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Dipartimento di Fisica, Sapienza, Università di Roma.
Feb 28, 2013
Wael S. Ahmad Prof. Paolo Calvani
Dott. Alessandro Nucara
Talk Outline
I. Introduction&motivation
• Interest in Perovskite Manganites.
• Doped manganese Perovskites
• Ga-substituted LaMnO3
II. Experiments& Data Anlysis
• Film growth
• Optical Measurements
• Data Analysis
III.Experimental results and Preliminary
Interpretation
IV.Future work
• the colossal magnetoresistance (CMR) effect
(1,2).
• There is a great potential of applications
related to the colossal magnetoresistance
(CMR) effect such as magnetic sensors or in
computer memory systems.
Continued
Interest in Perovskite Manganites
1- G. H. Jonker and J. H. Van Santen, Physica 16 (1950).
2- E.O. Wollan and W.C. Koehler. Phys. Rev. 100 (1955)
Manganese Perovskites
Perovskite Structure
[RE/AE] MnO3-type perovskite
structure: La or Ca atoms red, Mn
green, and oxygen blue).
Chemical formula
[RE/AE] MnO3
RE = trivalent rare earth
AE = divalent alkaline-
earth ion
Crystal Structure
Continued
Doped manganese Perovskites
3d4
Mn4+
3d3 eg
t2g
dx2
-y2
dz2
eg
Mn3+
t2g
Crystal field Crystal field + Jahn-Teller
electronic configuration of
the outer d state of Mn
Upon doping by divalent ions, Mn acquires
a mixed valence: Mn4+ and Mn3+
dxz
dxy
dyz
Parent Compound LaMnO3
• LaMnO3 exhibits rich and interesting physical properties
related to the interplay between lattice distortions,
transport properties, and magnetic ordering.
• LaMnO3 is stabilized in orthorhombic crystal structure,
which is a distorted form of the cubic perovskite structure.
Below TJT ≈780 K the long and short Mn-O bonds are
ordered (cooperative Jahn-Teller effect).
• LaMnO3 is an insulating
antiferromagnet with TN ≈ 140 K
(A-type antiferromagnetism).
Ga-substituted LaMnO3
• Ga is isovalent to La. Nevertheless, strong effects on the
perovskite properties are expected as Ga suppresses the Jahn-
Teller effect, is not magnetic, and thus reduces the exchange
terms between Mn ions (5).
• Our optical study on La0.5Ga0.5MnO3 film is aimed at verifying the
above effects on the electronic bands of Mn.
Ga3+ : [Ar]. 3d10
Ga (free atom): [Ar]. 3d104s24p1
Electronic Configuration of Gallium
5- J. Blasco, et. al, Phys. Rev. B 66, 174431(2002).
Film growth
• Pulsed Laser Deposition (PLD)
technique is used to grow the film
(F. Miletto, CNR-SPIN, Naples).
• Epitaxial films of 100 nm in
thickness. Grown on (LaAlO3)0.3
(SrAl0.5Ta0.5O3)O.7(LSAT)
substrates.
UV-VIS spectroscopy (200-800 nm) (electronic excitations)
High energy photons absorbed by bound electrons
Change of the electronic states of the molecules By transfer of electrons
from their ground state orbitals to the higher energy excited state
orbitals.
IR spectroscopy (1µm-1000 µm) (phonon excitations)
Excitation of vibrational motions of atoms in a crystal.
Excitation of free electrons in metals (Drude model).
Excitation of polarons
Optical Measurements
Experimental Setup
Spectrophotometer components:
•Light source
•Sample chamber
•Monochromator
•Detection system
Transmittance of LMGO+LSAT at different temperatures
Data Analysis
RAW DATA
Data Analysis
Two-layer transmittance
and are define through the
thickness ds,f of the substrate or
that of LMGO film, and through
their refractive indexes s(ω) and
f (ω) respectively.
2
)~~
(2exp(~~)~
2exp(~~)~
2exp(~~1
))~~
(exp(~~~
)(fsfsffsfssfs
fsfsfs
irrirrirr
itttT
fSfSfS nd ...~2
~
Sf
Sf
SfSfnn
nntr ~~
~~~
1~
S
SSS
n
ntr ~1
~1~1~
f
f
ffn
ntr ~1
~1~1~
JJ
J
i
S
22
2
~
Drude-Lorentz (DL) expansion of the
dielectric function is considered
The intensity
The width of the jth
DL contribution The central frequency
(*)
best fit to T (ω) at T = 10 K
Data Analysis
• A fit of the experimental T(ω) through equation (*) provides the
optical properties of the LMGO film, once a proper number of
DL contributions are included into the dielectric function.
Experimental results and Preliminary Interpretation
60/).()( 2
Peak frequencies
S1=13000 cm-1
S2 =16000 cm-1
S3=20700 cm-1
S4 =33500 cm-1
Optical conductivity of LMGO and LMO at 10 K (a) and 250 K (b)
with the individual DL contributions in LMGO (6).
6- A. Nucara, et al., Eur.Phys. J. B 7, 10945 (2011).
Experimental results and Preliminary Interpretation
Intensity and Peak frequency of S2
the intensity of S2 increases as T
crosses TN ≈ 75 K. This behavior
is in accordance with charge
dynamics between Mn+3 ions with
parallel core spins. To this extent,
we assign this band to the HS
transition.
S2 contribution around 16000
cm-1 and S4 band around 33500
cm-1 exhibit a more pronounced
temperature dependence.
Experimental results and Preliminary Interpretation
Intensity and Peak frequency of S4
Opposite to S2, a sharp decrease of
the intensity of S4 is observed as the
temperature approaches TN ≈ 75 K.
Since the onset of the FM-PM phase
inhibits the charge hopping between
neighbor Mn+3 ions with anti parallel
spin in the ab-plane, this band can be
assigned to the LS transitions.
Experimental results and Preliminary Interpretation
Intensity of S3
S3 contribution is weakly
affected by temperature. The
intensity S1 is not affected by
the temperature: it can be
assigned to the intrasite
transitions within Mn Ions levels
as well as to charge transfer
between O2p and Mn3d ions.
The central frequency of S2 in
LMO is 19500 cm-1 and in
LMGO is 16000 cm-1 . This
energy shift may be related to
absence of Jahn-Teller effect
which is suppressed by Ga
doping.(5,6)
Preliminary Interpretation
6- A. Nucara, et a., Eur.Phys. J. B 7, 10945
(2011)
5- J. Blasco, et. al, Phys. Rev. B 66,
174431(2002).
Future Work
In La deficient manganite films, Mn2+ ions substitute the La at
the A site of the lattice, therefore, Manganese of three formal
valence (Mn2+, Mn3+, Mn4+) are present in the lattice.
From the analysis of the optical conductivity: • we will deduce the dynamics of the insulator-to-metal
transition for different La deficient compounds
La (1-x) Mnx2+ Mn(1-y)
3+ Mny4+ O3 Mixed valence manganites
Double Exchange between Mn3+ and Mn4+
• Bismuth manganite is remarkable because strong
coupling between magnetic, electric and structural
order parameters occurs at once.
• We will investigate the spectra in the mid and far
infrared region with the aim to reveal the spectral
structures arising from the strong correlation between
magnetic and lattice parameters and their evolution
with temperature.
Bismuth manganite
FutureWork
Thanks for your attention