Stoichiometry and crystal quality in LuFe2O4-δ and YbFe2O4-δ
H. L. Williamson1,2, J. de Groot1, M. Herlitschke1,3, R. A. McKinnon2, M. R. Lees2, G. Balakrishnan2, R. P. Hermann1,3, M. Angst1
1
Jülich Centre for Neutron Science JCNS and Peter Grünberg Institut PGI, JARA-Fit, Forschungszentrum Jülich, D-52425 Jülich, Germany.
2 Department of Physics, The University of Warwick, CV4 7AL, Coventry, UK.
3 Faculté des Sciences, Université de Liége, B-4000 Liége, Belgium.
Motivation
Structure
complex coupling between electricity
and magnetism (Magnetoelectric
coupling) where a system can
become polarized by a magnetic field
and vice versa.
Multiferroic’s
The rhombohedral RFe2O4-δ (R =
Lu, Yb, Y, Tm, Ho and Er)
system:
• Average valance Fe2.5+
Fe2+/Fe3+ CO
CO/CO2=1:3
LuFe2O4-δ
CO/CO2=1:5
90 K
(009)
(2/3 2/3 l)
0.02
(2/3 2/3 l)
(006)
230 K
0.01
0.00
0.00
50
100
150
200
250
300
0
T(K)
hhl
Smeared out magnetic
transition and only a
small peak at the CO
transition.
200
180
C(J/mol K)
350 K
170
314 K
160
150
260
280
300
320
340
T(K)
50
100
150
200
C(J/mol K)
(009)
0
T(K)
250
300
350
T(K)
Single crystal x-ray data
(SCXR)
no 3D charge
order, only diffuse super
structure lines along the
(1/3 1/3 l).
200
250
300
Specific Heat
180
160
150
50
150
300 K
230K
100
100
(009)
Specific Heat
LuFe2O4
50
TN= 230 K
LuFe2O4
120
100
80
60
100
150
200
250
300
(006)
300 K
TCO= 314 K
1.85 K - 300 K
150 K - 400K
140
SCXR reveals 3D CO
superstructure at 90 K up
to 300 K. At T=350 K the
CO is destroyed and only
weak diffuse lines are left.
hhl
0
C(J/mol K)
Sharp magnetic transition at
TN= 235 K. Indication of
further transition at 175 K on
cooling in good agreement
with Christianson et al. [6].
TN= 235 K
ZFC
FC
0.03
230 K
TLT= 182 K
TLT= 169 K
LuFe2O4
0.04
0.01
0
100 Oe ||c
M(f.u)
M(f.u)
0.05
Very broad ferrimagnetic
transition at 202 K, no
evidence of the previously
reported transition at 175 K
[6].
TN= 202 K
FC
ZFC
0.02
Single Crystal X-Ray Diffraction
90 K
Magnetometry
LuFe2O4
0.03
Crystal growth of LFO and YbFe2O4-δ
(YbFO) via optical floating zone
method using gas ratios of
CO/CO2=1:3 and CO/CO2=1:5 to
tune O-stoichiometry and study
property changes.
Single Crystal X-Ray Diffraction
Magnetometry
0.04 100 Oe ||c
Off stoichiometric crystals without
long range magnetic order present
broad transitions compared to sharp
transitions of near stoichiometric LuFe2O4 crystal
crystals.
The specific CO configuration in the bilayers was initially thought to
produce a ferroelectric polarization due to an imbalance of Fe2+ and
Fe3+ in the two layers of the bilayer [4]. However our recent
investigations indicate that the CO configuration is LFO is actually
non-polar [3].
This has led to the investigations of
other members of the RFe2O4 series
(R=rare earth)
YbFe2O4 crystal
Recent studies on LFO report large
differences in crystal quality based on
oxygen stoichiometry [5]. A systematic
study is needed:
(a) Refined monoclinic
crystal structure C2/m of
charge ordered LFO. The
ferrimagnetic high-field
spin order and Fe3+/2+
charge order is
represented by arrows
and different colors
respectively [3].
• triangular Fe bilayers
separated by Lu monolayers.
which allow ferroelectricity through
charge order (CO), with particular
interest in LuFe2O4-δ (LFO) since the
discovery of interesting magnetic and
electrical characteristics .
[2]
Crystal Growth
350
400
450
(006)
Two sharp transitions, much
more defined than that of the
(1:3) crystal. In particular the
onset of CO at TCO=317 K,
which correlates nicely with
the SCXR data.
350 K
(006)
T(K)
(009)
hh0
hh0
YbFe2O4-δ
CO/CO2=1:3
Single Crystal X-Ray Diffraction
Mössbauer Spectroscopy
YbFO Mössbauer
LFO Mössbauer
90 K
230 K
Magnetometry
0.006
100 Oe ||c T= 145 K
T= 246 K •
T= 218 K
-3
emu/g (x10 )
YbFe2O4
Magnetic transition similar
to that seen in LFO but
shifted to 246 K.
0.004
CO/CO2=1:5
From LFO crystal growth- (1:5)
gas ratio produced better
stoichiometric crystals.
YbFO single phase powder
synthesized in (1:3).
(006)
FC
• Transitons at 218 K, 145 K
and
low
temperatures
require
more
detailed
investigation.
0.002
ZFC
230 K
YbFO crystals grown in (1:5).
1
50
100
150
200
250
300
T(K)
Specific Heat
200
TCO=305 K
YbFe2O4
C(J/mol K)
(006)
300 K
Charge order transition
similar to that seen in the
specific heat data of LFO
with
slightly
lower
temperature.
160
2500
YbFO Mössbauer data reveals similar Fe2+
and Fe3+ spliting between 250 K and 310 K
as the LFO data [7].
2000
(006)
350 K
140
120
150
200
250
300
T(K)
Outlook
350
Powder x-ray confirmed single
phase YbFO decomposed into 4
different phases:
•
•
•
•
[7]
180
Mitglied der Helmholtz-Gemeinschaft
Mossbauer spectroscopy
was carried out using a
57Co source.
400
CDW or will better stoichiometric
crystals show similar 3D CO
found in LuFe2O4-δ ?
[8]
• Mössbauer analysis of YbFO- peak fitting.
• Neutron scattering experiments to uncover origins of muliple transiton points
showin in magnetization YbFO data.
• Refine YbFO SCXR data at low and higher temperatures measured.
• Determine true oxygen content in both types of LFO crystals and YbFO (1:3).
• Grow YbFO in CO/CO2 = 1:3.5 to view stoichiometric changes.
Hearmon et al. [8]
describes charge ordering
as incommensurate
charge density waves
(CDW).
Intensity
0
hhl
0.000
YbFe2O4
YbFe2O4
YbFeO3
Yb2Fe3O7
Yb3Fe5O12
Sintered powder
Powdered crystal
CO/CO2=1:5
1500
1000
500
(006)
(2/3 2/3 l)
0
hh0
Preliminary SCXR data of YbFO.
10
30
40
2
50
60
70
References
Acknowledgement
B. Klobes is acknowledged for assistance during the
magnetometry measurements. MA and RW achknowledge the
Helmholtz Association for funding of their Young Investigator
Groups ‘Complex ordering phenomena in multifunctional oxides‘
and ‘Lattice Dynamics in Emerging Functional Materials‘. Work at
Warwick is supported by the EPSRC, UK.
20
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
.
S.-W. Cheong and M. Mostovoy, Nat. Mater. 6, 13 (2007).
Spaldin & Fiebig, Science 309, 391 (2005).
de Groot et al., Phys. Rev. Lett. in press; Preprint on arXiv: 1112.0978.
Ikeda et al., Nature 436, 1136 (2005).
de Groot et al., Phys. Rev. Lett. 108, 037206 (2012).
Christianson et al., PRL 100, 107601 (2008).
Xu et al., PRL 101, 227602 (2008).
Hearmon et al., Phys. Rev. B 85, 014115 (2012).