Wael S. Ahmad Prof. Paolo Calvani Dott. Alessandro Nucara · (1,2). • There is a great potential...

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