Phonon Spectroscopy Near Phase Transition Temperatures in Multferroic BiFeO3 Epitaxial Thin Films

0.4 MB·

by  R. Palai

R. Palai et al.
Phonon Spectroscopy Near Phase Transition Temperatures in Multferroic BiFeO
3
Epitaxial Thin Films
R. Palai1, J.F. Scott2, and R.S. Katiyar1
1Department of Physics and Institute for Functional Nanomaterials,
University of Puerto Rico, San Juan, PR 00931-3343, USA
2Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, UK
(Dated: January 12, 2010)
We report a Raman scattering investigation of multiferroic bismuth ferrite (BiFeO3) epitaxial
0 (c-axis oriented) thin films from -192 to 1000◦C. Phonon anomalies have been observed in three
1 temperatureregions: intheγ-phasefrom930◦Cto950◦C;at∼370◦C,Ne´eltemperature(TN),and
0 at ∼ -123◦C, due to a phase transition of unknown type(magnetic or structural). An attempt has
2 beenmade tounderstandtheorigin of theweak phonon-magnon couplingand thedynamicsof the
n phase sequence. The disappearance of several Raman modes at ∼ 820◦C (Tc) is compatible with
a the known structural phase transition and the Pbnm orthoferrite space group assigned by Arnold
J et al. [1]. The spectra also revealed a non-cubic β-phase from 820-930◦C and the same non-cubic
2 phaseextendsthroughtheγ-phasebetween930-950◦C,inagreementwithArnoldet al. [2],andan
1 evidenceofacubicδ-phasearound1000◦Cinthinfilmsthatisnotstableinpowderandbulk. Such
acubicphasehasbeentheoretically predictedin[3]. Micro-RamanscatteringandX-raydiffraction
◦
] showed nostructural decomposition in thin films duringthethermal cycling from 22-1000 C.
i
c
s PACSnumbers: 77.55.+f,78.30.-j,77.80.Bh,78.66.Nk
-
l
r
t I. INTRODUCTION roelectric up to ≈ 820◦C [14], and ferroelastic between
m
◦
820-930 C [15]. Bulk BFO crystallizes in a rhombohe-
t. dral (a = 5.58 ˚A and α = 89.50) structure at room
a Multiferroics are the materials which display a coexis- temperature (RT) with space group R3c (C6 ) and an-
m tence of at least two of the switchable states: polariza- 3v
tiferromagnetism of G-type [13, 16]. The structure and
- tion, magnetization or strain in the same phase [4]. In properties of bulk BFO have been studied extensively
d addition, they may also exhibit a magnetoelectric (ME)
n [13,16–18]andalthoughearlyvaluesofpolarizationwere
effect: magnetization induced by an electric field and
o electric polarization by means of magnetic field [5]. The low (Pr = 6.1 µC/cm2) due to sample quality, Pr = 40-
c 100 µC/cm2 was recently found in bulk by several dif-
[ current interest in multiferroics is largely based on engi- ferent groups [19, 20]. The epitaxially grown thin films
1 ntheeeirredphepyistiacaxliaplraonpderhteietesroasrteruacstugroeoddthaisnbfiulmlks,abnedcapuesre- of BFO on SrTiO3 (STO) substrates show very high
v mit technological applications in data storage, magnetic values of Pr(∼ 100 µC/cm2) [6] compared to the best
7 known ferroelectrics such as PbZrTiO (∼ 70µC/cm2)
recording,spintronics,quantumelectromagnets,andsen- 3
9 and BaSrTiO (∼ 30µC/cm2). This makes BFO a po-
9 sors [6–8]. Devices made up of multiferroic materials 3
tential material for novel device applications.
1 can perform more than one task and facilitate device
The motivationforthe presentstudy is manifold. The
. miniaturization. A weak ME effect has been observed
1 first objective is to test the recent space group deter-
0 in most multiferroics, generally showing a small change
mination of the γ-phase reported by Arnold et al. [1]
0 intheirspontaneouspolarizationunderappliedmagnetic
as being orthorhombic. Their definitive neutron study
1 field [5, 9, 10]. However, the complete switching of fer-
showed that the γ-phase is indeed stable (which in itself
: roelectric domains by applied magnetic fields has rarely
v had been controversial), and that it has the same or-
i been observed. Why and under what circumstances a
X thorhombic Pbnm orthoferrite symmetry as does the β-
large coupling should exist and how to control the cou-
phase. AcubicPm3mperovskitestructurewasdefinitely
r plingarestillopenquestions. Understandingthephysics
a ruledout,althoughabody-centeredorthorhombicspace
of the different possible interactions between magnetic
groupwasindistinguishablefromtheprimitivePbnm. A
and electric order parameters i.e. giving rise to magne-
main aim of our Raman study is to test the orthorhom-
toelectric (ME) coupling would be very useful.
bic crystal class for the γ-phase and see whether we can
Magnetism and ferroelectricity are involved with lo-
further distinguish between primitive and body-centered
calspinorderingandoff-centerstructuraldistortions,re-
orthorhombic structures.
spectively [11]. These arequite complementaryphenom-
ena that coexist in certain multiferroic materials. Cur-
rently, BiFeO (BFO) is one of the most widely studied
3
II. EXPERIMENTAL DETAILS
multiferroicsbecauseitisoneofonlytwoorthreesingle-
phase multiferroics at room temperature i.e an antifer-
romagnetic(AFM)incommensuratephasewithcycloidal We investigated300nm (001)BFO thin films onSTO
modulation (λ ≈ 60 nm) below ≈ 370 ◦C [12, 13], fer- (100) substrates with ∼ 25 nm thick SrRuO (SRO)
3
2
buffer layer by pulsed laser deposition (PLD). A Jovin that this is a rhombohedral-rhombohedral symmetry-
Yvon T64000 micro-Raman microprobe system with Ar preservingMotttransition,thatseemsquiteunlikely,be-
ionlaser(λ= 514.5nm) inbackscatteringgeometrywas causeHaumontet al. [22]havefoundseveralphasetran-
used for polarized and temperature dependent Raman sitions at lower pressure. Thus the symmetry of BiFeO
3
scattering. Sample deposition and experimental details is not rhombohedral on either side of the high-pressure
are given in [15]. metal-insulator transition. Whether the transition is a
The X-ray diffraction (XRD) pattern (see Fig. 2a) of Motttransitionorabandtransitionisunproven. Various
the BFO films taken using CuK (1.5406˚A) radiation theoretical models disagree: Vasquez et al. [3] get an ab
α
show c-axis (pseudo-cubic <001> direction perpendicu- initio Mott transition; Clark et al. [23] got a band tran-
lar to the substrate) orientation with a high degree of sition to a semimetal from a screened exchange model.
crystallinity. The c-axis length was found to be 3.95 ˚A, Figs. 1a and b show the temperature variation (from
which implies epitaxial strain is quite relaxed. This RT up to 1000◦C) of unpolarized Raman spectra of a
agrees with the reported values (c = 3.997 ˚A) [31]. BFO(001)thinfilm. Acloserobservationnearthephase
The comparison of the unpolarized (perpendicular to transitions reveals two noticeable changes in the signa-
the <001> of the substrate) Raman spectrum of BFO ture of the Raman spectra: the disappearance of several
thin film with STO and SRO/STO spectra (cf. Fig. 2a strongermodesat∼820◦Candthecompletedisappear-
in [15]) precludesany Ramancontributionfromthe sub- ance of all the modes above 1000 ◦C. This temperature
strateandbottomelectrode;tothecontrary,weobserved behavior implies that BFO maintains its room temper-
adip,ratherthanapeak,attheSTOstrongestpeakpo- ature structure up to ∼ 820 ◦C, indicating the a struc-
sition. As is evident from the intensity comparison, all tural (ferroelectric) phase transition, in agreement with
of these peaks are due to the BFO normal modes of vi- the earlierinvestigationson BFO bulk single crystaland
brations and none of them arose from the substrate. We polycrystallinesamples[24]. NotethatthinfilmsofBFO
verified our results using target materials, single crys- show first order phase transitions as in bulk, whereas
tals, and also by growing (001) BFO films on different STO and PbTiO PTO are known to be first order in
3
substrates. bulk but second order in thin films [25, 26].
The presence of the four peaks (∼ 213, 272, 820 and
918 cm−1) above 820◦C up to ∼ 950◦C (Fig. 1b) shows
III. RESULTS AND DISCUSSION thattheintermediatebeta-phaseisnotcubic(Pm¯3m)as
reported by Haumont et al [24]. In fact, the phase dia-
A. Phonons in the γ-phase gram of BFO [27], and its more recent revised versions
[15], show that BFO possesses a non-cubic β-phase be-
◦
Fig .1 shows the temperature dependent Raman spec- tween820to933 Cbeforeitgoestotheγ-phase,andthe
β-phasewasrecentlyshowntobeorthorhombicbyusing
traof(001)BFOfilm(300nmthick)onSrTiO substrates
3
with SrRuO buffer layer (25nm thick). As can be seen high temperature X-ray diffraction and domain struc-
3
from Fig .1b, four phonon features marked with arrows tures [15] and neutron diffraction [1]. The complete dis-
persist into the γ-phase from 930-950◦C. Therefore this appearance of peaks at above 950◦C – not at 930◦C –
confirms an existence of non-cubic γ-phase. No first- indicates that the γ high temperature phase also can-
order Raman scattering from phonons is allowed for cu- not be cubic (Pm¯3m), for which any first-order Raman
bic Pm3m(because eachionisataninversioncenter,all scattering is forbidden.
phonons are odd-parity). This is in agreement with the Fig. 2a shows the room-temperature XRD patterns of
◦
observationofanorthorhombicsymmetryfortheγ-phase anas-grownfilmandfilmafteritunderwent1000 Cther-
byArnoldetal. [1]. AlthoughtheRamanlinesarebroad mal cycle. As can be seen, the as-grown film is highly
and weak at these temperatures, they exhibit no signifi- epitaxialshowingonly (00l) peaks andbecame polycrys-
cant frequency shift at the β-γ transition at 930◦C; nor talline after thermal cycling. In principle it is possible
are there additional lines above or below 930◦C. There- that the specimen would melt at high temperatures and
fore it is very likely that the structure is Pbnm in both then recrystallize in the specimen holder (bottom of the
β and γ phases and that no primitive-to-body-centered Pt crucible). However, we monitored the sample surface
phase change occurs. Note that Fig .1 is for a thin film. continuously with an optical microscope and no thermal
◦
Therefore we conclude that the films do not differ from decomposition was observed up to 1000 C.
single crystals and powders previously studied with re- The Raman spectra (Fig. 2b) before and after heat-
gard to the γ-phase. ingshowexactlysamenumberofphononmodes,indicat-
◦
Thereisrenewedinterestinthisβ-γ transitionbecause ing either no decomposition up to 1000 C or complete
of the earlier discovery of a metal-insulator transition recrystallization, contrary to earlier studies [17], which
◦
at 931 C where the orthorhombic-cubic transition takes could be due to the reduced surface/volume ratio, mini-
place in bulk [15] and that the same transition may oc- malsurfaceimperfectionsanddefects,andincreasedsta-
cur at 47 GPa at RT. Earlier Mo¨ssbauer studies estab- bilization from the substrate. Note that the possibili-
lished that at this pressure the magnetization also dis- tiesofsubtlestructuralchanges(smallchangesonangles
appears [21]. Although Gavriliuk et al. [21] concluded and/orin-planelatticeparameters)cannotbecompletely
3

-1m)12 (c) PPeeaakk 21 2202 70 (d) TN (oTnset)
(c10 Peak 3 60 Peak 1 N
n 18 ) Peak 2
tio 8 16 -1m50 Peak 3
a T c
ari 6 N (onset) 14 M (40
v T
cy 4 N 12 WH30
n F
e 10
u 2 20
q
e 8
Fr 0 10
6
50 150 250 350 450 50 150 250 350 450
Temperature (oC) Temperature (oC)
◦ ◦
FIG.1: TemperaturedependentRamanspectraof(001)BFOfilmonSRO/STOfrom22-400 C(a)and550-1000 C(b). Thegraphs
in (b) were adapted from our earlier work [15]. The beginning of the dashed arrows pointing up in (b) shows the beginning of the
newphase. The existence of phononsin γ-phasein (b)aremarked with thesolid arrows pointingdown. Theα-phaseextends upto
◦ ◦
820 C;(c)and(d)temperaturedependence(RT-500 C)ofphononfrequenciesvariation andFWHMfor72(peak1),140(peak2),
and 171(peak 3) cm−1, respectively.
ruled out. However, the Raman frequencies before and B. Phonon anomalies near TN:
after thermal cycling remain unchanged makes this un-
likely. Thisfactfavorsfilmsoverbulkorpowdersamples
for very high temperature studies in the future. Reach- Therearediscrepanciesintheliteratureregardingboth
ing the tetragonal and cubic phases extrapolated from the crystal structure of (001) BFO thin films e.g. with
the powder study of Arnold et al. [2] does not seem im- several reports claiming tetragonal [6, 28], rhombohe-
possible with thin films. dral [29, 30], and monoclinic [31, 32] structure, and
its phonons. Of particular interest regarding phonon-
magnon coupling in BFO was the report [24] of a very
4

(a) phase transition, we followed very closely the temper-
BFO/SRO/STO(001), RT O O
s) SA ATsf-tOger r(o 01w00n10)00C heating Int. (a. u) (003) BF (300) ST (paPtliuecraiteky)d1,e,p1we4n0edec(nPo◦cneesaikdoef2ref)e,dwaitnindatse1n7ssi1engp(lPheoepankeoank3)mfoocrmdet−sh1ei.fesriom7m2-
RT up to 500 C(Fig. 1a). In general, the change in
t
ni phononfrequencybandandwidthwithtemperature can
u
66 68 70 72 74 76 78 80
. 2q (degree) be caused by several factors, such as anharmonic scat-
b
tering, spin-phonon coupling, lattice expansion and/or
r
a
contraction due to anharmonicity and/or magnetostric-
(

. tion effects, and phonon renormalization resulting from
t
n
I electron-phonon coupling [34]. The latter one is not ap-
plicable here as BFO is a highly resistive material, and
the carrier concentration is low. The change in ionic
bindingenergieswithtemperaturealsoaffectsthechange
inphononbandwidthinioniccompounds. However,this
20 30 40 50 60 70 80
2q (degree) is not applicable here as BFO insulating. Figs. 1c and
d reveal the fluctuation of phonon frequency and full-
◦
(b) BFO/SRO/STO(001) RT width at half maxima (FWHM) around 370 C (onset
As-grown could be ∼360◦C), which happens to be the T of BFO.
N
after 1000oC heating NearT we seesmall(1 or2cm−1)changesinbothpeak
N
frequency and linewidth for three phonon modes at 72,
ts) s) RT B AFsO-g/SroRwOn/STO(001) 140, and 171 cm−1. These satisfy the non-mean-field
ni nit after 1000oC heating predictions of Nugroho et al [33]. This behavior could
b. u arb. u bvaentihsheemsaanbiofvesetaTtio,n37o0f◦pCh.oTnohnis-mwaegankoinntienrtaecrtaicotniocnanthbaet
ar nt. ( explained with thNe fact that the magnetic phase transi-
( I
. tion is not accompanied by a structural phase change.
t
n 300 400 500 600 700 The observation of a rather weak phonon-magnon inter-
I Raman shift (cm-1)
actionisalsoconsistentwiththetheoreticalpredictionof
weak magnetization and ME coupling in BFO thin film
by Ederer et al [35].
100 200 300 400 500 600 700
Raman shift (cm-1)
C. Phonon anomalies near 140-150K:
FIG. 2: Room-temperature XRD patterns taken using CuKα In order to study the cryogenic behavior of BFO thin
(1.5406˚A) radiation (a) and Raman spectra (b) of (001)BFO
films, we carried out scattering measurements (Fig. 3a)
◦
filmonSRO/STObeforeandafterthermalcycleupto1000 C.
down to 81 K; no significant change in Raman spectra
XRD pattern of STO is given for comparison.
hasbeenobserved,indicatingtheRTstructureremained
unchanged down to 81K. However, a close observation
shows phonon anomalies around 150K (Figs. 3b and c).
This agrees with the observation of change in magnetic
large(40cm−1)changeinthefrequencyofonelongwave-
order at 150K by Pradhan et al. [36], but the nature of
length phonon branch near T . We emphasize in the
N this phase transition remains moot.
present workthat we see no such phenomenon (Fig. 1a).
Figs. 3b and c show small anomalies in the frequency
Instead we see in Figs.1c and d very small changes in
and linewidth of two phonon branches at 140 and 171
frequency (1 or 2 cm−1) and linewidth of several polar cm−1near 140K,a temperature at which anomalies have
modes, and we model them according to the non-mean-
previously been reported. Although the changes are
field theory of Nugroho et al [33].
small and the data sparse, they are highly reproducible.
In general, there are three kinds of phonon anoma- We are aware of similar observations on single crystals
lies observed near the phase transition temperatures: a by BrahimDkhil [37], and we thank him for preprints of
sigmoidalS-shapedchangeinfrequency(suchasthatre- his work. The present data merely show that the same
portedbyHaumontetal. [24]butnotfoundinourwork); effects are present in thin films.
a step discontinuity; or a small “bump” (increase) that Thenatureofthephasetransitionat140Kremainsun-
returns to the background level a few degrees above or known. There are anomaliesin magnonscatteringcross-
below the transition temperature. sections[38,39]andlinewidth[40,41],inmechanicalloss
In order to study the evolution of Raman signature tangent [38, 42] and 140K is the end-point in Almeida-
around the antiferromagnetic-paramagnetic (AFM-PM) Thoulessdataplots[40,41]. However,theearliersugges-
5
tion by our group [43] that 140K is a spin-reorientation T ,itisnotsufficientforphononbehaviorinBiFeO . In
N 3
transition temperature is not confirmed by very recent this case it is necessary to go to fourth order in L. The
neutron scattering studies [44] and spin-glass effects[45] reasonsareexplainedbyNugrohoetal. [33]intheirwork
have also been suggested but are unproven. onYbMnO . Inthis casethe keyterminthefreeenergy
3
is of the form gP2L2, which for weak coupling gives an
explicit interaction of electric field to L that results in a
D. Weak phonon-magnon coupling: magnetocapacitance of
In our Raman spectra (Figs. 1c and d) we observed (g2P2/kT) [<L2(x)L2(0)>−<L2 >2]dx. (2)
small (1 or 2 cm−1) increases or decreases in phonon Z
frequencies very near T . Note that this is observed
N Althoughthis fourth-ordertermis higher orderthan the
for severaldifferent phononsymmetries. The symmetry- terms in g<L2> considered by Fox et al. [48], it is sin-
independence ofthe phonon-magnoncouplingimplies an
gularatT ,because it is proportionalto the cube ofthe
interaction of form P2M2 (where P and M are the po- N
correlationlength (η) that diverges at T .
N
larizationandmagnetization,respectively)inthefreeen-
The result is that the phonons in Raman effect in
ergy, as first suggested for magnetoelectrics by Smolen-
BiFeO of any symmetry will be expected to have small
3
skii and Chupis [13]. In general the coupling of phonons
anomalies in their frequencies at T . These small dips
N
and magnons can occur through several different micro- or jumps will be proportionalto temperature t(α−1) [33]
scopicphysicalmodels: TheTorrance-Slonczewskimodel
[51],wheretisreducedtemperature,t=[T -T]/T andα
N N
[46] involves modulation of the crystal field at the spin
is the critical exponent describing divergence of the spe-
sitebytheopticalphononandissignificantforionswith
cific heat[52]. Since the α is typically small, the phonon
unquenched orbital angular momenta, such as Co+2 or
frequenciesshouldvaryapproximatelyasT /[T -T]near
Fe+2; the model of Buyers et al. [47] is an angular mo- N N
T andin principle couldbe usedto evaluate criticalex-
N
mentum coupling of spins in octahedra where the opti-
ponent α. However in the present work the data are too
cal phonon eigenvector is rotation-like as viewed from
impreciseforthischore,andevenhigherresolutionwould
the magnetic ion. However, such models do not give
by insufficient due to phonon damping. The bump in
frequency anomalies near T like those we observed in
N phonon frequencies and linewidth are qualitatively pre-
BiFeO . Aratherdetailedmodelofmagneto-capacitance
3 dicted from the non-mean field theory of Nugroho et al.
was given by Fox et al. [48] for BaMnF near T , and
4 N [33]. However, their model does not predict magnitudes
related models by Scott [49] and by Glass et al. [50] for
forthe height(increaseinfrequency)orwidth(hownear
the BaMF family nearT(2D),the two-dimensionalspin
4 the transition the increase occurs) of the bump. No
ordering temperature [typically ca. 3T in that family].
N anomaly at all is predicted by the mean-field theory of
The free energy of Fox et al. can be defined as:
Fox et al. [48], which does not consider terms in the
G=f(L2,L2)+BM2+(b +b p+b p2)M L +..., (1) free energy introduced by Nugroho et al. [33]. A similar
z 0 1 2 x z behavior has been observed at 150 K (Figs. 3b and c)
where P is along y, the polar axis and z is the sublat- couldbedue tothechangeinmagneticordering[36]. As
tice magnetization direction; L is (1/2)gµ S1/2 (S+1)1/2 matter of coincidence the bottom electrode SRO has a
[ (Sj up - Sj down)] and M is the weak magnetization ferromagneticphase transitionat 150 K. Note that none
MP= gµ S1/2 (S+1)1/2 [ (Sj up + Sj down)]. Note of these peaks is related to SRO and a modulated effect
that p is not the total poPlarization P but only the part is highly unlikely, but not impossible.
induced by the magnetoelectric coupling: P = Pr + p Insummary,ourRamanfrequenciesnearTNand150K
and it was carried out to second order in polarization show small peaks or dips for all phonon modes that are
P, sublattice magnetization M, and weak ferromagneti- qualitativelysimilartothosepredictedbyNugrohoet al.
zation L, with the result that the magneto-capacitance [33],implyingageneralinteractionofformP2L2,andthe
varies with temperature as (b - b b )L (T), where b , need for a fluctuation term neglected in the mean-field,
0 1 2 2 0
b , and b are respectively the coefficients of magneto- weak ferromagnetism model of Fox et al [48].
1 2
electric free energy terms independent of, linear in, and
quadratic in polarization P. The authors noted that the
mean-fieldtheory,althoughgenerallynotsatisfactoryfor IV. CONCLUSION
magnetic transitions, works well for weakly canted fer-
romagnets because the expansion parameter L is small In conclusion, high quality epitaxial (001)BFO films
at all temperatures. Note that the sign of the magneto- have been grown on (100) STO substrates using PLD.
capacitance term can be positive or negative depending The XRD studies showed that films are c-axis oriented
upon the magnitude of (b - b b ). Because they used with high degree of crystallinity. The RT polarized
0 1 2
mean field theory, their work neglected the small term Raman scattering of (001)BFO films showed pseudo-
near T due to fluctuations considered below. orthorhombic monoclinic crystal structure contrary to
N
Althoughthesecond-ordertheoryofFoxetal. [48]was the rhombohedral and tetragonal symmetries reported
satisfactory for describing all the data in BaMnF near earlier. We observed the ferroelectric phase transition
4
6
(a) BFO(001) thin film
81-273 K
273
nits) 223 -1m)4 (b) PPeeaakk 12 10 9 (c) PPeeaakk 12 24
Int. (arb. u 111111653742333333 ency variation (c123 Peak 3 468 -1FWHM (cm)678 Peak 3 112581
81 qu 2 5
e
Fr0 12
50 150 250 350 450 550 650 0 4
Raman shift (cm-1) 50 100 Te15m0pera2tu00re (K)250 300 50 100Tem1p5e0ratur2e0 0(K) 250 300
FIG. 3: (a) Temperature dependent Raman spectra of (001)BFO film on SRO/STO from 81K-RT; (b) and (c) temperature
dependence of phonon frequencies variation and FWHM for 72 (peak 1), 140 (peak 2), and 171 (peak 3)cm−1 down to 81K from
RT, respectively.
◦
at around 820 C and no softening of Raman modes was the comparative study with the thin film and G. Cata-
observed at low frequencies, as in BFO single crystals. lan,UniversityofCambridgeforhisusefulcommentsand
◦
The AFM-PM phase transition at around 370 C caused suggestions. We thank Brahim Dkhil for sharing his Ra-
somesmallchangesinthephononfrequencies,linewidth, man data on phonon anomalies in BFO single crystals
and/or intensities of several low frequency modes, in- near 140K prior to their publication.
dicating ME coupling in the material. A non-cubic γ-
◦ The authors would like to acknowledge financial sup-
BiFeO phase was observed between 931-950 C in the
3
port from DOE DE-FG02-08ER46526grant. RP thanks
BFO thin films, in agreement with the accepted BiFeO
3
Institute of Functional Nanomaterial (IFN), UPR for fi-
phase diagram. The spectra also revealedan evidence of
◦ nancial support.
a cubic δ-phase around 1000 C in thin films that is not
stable in powder and bulk.
ACKNOWLEDGEMENT
The authors are grateful to Prof. Hans Schmid, Uni-
versity of Geneva for supplying BFO single crystal for
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