Studies of Charge Order in Single Crystals of LuFe2O4 and Fe2OBO3

David Mandrus

Oak Ridge National Laboratory

Work supported by BES Division of Materials Sciences and Engineering

Manuel Angst

Collaborators

ORNL -- M. Angst, B.C. Sales, D.-H. Kim, M. Lumsden,S. E. Nagler, A. Christianson, G. Veith, W. Tian, D. Mandrus

UT – J. Musfeldt, Y. Luan, V. Keppens

UMass -- P. Khalifah

Julich – R. Hermann, W. Schweika

N.C. State – H. J. Xiang, M.-H. Whangbo

Ames -- J. W. Kim

UK – V. Varadarajan, J. W. Brill

Special thanks to Art Sleight

Andy Christianson

Motivation

• Search for model charge order systems in which electrostaticsplay an important role

• Charge ordering is an intriguing route to ferroelectricity, and there may be strong coupling between magnetic & ferroelectric order parameters

• ORNL has good tools for these studies – i.e., neutron scattering,electron microscopy, piezoelectric force microscopy

Outline

LuFe2O4 – snapshot of work in progress• Magnetization & thermal properties• Neutron scattering• Resonant Ultrasound Spectroscopy• IR spectroscopy

Fe2OBO3 – summarize results presented in 2 papers*• first growth of single crystals• first observation of C.O. superstructure• BVS analysis showing integer iron valence• discovery of incommensurate phase

*M. Angst, et al. PRL 99, 086403 (2007)M. Angst, et al. cond-mat: 0707.3127

M. Subramanian et al., Adv. Mater. 18, 1737 (2006)

Large Magneto-Dielectric Effect in LuFe2O4

M. Subramanian et al., Adv. Mater. (2006)

Structure of LuFe2O4

Iida, et al. JPSJ (1993)

Proposed Charge Ordering Pattern (Ikeda)

Yamada & Ikeda, JKPS (1998)

Ikeda, Nohdo, Yamada JKPS (1998)

LuFe2O4 Optical Floating Zone Growth

0 50 100 150 200 250 3000.00.20.40.60.81.01.21.41.61.82.02.22.42.62.83.03.2

2 T 1.5 T 1 T 0.1 T 0.03 T 0.07 T 0.05 T

M (μ

B /f.

u.)

T (K)

H//c

LuFe2O4 Magnetic Response

0 50 100 150 200 250 3000.0

0.2

0.4M

(μB

/f.u.

)

T (K)

0.07 T || c (field cooled)

100 150 200 250 300 350

6

8

10

12

14

16| c

orre

cted

hea

t flo

w |

(mW

)

T (K)

40 C / min

Thermal Properties

TCOTN

140

150

160

170

180

190

250 300 350 T (K)

Cp (J

/mol

/K)

preliminaryheat capacity(PPMS)

3D Charge Order below 320 K

• 3D charge order develops below 320 K and is observed with propagation vector (1/3 1/3 1/2)

290 300 310 320 3300

40

80

120

160

200

Inte

nsity

(~C

ount

s/10

sec

onds

)

T (K)

(1/3 1/3 10.5)

-2 0 2 4 6 8 10 12 14 16 18 20 22 24

0

20

40

60

Inte

nsity

(~C

ount

s/se

cond

)

(1/3 1/3 L)

280 K

Neutron work byAndy ChristiansonShull Fellow, ORNL

Magnetic Ordering In LuFe2O4

• Two Magnetic transitions-240 K (previously observed) and 180 K (new).• Both transitions are 3D, however the peaks are not resolution limited (indicating a

finite correlation length).• Intensity occurs on peaks indexed as (1/3 1/3 L) where L is either an integer or half

integer. The L integer peaks are the fundamental magnetic peaks and the half integer peaks are a consequence of the charge order at 320 K.

0 50 100 150 200 250 300

0.0

0.1

0.2

0.3

0.4

0.5

Inte

grat

ed In

tens

ity (A

rb. U

nits

)

T (K)

LuFe2O4

(1/3 1/3 0)

-6 -4 -2 0 2 4 60

40

80

120

160

Inte

nsity

(~co

unts

/sec

ond)

(1/3 1/3 L)

280 K 220 K 110 K

Magnetic Structure at 220 K

• Peaks at Integer L are purely magnetic while peaks at ½ integer L are related to both the charge order and magnetic order.

• The magnetic contribution to the L=1/2 integer peaks arises solely due to the charge order.

-8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 200

40

80

120

160

In

tens

ity (c

ount

s/se

cond

)

(1/3 1/3 L)

220 K

Magnetic Structure of LuFe2O4 at T = 220 K

Spins point along c-axisOrdering wavevector (1/3, 1/3, 0)3 symmetry equivalent mag. structures2/3 moments point up, 1/3 downConsistent with magnetization

0 100 200 3000

1

2

3M

(μB

/f.u.

)

T (K)

2 T || c (field cooled)

~ consistent with the ferrimagnetic spin model:

• on average 4.5 μB / Fe atom• two Fe / formula unit• 2/3 of spins pointing in one direction,

1/3 in the opposite direction→ net moment is 1/3 of the total

(3 μB )

180 K Transition

• Significant broadening occurs in magnetic peaks below 180 K.• A 2D component to the scattering becomes easily observable below 180 K, however,

this appears to be the remnants of unordered parts of the sample as this 2D diffuse scattering is stronger in samples with worse oxygen stoichiometry (comparison above). This is consistent with the old work which shows only 2D scattering.

-9 -6 -3 0 3 6 90

4 0

8 0

1 2 0

1 6 0

0

4 0

8 0

1 2 0

1 6 0

(1 /3 1 /3 L )

(b ) 2 2 5 K 1 1 0 K

In

tens

ity (a

rb. u

nits

)( a )

2 2 0 K 1 3 0 K

Low Temperature Satellites

• New satellites appear below 180 K. At present the origin is unknown, however they appear to not be magnetic in origin.

-0.36 -0.32 -0.28

0

1

2

3

4

-4 -2 0 2 40.0

0.1

0.2

0.3

0 40 80 120 160 2000.000

0.005

0.010

0.015

Intensity (Arb. U

nits)

(h h 0)

130 K 220 K

Inte

nsity

(Arb

. Uni

ts)

(0.3083 0.3083 L)

Inte

grat

ed In

tens

ity (A

rb. U

nits

)

T (K)

Warming (0.306 0.306 0)

advantages of RUS: all elastic constants can be obtained in one measurement

small samples (mm3)

The University of Tennessee

1.0x106 1.1x106 1.1x106 1.1x106 1.2x1060.0

0.5

1.0

1.5

2.0

2.5

Ampl

itude

(V)

Frequency (Hz)

Veerle Keppens

0

T

Mo

du

lus

0T

Strain-Order Parameter Coupling at Phase Transitions

Bi-Linear coupling:Fc = βQε

Quadratic coupling:Fc = βQ2ε

Mo

du

lus

θ−−=

Tacc 0

c

c

TThcc

TTcc

<−=

>=

β

20

0

2

Magnetite Displays Bi-Linear Coupling at Verwey Transition

Schwenk et al., Eur. Phys. J. B 13, 491 (2000)

Elastic strain induced by the ultrasonic wave couples to the charge fluctuation modes

0 50 100 150 200 250 300 350 4000.82

0.83

0.84

0.85

0.86

0.87

0.88

Fr

eque

ncy

(MH

z)

Temperature (K)

0 Tesla 5 Tesla (perp. c) 5 Tesla (// c)

LuFe2O4 RUS Measurements in Magnetic Field

0 5000 10000 15000 20000

738.8

739.0

739.2

739.4

739.6

739.8

740.0

740.2

740.4

740.6

Freq

uenc

y (k

Hz)

Field (Oe)

260 K

RUS Measurements in Magnetic Field

Xiang and Whangbo, PRL (2007)

Optical Properties of LuFe2O4: Musfeldt Group, University of Tennessee

Fe2+ → Fe3+ charge transfer, minority channel

Fe2+ → Fe2+ on-siteexcitation, minority channel

O p → Lu excitation,majority channel

O p → Fe d chargetransfer, minority channel

• Optical properties are dominated by excitations in the minority spin channel• √3 x √3 and chain structures have similar density of states pictures. Possibility of extracting

domain distribution information via temperature dependence of 1.5 eV excitation. • Magneto-optical work in progress.

Assuming 50% Fe2+ and 50% Fe3+ is compatible with the spectra.

Upon heating, the charge order vanishes with an increase in the electron hopping frequency.

Below 260 K the hopping is slower than resolvable, ie than 0.1 MHz.

Mössbauer Spectroscopy (R. Hermann)

Fe2OBO3

Fe2OBO3→ monoclinic distortion below 317 K

Half-width of the combinedmonoclinically split(102)/(–102) doublet [Attfield et al., Nature 396, 655 (1998)]

Two structurally distinct metalsites with octahedral coordination(I, II)

Warwickite type MIM`IIOBO3 usually orthorhombic

O

B

MI

MII

ac

b

ribbonsβ

Fe2OBO3Charge order surmised based on Mössbauer spectroscopy and resitivity:

[Attfield et al., Nature 396, 655 (1998)]

Rapid hopping on two structural sites: Fe2.5+

Four doublets assigned to Fe2+/Fe3+ on two struct. Sites,no discernible hopping

Fe2OBO3Should lead to unit cell doubling in a direction[weak (h+0.5 k l) reflections]

not observed (on polycrystalline material)

• No superstructure observed &• Resistivity feature associated

with CO very broad

→ Single-crystal study highly desirable

a

possible charge order model[Attfield et al., Nature 396, 655 (1998)]

One ribbon

Fe3+

Fe2+

Single crystals of Fe2OBO3Try flux growth in Fe – B – O system Starting point : growing FeBO3 fromFe2O3 – B2O3 – PbO – PbF2 flux[following Kotrobova et al.,J. Crystal Growth 71, 607 (1985)]

Then tune growth parameters to favor crystallisation of Fe2OBO3

Fe2OBO3 crystallised as thin needles (||a) of weight up to ~1 mgPhase confirmed by powder x-ray diffraction (and various phys. propert.)

Fe2OBO3 : electron diffraction ([001] zone)

T = 350 K T = 300 K T = 117 K

(040)

(200)

a*

Experiments performed by Jing Tao on Urbana TEM

0

2

4

6

-2 -1 0

000

200110020

355 K

k =

h =

100 K

a*

b*

X-ray diffraction:Doubling of unit cellalong a at low T

average structure

Fe1a

Fe1b

Fe2b

Fe2a

2a

Fe1d

Fe1c

Fe2c

Fe2d

100 K : “up” - domain

=

= “down” -domain +

+

Structurerefinement

2.0

2.2

2.4

2.6

2.8

3.0

100 K

EuBaFe2O5

100 K 295 K

Fe3O4Fe2OBO3

Fe1aFe2a

Fe1bFe2b

90 K

Bond

-Val

ence

-Sum

Fe2OBO3 seems best example so far of ionic charge order

F. Menil, J. Phys. Chem. Solids 46, 763 (1985)

“…the inductive effect of the antagonistic bond…”

From Art Sleight:

In EuBaFe2O5, the impact of the Eu and Ba would be to increase Fe-O covalency relative to Fe3O4. In Fe2OBO3, the impact of B would be to increase ionicity relative to Fe3O4. And I am convinced that it is this increased ionicity that gives basically the ideal values in Fe2OBO3.

Fe – O – T

10

100

1000

10000

260 280 300 320 340 360 380 400

-5

0

5

10

ρ (Ω

cm)

T (K)

heat

flow

(W / m

ol)

Fe2OBO3 : intermediate phase

Resistivity and Differential scanning calorimetry :Two phase transitionsI

II

20 K / min

Ea ≈ 0.16 eV

Ea ≈ 0.36 eV

Ea ≈ 0.4 eV Activated transport :(ρ ∝ eEa /kT )

10

100

1000

10000

-5

0

5

10

260 280 300 320 340 360 380 40090.090.190.290.390.4

ρ (Ω

cm)

heat

flow

(W / m

ol)

T (K)

β (°

)

III

Structurally, IIis monoclinic-orthorhombictransition

20 K / min

Fe2OBO3 : intermediate phase

Hysteresis → II (weakly) first ordertoo

0.5 K / min

(Viji Varadarajan, Joe Brill, UK)

II

Fe2OBO3 : intermediate phase

Intensity (counts/mon.)

0

100

200

300

2.95 3.05

2.9

3.0

3.1

100 K(1/2, k, l )

k (r.l.u.)

l (r

.l.u.

)

260 280 300 320 340

2.5

3.0

3.5

T (K)

l (r.

l.u.)

(3/2, 3,l )(cooling)

260 280 300 320 3400.0

0.5(3/2, 0,l )(cooling)

T (K)

l (r.

l.u.)

1.2 1.5 1.80.02

0.04

0.06

2.6 2.8 3.0 3.2 3.4 2.0 2.2 2.4 2.6 2.8 3.0

int

ensi

ty (c

ount

s/m

on.)

ξa~ 27a

(h,3,5/2)

h (r.l.u.)

ξb ~ 2b

k (r.l.u.)

(3/2,k,5/2)

400 K

ξc ~ 2c

350 K(3/2,3,l )

k (r.l.u.)

Correlations Above CO Transition

Intensity (counts/mon.)

0

100

200

300 260 280 300 320 340

2.5

3.0

3.5

T (K)

l (r.

l.u.)

(3/2, 3,l )(cooling)

commensurateCO(static)

incommensurateCO(partly dynamic)

dynamicshort-rangeorder

2.95 3.05

2.9

3.0

3.1

100 K(1/2, k, l )

k (r.l.u.)

l (r

.l.u.

)

Incommensurate Phase Belongs to Class of Phenomena Known as “Modulated Phases“

Malis, Ludwig, PRB 60, 14675 (1999)

“the origin of the modulated phase is still under debate”

Cu-Au 50-50 at. %

Above 410 °C stable phase disordered

Below 385 °C equilibrium phase CuAu I, in which Cu and Au occupy alternating atomicLayers

Between these 2 transition temperatures CuAu orders into a one dimensional longPeriod superlattice called CuAu II, whichconsists of a periodic array of antiphaseboundaries with an average modulationwavelength approx. 10 times the sizeof the underlying CuAu cell

Future Work

Optics – Willie Padilla,Boston College

“Switching” Effects in IV curves -- Ana Akrap, L. Forro, EPFL

Impedance Spectroscopy, search for ferroelectricity

TEM work

Summary

Crystal growth of Fe2OBO3

Found C.O. superstructure reflectionsShowed that Fe2OBO3 is best example to date of ionic C.O.Found the incommensurate phase, assigned it to the same

class of phenomena as the CuAu II “modulated phase”

Papers: M. Angst, et al. PRL 99, 086403 (2007); M. Angst, et al. cond-mat: 0707.3127

Fe2OBO3

LuFe2O4

Oxygen stoichiometry strongly affects physical propertiesObservation of thermal anomaly at 330 K3D magnetic structure solvedNew phase transition observed at 180 K