ptio

n

658065706560655065406530

Photon Energy (eV)

MnMn charge state charge state in Mn:ZnO/in Mn:ZnO/--AlAl22OO33(001)(001)XX--ray absorption nearray absorption near--edge spectroscopy edge spectroscopy

(XANES)(XANES)

Titov et al. PRB 72, 115209 (2005)

Mn metal

Mn(II)

Mn:ZnO film

nanoparticles

Mn3O4

Mn(IV)

Mn(III)

GaMnN (Mn3+)expt

ZnMnTe(Mn2+)calc

ZnMnTe(Mn2+) expt

1s Mn 3d (t2) + Mn 4p (t2) + O 2p

tetrahedral coordination

Mn(II) e↑

Mn 3d + 4p + N 2p10Dq

t2↑

e↓

t2↓Mn 3d + 4p + N 2p10Dq

Eexc

MnMn local structural environment local structural environment in Mn:ZnO/in Mn:ZnO/--AlAl22OO33(001)(001)Extended xExtended x--ray absorption fine structure (EXAFS)ray absorption fine structure (EXAFS)

Mn and Zn K-shell EXAFS

2.0

1.5

1.0

0.5

0.0

|(R

)|(Å-

3 )

86420

R (Å)

Mn:ZnO nano (Mn K)ZnO powder (Zn K)

Mn(II) substitutes for Zn

Mn:ZnO film (Mn K)

DopantDopant magnetic properties magnetic properties –– MnMn0.050.05ZnZn0.950.95OOXX--ray magnetic circular ray magnetic circular dichroismdichroism (XMCD)(XMCD)

e-h

ITEY

H

MIFY55x10-3

50

45

40

Flou

resc

ence

Yie

ld (a

rb. u

nits

)

640638636634632

Incident Photon Energy (eV)

8x10-3

6

4

2

0

-2

Mn

Dichroism

Mn L3 XASMn L3 XMCD

5K, 5T

MnMn LL33 XMCD XMCD -- MnMn0.050.05ZnZn0.950.95OO

35

e-h

ITEY

H

MIFY30x10-3

20

10

0Flou

resc

ence

Yie

ld (a

rb. u

nits

)

640638636634632

Incident Photon Energy (eV)

XAS(FY)

XMCD(FY)

B

T = 5K

Mn(II) is paramagnetic in as-grown films (highly resistive)

Field dependence of Field dependence of MnMn dichroismdichroism for for different different dopantdopant concentrationsconcentrations

1.0

0.8

0.6

0.4

0.2

0.0

Nor

mal

ized

Mn

XM

CD

/XA

S (a

rb. u

nits

)

543210

Magnetic Field (Tesla)

Mn0.002Zn0.998OMn0.025Zn0.975OMn0.05Zn0.95O

MC simulations

Lower rate of growth of dichroism with field reveals nonrandom dopant distribution (correlated substitution or spinoidal decomposition)

Presumed caused – lowering of total energy by closer proximity

MC simulations -- Droubay et al., PRB 79, 075324 (2009)

Does Does pp--type doping of type doping of Mn:ZnOMn:ZnO activate highactivate high--TTcc ferromagnetism?ferromagnetism?

Concentration calibration is tentativeAll films are highly resistive and paramagnetic

Secondary ion mass spectrometry (SIMS) depth profiles

Growth in N2

0 100 200 300 400 500 60010-1100101102103104105106107

10181019102010211022102310241025NO

- ZnO-

Cal

ibra

ted

NO

-in

tens

ity (a

.u.)

Depth (nm)N

concentration (atoms/cm

3)

Growth in N2O

0 200 400 600 800 1000 120010-1100101102103104105106107

10181019102010211022102310241025NO

- ZnO-

Cal

ibra

ted

NO

-in

tens

ity (a

.u.)

Depth (nm)

N concentration (atom

s/cm3)

DopantDopant distributionsdistributionsAre bulk models correct for Are bulk models correct for nanoscalenanoscale materials?materials?

singles (n=1) dimers (n=2) open trimers (n=3) closed trimers (n=3)n-mers – a lattice cluster containing n dopants (n = 1, 2, 3,...)

cited 173 times

Monte Carlo simulations Monte Carlo simulations –– MMxxZnZn11--xxOO(M = generalized metal (M = generalized metal dopantdopant))

Wurtzite crystal size & dopant mole fraction (x) defined Dopants randomly placed at cation sites (consistent with x) Dopant “connectivity” through intervening oxygens examined and cataloged

throughout crystal Simulation done 1000 times for each crystal size and x value Statistics collected and analyzed

5 nm radius MxZn1-xO nanocrystal(~20,550 cation sites)

Dependence on S/V ratio for MDependence on S/V ratio for M0.090.09ZnZn0.910.91OO

0.318

singles

0.197

dimers

Higher fraction of singles and dimers as S/V ratio increases because of reduction in dopantcoordination number at the surface

Effect on magnetic properties of MEffect on magnetic properties of M0.100.10ZnZn0.900.90O O grown on grown on --AlAl22OO33(001)(001)

0.2 m

Mn:ZnO

Mn:ZnO

100 nm5 mMn:ZnO

-Al2O3(001)

0.2 m

Mn:ZnO

Mn:ZnO

100 nm5 mMn:ZnO

-Al2O3(001)

0.324Behringer’s eqns.

100 nm

7.5 nm

S/V = 5.4 x 10-2 Å-1

aZnO || asubcos30

OZnO

OAl2O3

a/a = +18%

aZnO || asubcos30

OZnO

OAl2O3

aZnO || asubcos30

OZnO

OAl2O3

a/a = +18%

assume

for even n-mers

assume

for even n-mers

0.367Monte Carlo

For isolated Mn(II), moment = 5.9 B (Kittel)MC 2.2 B & Behringer 1.9 B (~15% difference)

General result...General result...

xeff = 1 – [(1 – x)12 + 0.39(S/V)]1/12

For spherical nanoparticles, thin nanodisks, nanorods, and granular epitaxial films with S/V ≥~5 x 10-4 Å-1 and ~0.05 ≤ x ≤ ~0.15,

where xeff can be used in place of the actual x in bulk probabilistic formulae (bionomial thm or Behringer’s eqns).

Droubay et al., PRB 79, 075324 (2009)

SummarySummarySummary Must perfect the growth and do the right characterization to make defensible conclusions. Structurally excellent Co:ZnO & Mn:ZnO are highly resistive and paramagnetic as grown. Same is true for structurally excellent Co: and Cr:TiO2 with dispersed dopants. Paramagnetic Co spins + abundant itinerant electrons from AlZn and Ov ≠ ferromagnetism. Can’t grow p-Mn:ZnO by co-doping with N to see if hole-mediate exchange interaction is strong. Outlook for magnetically doped oxides?

Not high-Tc DMS as originally envisioned and predicted.Defect-mediated ferromagnetism can occur in poorly ordered materials (crummy films &nanoparticle assemblies).Is spinoidal decomposition the key to high-Tc DMS? Maybe – example Co:TiO2.

1 m

0.2 m

AFM MFM

1 m

0.2 m

1 m

0.2 m

1 m

0.2 m

AFM MFM

1 m

0.2 m

1 m

0.2 m

1 m

0.2 m

AFM MFM

1 m

0.2 m

1 m

0.2 m

1 m

0.2 m

1 m

0.2 m

AFM MFM

36% remanence

H || film plane --room temperature

H (Oe)

Mag

netiz

atio

n (x

10-6

emu)

1.2B per Co

36% remanence


-40

-20

0

20

40

400020000-2000-4000H (Oe)

Mag

netiz

atio

n (x

10-6

emu)

1.2B per Co

36% remanence


H (Oe)

Mag

netiz

atio

n (x

10-6

emu)

1.2B per Co

36% remanence


-40

-20

0

20

40

400020000-2000-4000H (Oe)

Mag

netiz

atio

n (x

10-6

emu)

1.2B per Co

EXTRAS

1 m

0.2 m

AFM MFM

1 m

0.2 m

1 m

0.2 m

1 m

0.2 m

AFM MFM36% remanence


H (Oe)

Mag

netiz

atio

n (x

10- 6

emu )

1.2B per Co

36% remanence


-40

-20

0

20

40

400020000-2000-4000H (Oe)

Mag

netiz

atio

n (x

10- 6

emu )

1.2B per Co

photon energy (eV)810800790780770760

Co L-edgeXMCDALS B = -0.5T

B = +0.5T

Inte

nsity

(arb

. unt

is)

(I +-I

-)/(I +

+ I -)

(0.1

%)

Magnetic Field (KOe)

300K350K380K

Magnetic Field (KOe)

1.0

0.5

0.0

-0.5

80400-40-80

300K350K380K

300K300K350K380K

H(B

) – H

(0) (

m

-cm

)

Spinoidal decomposition in Co:TiO2/LAO(001)SpinoidalSpinoidal decomposition in Co:TiOdecomposition in Co:TiO22/LAO(001)/LAO(001)

Magnetic properties with & without surface CoxTi1-xO2 clusters

Magnetic properties with & without Magnetic properties with & without surface Cosurface CoxxTiTi11--xxOO22 clustersclusters

172Å Co0.04Ti0.96O1.96/LaAlO3(001)( = 0.35 -cm – Co-enriched CoxTi1-xO2-x clusters present)

222Å Co0.06Ti0.94O1.94/LaAlO3(001)( > 4 K-cm – flat film & distributed Co)

1.2 B/Co1.1 B/Co

Magnetic Field (Gauss)

Mag

netiz

atio

n (

Bpe

r Co)

-4000 0 4000

B || surface

Magnetic Field (Gauss)

Mag

netiz

atio

n (

emu)

-60

-40

-20

0

20

40

60

-4000 0 4000

B || surface

Co K-edge x-ray absorption – Advanced Photon Source

Co speciation in CoCo speciation in CoxxTiTi11--xxOO22/LAO(001)/LAO(001)

Co(II) environment morelike CoTiO3 than CoO

No evidence for Co metal

No evidence for CoO

Co:TiO2 (no particles)Co:TiO2 (small particles)Co:TiO2 (small particles)CoTiO3 (powder)CoO (powder)Co (metal foil)

-20 -10 0 10 20 30 40 50E – Eo (eV)

Eo = 7708.8 eV

Nor

mal

ized

abs

orpt

ion

(arb

. Uni

ts)

Co(IICo(II) local coordination ) local coordination ---- CoCoxxTiTi11--xxOO22/LAO(001)/LAO(001)

N = 5.82 (5.00) for randomly distributed (exactly correlated) O vacancies

Co(II) in slightly undercoordinatedoctahedral environmentCo(II) substitutes for Ti(IV)

RCo-O = 2.04 +/- 0.01 (2.01 +/- 0.01) Å in the ab plane (c direction) (1.94 & 1.97Å in pure anatase)

O vacancy forcesCo to be Co2+ CoxTi1-xO2-x

These oxygen vacancies are not electrically active

N = 5.4 +/- 0.3. N < 6 O vacancy (strain?)

Co K-shell EXAFS (APS)

s polarization

p polarization-1.5

1.5

1.0

0.5

0.0

-0.5

-1.0

12108642

k2*

(k)

k (Å-1)

s polarization

p polarization-1.5

1.5

1.0

0.5

0.0

-0.5

-1.0

12108642

)

-1)

-1.5

1.5

1.0

0.5

0.0

-0.5

-1.0

12108642

)

-

Semiconductor spintronicsSemiconductor Semiconductor spintronicsspintronics

carrier mediated exchange

diluted magnetic semiconductor (DMS)

sub. mag. dopant(Co, Fe, Ni, Cr, Mn)

ferromagneticn-semiconductor

quantum well structure

basesemiconductor

metal contact

metal contacte↑ h↑ + h↓

spin LED

“Is it possible to create magnetic semiconductors that work at room temperature?”Editorial Staff, Science 309, 82 (2005)

Tc ~ xNo( or )S(S + 1)(Ef)x = magnetic dopant mole fractionNo = cation density = s-d exchange parameter = p-d exchange parameterS = total valence spin on dopant(Ef) = density of free carriers at Ef

s

p

d

d

crystal momentumen

ergy

Previous work on Zn diffused Previous work on Zn diffused aa--CoCo0.090.09ZnZn0.910.91O/ O/ --AlAl22OO33(012) that we grew(012) that we grew……

Kittilstved et al., PRL 97, 037203 (2006)

Interstitial Zn (Zni) – a shallowdonor in ZnO

= 107 ~0.3 -cm[n] = ~2 x 1019 cm-3[Co(II)] = ~1021 cm-3

as growninsulating

x=0.09T = 300K

-0.10

-0.05

0.00

0.05

0.10

Sat

urat

ion

Mom

ent p

er C

o -

B

-2000 -1000 0 1000 2000Field [Oe]

+Zni

Zn diffused

Reversible chemical activation of FM & conductivityReversible chemical activation of FM & conductivity

Co:ZnOCo:ZnO + ZniCo:ZnO + Zni + O2

Abso

rban

ce LF

Co:ZnO

Zn vapor

Co:ZnO + Zni

O2

Co:ZnO + ZnO

, t

full recovery of Co(II)tet LFpeak manifold upon reoxidation

Highly correlated kineticsHighly correlated kineticsHighly correlated kinetics

power lawhopping conductivitypercolation theory

log() ~ n-1/3

Present problem – can’t reproduce the Zn diffusion process by Kittilstved:Similar magnetic behavior, but...Magnetization not “turned off” by heating in O2

Zn diffusion….. = 107 ~0.3 -cm[n] = ~2 x 1019 cm-3[Co(II)] = ~1021 cm-3

Zn diffusion….. = 107 ~0.3 -cm[n] = ~2 x 1019 cm-3[Co(II)] = ~1021 cm-3

Effect of Zn diffusion* on magnetizationEffect of Zn diffusion* on magnetization

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

Mom

ent (

µB/C

o)

-10000 -5000 0 5000 10000Field (Gauss)

as-grown(insulating)

3100 Å a-Co0.1Zn0.9O / r -sapphire

Zn-diffused600°C, 5 hrs(0.03 ·cm)

*Following process by Kittilstved et al., Adv. Mater. 16, 2115 (2004)

Zn

insulatingCoxZn1-xO on

Al2O3(012)@ 600oC

sealed quartz tube

Discovery of trace amounts of Co(0) after Zn diffusionDiscovery of trace amounts of Co(0) after Zn diffusionDiscovery of trace amounts of Co(0) after Zn diffusion

0.05 B/Co ~3% of Co is Co(0)no

rmal

ized

abs

orpt

ion



norm

aliz

ed a

bsor

ptio

n



Zn diffused Co:ZnO0.150Co + 0.850Co:ZnO

Cobalt: Kobald (German), goblin or evil spirit, from cobalos (Greek)

-- poly ZnO*-- Co metal

glancing incidence XRD

Nor

mal

ized

inte

nsity

7760775077407730772077107700Energy (eV)

Co metal

310 nm a-Co:ZnO+ Zn diffusion

310 nm a-Co:ZnO

X-ray photoemission depth profilingXX--ray photoemission depth profilingray photoemission depth profiling

500 eV Ar+e-

770780790800810Binding Energy (eV)

Cycles 1-2

4

6

810

1214

Co0Co2+

interface


Cycles 1-2

4

6

810

1214

Co0Co2+

interface

0 5 10 15 200

10

20

30

40

50

60

70

80

Sputter Depth (nm)

Ato

mic

Con

cent

ratio

n (%

)

Co 2p

Zn 2p3/2

O 1s

C 1 s

Al 2p

0 5 10 15 200

10

20

30

40

50

60

70

80

Sputter Depth (nm)

Ato

mic

Con

cent

ratio

n (%

)

Co 2p

Zn 2p3/2

O 1s

C 1 s

Al 2p

45Å Co:ZnO/Al2O3(001) – as grown

Zn Zn indiffusionindiffusion –– does it reduce structural does it reduce structural Co(IICo(II)?)?

70Å Co:ZnO/Al2O3(001) -- Zn diffused

45Å Co:ZnO/Al2O3(001) – as grown


Cycles 1-2

4

6

810

1214

Co0Co2+

interface


Cycles 1-2

4

6

810

1214

Co0Co2+

interface

0 5 10 15 200

10

20

30

40

50

60

70

80

Sputter Depth (nm)

Ato

mic

Con

cent

ratio

n (%

)

Co 2p

Zn 2p3/2

O 1s

C 1 s

Al 2p

0 5 10 15 200

10

20

30

40

50

60

70

80

Sputter Depth (nm)

Ato

mic

Con

cent

ratio

n (%

)

Co 2p

Zn 2p3/2

O 1s

C 1 s

Al 2p

0 5 10 15 200

10

20

30

40

50

60

70

80

Sputter Depth (nm)

Ato

mic

Con

cent

ratio

n (%

)

Co 2p

Zn 2p3/2

O 1s

C 1 s

Al 2p

0 5 10 15 200

10

20

30

40

50

60

70

80

Sputter Depth (nm)

Ato

mic

Con

cent

ratio

n (%

)

Co 2p

Zn 2p3/2

O 1s

C 1 s

Al 2p

Co2+ Co0


Cycles 1-2

14

46

8

10

12

16

interface

Co2+ Co0


Cycles 1-2

14

46

8

10

12

16

Co2+ Co0


Cycles 1-2

14

46

8

10

12

16


Cycles 1-2

14

46

8

10

12

16

interface


Result from peak fitting: 36% Co(0)/64% Co(II)


Result from peak fitting: 36% Co(0)/64% Co(II)

Zn indiffusion – does it reduce structural Co(II)?Zn Zn indiffusionindiffusion –– does it reduce structural does it reduce structural Co(IICo(II)?)?


Co0Co2+

4

68

10

12

14-18}interface


Co0Co2+

4

68

10

12

14-18}interface

0 10 20 30 40 50 600

10

20

30

40

50

60

70

80

Sputter Depth (nm)

Ato

mic

Con

cent

ratio

n (%

)

Co 2p

Zn 2p3/2

O 1s

C 1 s

Al 2p

O 1s

0 10 20 30 40 50 600

10

20

30

40

50

60

70

80

Sputter Depth (nm)

Ato

mic

Con

cent

ratio

n (%

)

Co 2p

Zn 2p3/2

O 1s

C 1 s

Al 2p

O 1s

500Å Co:ZnO/Al2O3(001) – Zn diffused

Kaspar et al., PRB 77, 201303(R) (2008)

XANES and EXAFS, revisitedXANES and EXAFS, revisited

Co0.5Zn0.5 intermetallic-Mn structure Co-Zn long bonds are longer than for hcp Co metal (2.6Å in CoZn vs. 2.5Å in Co)

Co orders preferentially on c sites (x8)1/6 of Co also on d sites (x12)Co anisotropy possible if CoZn crystallites are oriented

Ferromagnetic at room temp0.8 – 1.2 B/CoTC ~ 400 – 450 K

Co - Mn?

310 nm a-Co0.1Zn0.9O

x nm a-Zni:Co0.1Zn0.9Ox = 310, 32, 7 nm

10

8

6

4

2

0

|(R

)| (Å

-3)

43210R (Å)

parallelpolarization

Co:ZnO Co:ZnO

CoZn

perpendicularpolarization

model – Co:ZnO+ CoZn(111)

singles

dimers

trimers

0 0.05 0.10 0.15 0.20x

Predicted Predicted dopantdopant distributions distributions –– MMxxZnZn11--xxOO

singlesdimerstrimers

P(x) = Cxn(1 – x)kC, n, k – varied for best fit, but not equal to binomial theorem values

Fits –Gaussians

5 nm radius MxZn1-xO nanocrystal

1.0

0.8

0.6

0.4

0.2

0.0

P(x

,n)

0.200.150.100.050.00x

singles

dimers

trimers

binomial theorem

x = 0.09

Correcting for finite size effects in MCorrecting for finite size effects in MxxZnZn11--xxOO

100 nm

7.5 nm

S/V = 5.4 x 10-2 Å-1x = 0.01

x = 0.05

x = 0.10

log

of p

roba

bilit

y of

N-m

erfo

rmat

ion

Pro

babi

lity

109876543210

N-mer

M0.01Zn0.99O

M0.05Zn0.95O

M0.10Zn0.90O

prob

abilit

y of

N-m

erfo

rmat

ion

(offs

et fo

r cla

rity)

200

100

0

A

108642Dopant Concentration (%)

1.5

1.0

W

no = 0.57

Documents

Intrinsic properties of phase-pure Co- and Mn-doped ZnO ...info.ifpan.edu.pl/spintech5/presentations/chambers...Mn-doped ZnO/ -Al 2O 3: 1. Epitaxial growth (PLD) & properties 2. Indirect