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Multiferroicity from charge ordering?a case study

Manuel AngstPeter Grünberg Institut PGI and Jülich Centre for Neutron Science JCNS,

JARA-FIT, Forschungszentrum Jülich GmbH

5th Georgian-German School and Workshop in Basic Science, Tbilisi, Aug 6, 2012

brokenspace-inversion

Multiferroics

NS

M

Magnetism: Spins

+

P

Ferroelectricity: Charge (Dipoles)

M P

H E

M,P

H,E

M,P

H,E

Multiferroicity: Spins and Dipoles

brokentime-reversal

Multiferroics : Cross-coupling

NS +

M P

Magnetism: Spins Ferroelectricity: Charge (Dipoles)

M P

H EHE

Multiferroicity: Spins and Dipoles

Readwith GMR

MRAM

Write : requires remagnetization – high currents (slow, high power consumption)

Multiferroics for non-volatile memories

Multiferroic

Readwith GMR

Write with aVoltage

MF-RAM :

MRAM

Write : requires remagnetization – high currents (slow, high power consumption)

Multiferroics for non-volatile memories

[M. Bibes and A. Barthélémy, Nat. Mater. 7, 425 (2008)]

Multiferroics : only few materials

NS

M

Magnetism: Spins

+

P

Ferroelectricity: Charge (Dipoles)

M

H

P

E

M,P

H,E

M,P

H,E

Multiferroicity: Spins and Dipoles

Very small overlapp !

+

P

Ferroelectricity: Charge (Dipoles) Any charge order breakinginversion-symmetry may be expected to be polar.

Can in principle lead to very large polarizations Spins are for free ! same electrons/sites involved in charge and spin order

sizeable magnetoelectric coupling possible

Multiferroicity from charge order

Examples ???

LuFe2O4 classical magnetite

E || c

Temperature (K)

P

LuFe2O4

Pol

ariz

atio

n (m

C/c

m2 )

Charge Order below TCO~320 K

Room temperature

hh

350 K

0-3-6-9

369

-12

0-1 1

(t,t,3/2)

(1/3+d,1/3+d,3/2)

1/3 2/3

100 keV x-ray diffraction at APS 6ID-D (normal Bragg-peaks are masked with lead)

• Sharp superstructure reflections require high degree of stoichiometry & homogeneity• Still fairly long-ranged correlations within the layers above TCO

Symmetry Analysis

G2 G1

Charged bilayersPolar bilayers(antiferroelectrically stacked)

[M. Angst et al., PRL 101, 227601 (2008)]

Refined Structure

210 K, Mo-Ka 1285 unique reflections, 104 parametersR>4[F2] = 5.91% - space group C2/m

O

Lu

[de Groot et al., and MA, PRL 108, 187601 (2012)]

100 200 300

2.0

2.5

3.0

Bond

-Vale

nce-

Sum

Temperature (K)

Fe sites with valences

TCO

Refined Structure

210 K, Mo-Ka 1285 unique reflections, 104 parametersR>4[F2] = 5.91% - space group C2/m(structure model with polar bilayers (G2) has R>4[F2] ~ 15%)

O

Lu

Bond-Valence-Sum

G1

[de Groot et al., and MA, PRL 108, 187601 (2012)]

0 100 200 3000

1

2

3

4

5

6

7

Inte

nsity

(arb

.u.)

T (K) TN

TLT

(1/3,1/3,0)

+ in-H neutron diffraction, soft x-ray resonant diffraction, …

3D magnetic order below 240 K

Spin-flip Non-Spin-flip

DNS@FRMII

[de Groot et al., and MA, PRL 108, 037206 (2012)]

0 50 100 150 200 250 300 3500

1

2

3

4

5

6

7

m 0 H

(T)

T (K)

H || c

TCO

TN

TLT

Short-rangecorrelations

Paramagnetic charge ordered

Ferrimagnetic charge ordered

6ID-D@APS

6ID-D@ APS

Inhomogeneous“glassy” phase

700 710 720 730 740 750-0.5

0.0

0.5

1.03+Fe

L2

Inte

nsity

(nor

mali

zed)

Energy (eV)

m+ m x 4 m

120 K, 4 TFe2+

L3

Relation between Spin- & Charge OrderX-ray Magnetic Circular Dichroism (XMCD) @ APS 4ID-C

+

Fe2+ net spin along H

Smaller Fe3+ netspin opposite to H

Similar XMCD results, but on not long-rangespin- & charge ordered LuFe2O4:Ko et al., PRL 103, 207202 (2009);Kuepper et al., PRB 80, 220409(R) (2009)

E || c

Temperature (K)

P

LuFe2O4

Pol

ariz

atio

n (m

C/c

m2 )

0 2 4 6 80

10

20

30

V (V

)

I (mA)

At 250 K: current develops

Change charge configurationwith an electric field ?

0 mA 50 mA

2Θ

260K

L=0.00745 Å

Real T is ~420 K

At 150 K: 15 kV/cm (3000 V) no influence on CO 6ID-B/D@APS

Similar conclusions by in-situ neutron diffraction:

• Christanson et al., unpublished• Wen et al., PRB 81, 144121 (2010)

Silver electrodes:lattice parameters as thermometer

E || c

Temperature (K)

P

LuFe2O4

Pol

ariz

atio

n (m

C/c

m2 )

[Niermann, Waschkowski, de Groot, MA, and Hemberger, PRL 109, 016405 (2012)]

Conclusion

• Structure refinement: NO polar bilayers

• XMCD combined with spin structure supports this

• Correspondingly, dielectric spectroscopy suggestsabsence of (anti)ferroelectricity

LuFe2O4 is not ferroelectric

A clear example of an oxidemultiferroic due to charge order

has yet to be found

Complex ordering phenomena in multi-functional oxides

Young-Investigators-Group funded by Helmholtz association, part of the institute of scattering methods JCNS-2 & PGI-4 (director Th. Brückel)

Joost de GrootPhD student

Thesis subject LuFe2O4

Manuel AngstGroup leader

Shilpa AdigaPhD student

Thomas MüllerDiploma student

Pankaj ThakuriaPhD student

Main external collaborations on the results presentedSoft resonant x-ray diffraction

Stew R. Bland, Peter D. Hatton

Racquel de Souza, Urs Staub

Non-Resonant XRDRaphaël P. Hermann

Jong-Woo Kim , Phil Ryan, Zahir Islam,Douglas Robinson

Neutron diffraction

Andy D. Christianson, Mark D. Lumsden,Karol Marty, Steve E. Nagler

Zahra Yamani

Karin Schmalzl Werner Schweika, Yixi Su, Walter Borghols

XMCDRichard A. Rosenberg, David J. Keavney

Dielectric spectroscopyDaniel Niermann, Joachim Hemberger

Hailey WilliamsonMaster student

3 6 9 12 15 180

1

(1/3 1/3 l) (r.l.u.)

Crystal 1 Crystal 2

Inte

nsity

(arb

.u.)

L

hh

0-3-6-9

369

-12

0-1 1

Refinement of CO crystal structure

3 domains of CO:A qA = (1/3,1/3,3/2)B qB = (-2/3,1/3,3/2)C qC = (1/3,-2/3,3/2)

The domains correspond to3 monoclinic cells rotated by120°1/3 2/3

3/2

CAB

A A

A A

B

B B

B

CC

C C (part of hex cell)

Supernova(210 K, Mo-Ka )

7.6 mg

0.25 x 0.17 x 0.03 mm

RFe2O4

nominal 2.5+

Lu

FeO

triangularFe/O bilayers

c ~ 25 Å

a ~ 3.4 Å

rhombohedral ()

Charge-frustration(V electrostatic interaction)

V

V

V

low valence

high valence

?

Spin-frustration

V

V

V

?

Spins strongly Ising ||c

±dLuFe2O4

±dFloating Zone Growth of LuFe2O4

CO/CO2 mixturecontrol O partial pressure

~10-9 bar (& O-stoichiometry)