Local Structural Properties of Magnetoresistive Materials

Outline :

Magneto-Resistive materials

I - Manganites

II - Double-Perovskites

Fabrizio Bardelli

G.I.L.D.A

Magnetoresitive materials

Conductive phase :

• external applied magnetic field

• magnetic order

Insulating phase :

• high temperatures • paramagnetic phase

MR% = x 100(H) - (H=0)

(H=0)

• Magnetoresistance (MR) :

magnetization resistivity

Sr2FeMoO6

300K

Sr2FeMoO6

4.2K

Interest in Manganites and Double-Perovskites

• High Ferromagnetic Curie temperature (TC),

(up to 450 K in double-perovskites)

Attractive both in terms of basic investigations and technological applications

• Half-Metallic Ferromagnetic (HMFM) ground state electrical current is 100% spin polarized

• TC can be raised by changing doping species and

concentration, pressure, magnetic field…

• Manganites : prototype of strong electron correlated systems

• Double-perovskites : new mechanism at the origin of magnetotransport properties

Technological applications

• Magnetic storage technology :

MR materials have been usedfor years in reading-heads of hard disks

• Future spintronic devices :

Spin-driven electronic devices :

• spin-valves• spin-injectors• tunnel junctions

Magnetic layer

Magnetic layer Magnetic layer

Magnetic layer

Non-Magnetic layer Non-Magnetic layer

Low resistance High resistance

I Manganites

Theory

Doped manganites

O2-

Mn3+A = La3+, Y3+…

B = Sr2+, Ca2+

…

Perovskite cellChemical formula

A1-x Bx MnO3

A = trivalent alkaline ion

B = divalent rare earth

Mn mixed valence :

Mn4+

The Ca-doped series : La1-xCaxMnO3

LaMnO3 CaMnO3

Mn3+ Mn4+

Jahn-Teller active ion Non Jahn-Teller active

xCa

Regularoctahedron

Axially elongatedoctahedron

• La3+ and Ca2+ are subsitutional

• La1-xCaxMnO3 solid solution can be obtained with 0 ≤ x ≤ 1 :

(M

·

cm

)

M/Ms

T (K)

Local structure and magneto-transport propertiesMagnetic

transition (TC)

Metal-to-Insulator

transition (TMI)

Structural

transition (TS)

(TC TMI TS) Local structure :

• Mn-O bond lengths

• Mn-O-Mn bond angles

ferromagnetic

conductive

reduced distortion

paramagnetic

insulating

enhanced distortion

TC 260 K

Radial distribution

of atoms around

the absorber

Firstshell

FourierTransform

R(Å)

|FT|

(a.u

.)

Extendend X-ray Absorption Spectroscopy (EXAFS)

Selective and local probe

suitable to investigate the local structure around the absorber atom

XANES valence state and geometry

around absorber

EXAFS coordination numbers (N)

bond distances (R) local lattice distortions

(2)

XANES EXAFS

AbsorptionK-edge

Experimental

I Manganites

Na-doped manganite thin films

Sample thickness (Å) TC (K) c (Å) MR (%)

750 260 3.87 45

250 260 3.87 78

125 248 3.85 78

50 insulating at any T

La0.87Na0.13MnO3

PLD grown on STO substrate

xNa= 0.13 Max. MRP.Ghigna,

University of Pavia

Substrate affects the structure of thin films :

out-of-plane

a

substrateSTO

MRfilm

tensile stress compressive stress c < a c > a

film-plane

cSubstrates :

STO = SrTiO3 (cubic 100)lattice mismatch = 0.5%

NGO = NdGaO3 (cubic 110)lattice mismatch = -0.54%

Lattice mismatchasub-afilm

afilm

substrateNGO

MRfilm

100x

Aim of this work is to study the evolution of the local structure as a function of the thickness

Strong signal from the STO substrate prevented fluorescence acquisitions

TEY has limited penetration depth => lower signal from the substate

Total Electron Yield (TEY) detectorChallenging measurements :

TEY detector design goals:

• Signal amplification in gas phase

• Low temperatures (down to 4.2 K)

• Possibility to smear-out eventual Bragg peaks from the substrate

e-

polarized electrode

He2 X-rays

to amplifier (1010)

TEY current

ground

sample

insulatingholders

Bragg condition: n= 2sin

Total Electron Yield (TEY) detector

Incident beam

Scatteredbeam

d

X-raysSample+

-

• oscillation period < 1s

• theo 10-2rad

• exp < 1°

Still sample

Oscillating sample

Fouriertransfor

m

Mn-O Mn-LaMn-Mn

EXAFS signalFIT

EXAFS : results

Increasing Mn-O distance with

decreasing the film thickness

50 Å

125 Å

250 Å 750 Å

First shell

Average Mn-O

bond lengths

RM

nO(Å

)

50 Å

125 Å

250 Å

750 Å

LatticeMismatch ?

(0.5 %)

EXAFS : discussion

Insulating behavior

Strong static Jahn-Teller distortion

of the MnO6 octahedra

Origin of the structural change :

Mn-O bonds elongation 2%

50 Åfilm

XANESpre-edge features

An increased A1-A2 pre-edge peak splitting is the signature of a

enhanced Jahn-Teller distortion (Elfimov et

al.)

A1

A2

Reduction of the out-of-plane parameter (lattice mismatch)

apical JT component constrained in the film growth plane

X-ray beam is polarized in the growth plane of the film

We are sensitive onlyto in-plane bond distances !

Film growthplane

EXAFS : discussion

50 Å film(insulating)

RMn-O 1.995 Å

JT

2 x 2.07 Å

2 x 1.92 Å

Rexp. 1.992

Bulk powder sample

insulating phaseRMn-O 1.98 Å

JT

2 x 2.07 Å

4 x 1.92 Å

Large Jahn-Teller distortion withapical component oriented in the plane of the film

• Thinnest film (50 Å) :

Fully strained structure (dead-layer)

• Thicker films (250 and 750 Å) :

Fully relaxed structure (bulk values)

As the structure relaxes thereis no more a preferred orientationfor the Jahn-Teller distortion

• Intermediate thickness film (125 Å) :

Contributions from both fully strained ( 40%) and fully relaxed structures

Manganite thin films : conclusions

II Double-Perovskites

Theory

Double-perovskite cell

• Two interpenetrating FCC sublattices

Sr2+

Fe3+

Mo5+

O2-

Sr2FeMoO6 : crystalline structure

Sr2FeMoO6 : mis-site disorder

Sr2FeMoO6

H(T)

e-

FeS=5/2

Mo Fe MoS=1/2

Mo Fe Mo Fe

FM AFM FM

Mis-site disorder : Non perfect ordering of Fe and Mo ions

Mis-site disorderreduces MR

• Subsituting Mo5+ with W6+ in Sr2FeMoO6 we obtain

the solid solution Sr2FeMoxW1-xO6 with 0 x 1

• W-doping reduces the mis-site disorder rising TC

x Mo

Mo W

Sr2FeMoO6

- Half metallic

ferromagnet

- High Curie

temperature

- Large negative MR

between 5 and 300 K

Sr2FeWO6

- Insulating at all

temperatures

- Antiferromagnetic

below 37 K

A Metal to Insulator Transition (MIT)

is expected at a certain value of x

The W-doped series : Sr2FeMoxW1-xO6

Experimental

II Double-Perovskites

Resistivity measurements indicate a critical concentration (xc) in the interval 0.2 < xc < 0.3

MIT (xc 0.25)

Insulators

Conductors

Aim of the work :

Study of the evolution of the local structure as a function of the doping level

Sr2FeMoxW1-xO6 samples

Sr2FeMoxW1-xO6

Powder bulk samples (D.D.Sarma, Bangalore) :

x = 0.0 Sr2FeWO6

x = 0.05

x = 0.15

x = 0.3

x = 0.6

x = 0.8

x = 1.0 Sr2FeMoO6

1st RSr-O N=12

shell path deg.

1st RMo-O N = 6

EXAFS results

Mo

Mo

Mo

Fe

Fe K-edge

Mo K-edge

W LIII-edge

Sr K-edge

Measured in transmission mode at 77K using Si 311monocrhomator crystals

OO

O

O

O

O Fe

Fe

Mo

O

Fe

OO

O

O

O

O

Sr

O

O

O

O

O

• Abrupt change in the local structure crossing xc

• expansion of the FeO6 octahedra Fe3+ Fe2+

• Contraption of MnO6 octahedra and of the Sr-O bonds

EXAFS : first shell results

Fe-O

Mo-O

W-O

Sr-O

XRD Fe-O

XRD Mo/W-O

XRD Sr-O

• XRD data (Sanchez et al.) report a smooth evolution with x

AFMinsulating

FMmetallic

FMmetallic

AFMinsulating

Energy (eV) Energy (eV)

XANES spectra : Fe and Mo K-edges

x = 0.3

x = 0.6x = 0.8

x = 0.15

x = 0.05

x = 0.0

x = 1.0

x = 0.3

x = 0.6

x = 0.8

x = 0.15

x = 0.05

x = 1.0

Huge and abrupt change of the charge distribution crossing xc

Fe edge : change in the valence state (edge position)

Mo edge : evidence of localization of the charge carrier in the insulating phase

xc

xc

insulating insulating

metallic metallic

XANES spectra : W LIII- and Sr K-edges

x = 0.3

x = 0.6

x = 0.8

x = 0.15

x = 0.05

x = 0.0

x = 0.3

x = 0.6

x = 0.8

x = 0.15

x = 0.05

x = 0.0

x = 1.0

W edge : No detectable changes, neither in the local structure nor in the valence state

xc

xc

insulating

metallic

insulating

metallic

XANES : Fe edge considerations

XANES spectra of doped

compounds can be

fitted by a linear

combination of the two

end compounds

( Sr2FeMoO6 -

Sr2FeWO6 )

with as fitting

parameter.

fit(x) = • exp(Sr2FeMoO6) + (1 - ) •

exp(Sr2FeWO6)

1.2

0.4

(E)Fit

x = 0.6

• Excess of the metallic Sr2FeMoO6 - like structure in the FM

phase

• The sistem does not change structure up to the critical

concentration

Fe3+ Fe2+

FM Fe-Mo clustersare isolated by non

magnetic Fe-W clusters

2. Percolative transition

Mo5+ W6+

metallic Fe-Mo FMclusters connects each

other permitting conduction

Metal to Insulator Transition : two hypothesis (Kobayashi)

Fe2+ Mo6+ W6+

1. Valence transitionHMFM region Insulating region

Fe3+ Mo5+ W5+

XANES W does not change its valence state !

XANES excess of metallic/Sr2FeMoO6-like structure in the FM phase

Neither the valence transition nor the percolation scenario can describe the system !

Double-perovskites : conclusions

• EXAFS and XANES data depicts the microstructural counterpart of the Metal to Insulator Transition

• Contrary to XRD results we see an abrupt change of the local structure crossing the critical concentration

• XANES data show that neither the percolative nor the valence transition are good models to describe the system

• More quantitative analisys is needed on the XANES spectra

Acknowledgements

GILDA scientific group :

Prof. S. MobilioDr. F. D'AcapitoDr. C. Maurizio

M. Rovezzi

Gilda technicians group:

F. D'AncaF. LamannaV. SciarraV. Tullio

Collaborators :

C. Meneghini – University of "Roma Tre"P. Ghigna – University of Pavia

D.D. Sarma – Bangalore Institute of Science

Sr2FeMoO6 : kinetic driven mechanism (D.D. Sarma 2001)

3d5

4d1Ecry

Ecry

eg

eg

t2g

t2g

Eex

eg

eg

t2g

t2g

Ecry

Ecry

Eex

t2g EF

Eex > Ecry

Eex < Ecry Fe-Mo hybrid levels

in presence of hopping interaction

sAFM coupling between Mo delocalised and Fe localised electrons leads to FM coupling of the Fe

sublattice

Fe3+ S=5/2

Mo5+ S=1/2

Mo and W charge carriersbelong to the Fe-Mo hybrid

band

Adding W changes neither the structure nor the charge

distribution

Below a critical concentrationconduction band disappears due to

the low level of Mo ions

Charge carriers localize on Mo and Fe sites

Charge localization induces a change of the Fe valence state (Fe3+ Fe2+)

The greater ionic radius of Fe2+ drives theobserved transition of the local structure

Metallic phaseInsulating phase

Sr2FeMoO6 : ground state 1

Ground state is Semimetallic

:

• Up-spin :

gap at the

Fermi level (EF)

• Down-spin :

finite DOS at EF

Up-spin states () insulator

Down-spin states () : conductor

Fully spin-polarized

mobile charges !

O2p Mo/Fe t2g

O2p

3.9 eV

EF

Up Spin

Down Spin

Fe eg Mo t2g 0.5 eVFe eg

La1-xCaxMnO3

xCa

T(K)

paramagneticinsulating

350

300

250

200

150

100

50

00 0.2 0.4 0.6 0.8 1

LaMnO3 CaMnO3

Temperaturevs doping phase diagram

Doped manganiteshave complexphase diagram

FM = FerroMagnetic

AF = AntiFerromagnetic

CAF = Canted AF

FI = FM Insulator

CO = Charge Ordered

Maximum MR at x = 0.25

FM-MR conductive

phase

Jahn-Teller polaron = charge carrier + Jahn-Teller lattice distortion

Enhanced effective mass reduced mobility

Transport properties : antagonist mechanisms

Electron-phonon coupling (Millis, 1994 …forty years later

!)

Strong on-site Hund coupling

Transfer integral cos()

Predicted Tc is too high !

Double-Exchange (Zener, 1951)

: Mn3+ - O2- - Mn4+

eg

s=1/2

t2g

S = 3/2Mn4+ Mn3+ O2-

t2g

S = 3/2

: Mn4+ - O2- - Mn3+

eg

s=1/2

t2g

S = 3/2Mn3+ Mn4+ O2-

t2g

S = 3/2

Summarising

XANES and EXAFS :• Abrupt change of the local structure crossing xc

• System does not change adding W until xc is reached• W local structure does not change in the whole x range

XANES :• Excess of metallic/Sr2FeMoO6-like clusters in the FM phase• Evidences of charge localization on Mo and Fe sites in the AFM phase• W does not change valence !

Valence state model predicts a change of the W valence state

Percolative model predicts Sr2FeMoO6 and Sr2FeWO6 changing in weight according to the nominal concentrationsNeither the percolative nor the

percolation scenario can describe the system !

OMn

DE : delocalised Mo 4d1 electron plays the role

of the delocalised electron in manganites

SE : the interaction ismediated by virtualelectron hoppings intounoppupied Mo d

states

Double-Exchange (DE) vs Super-exchange (SE)

Other mechanism ?

But :Localised up-spin band at Fe site is fully filled=> Delocalised electron must be down-spin !

Therefore :

Strong on-site Hund strenght, which couples FM localised and delocalised electrons in manganites, cannot be invoked in the case of double-perovkites

vs

Transport mechanism 1Electronic levels

1. t2g Localised electrons form a core with S = 3/2

2. eg conduction electron belongs to the Mn 3d – O 2p hybrid derived states

3. Strong on-site Hund strenght couples FM the localised and delocalised electrons

3d4

Mn4+-site

3d3

eg

t2g

dx2

-y2

dz2

eg

Mn3+-site

t2g

crystal field crystal field + Jahn-Teller

Peaks splitting originatesfrom the crystal field which is influenced bythe Jahn-Teller distortion

eg

t2g

A2A1

E(t2g - eg)

E(A1 - A2)

A1

A2

A1A2

bulk

50 Å

6536 6538 6540 6542 6544

Energy (eV)

ab

sorp

tio

n

0.20

0.15

0.10

0.05

0.00

The large A1 – A2 energy splitting in the thinnest film is the signature of alarge Jahn-Teller distortion

E(A1 - A2) E(t2g - eg)

XANES