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Multiferroicity from charge ordering ? a case study. Manuel Angst. Peter Grünberg Institut PGI and Jülich Centre for Neutron Science JCNS, JARA-FIT, Forschungszentrum Jülich GmbH. 5 th Georgian -German School and Workshop in Basic Science, Tbilisi , Aug 6, 2012. Magnetism: Spins. - PowerPoint PPT Presentation
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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)