Enhanced magnetoresistance in La0.82Sr0.18MnO3-p-conjugated

semiconducting polymer heterostructure

Jitendra Kumar a, Rajiv K. Singh a, P.K. Siwach b, H.K. Singh a,*, Ramadhar Singh a,

R.C. Rastogi c, O.N. Srivastava b

a National Physical Laboratory, Dr K.S. Krishnan Road, New Delhi 110012, Indiab Department of Physics, Banaras Hindu University, Varanasi 221005, India

c Department of Chemistry, University of Delhi, Delhi 110007, India

Received 23 February 2006; accepted 16 March 2006 by E.V. Sampathkumaran

Available online 7 April 2006

Abstract

We report a large enhancement (w90%) in magnetoresistance in La0.82Sr0.18MnO3 (LSMO) layers by incorporating a p-conjugated

semiconducting polymer layer in between them. The epitaxial LSMO layers were deposited by DC magnetron sputtering on SrTiO3 single crystal

substrates and have FM transition temperature (TC)w310 K. A semiconducting polymer poly(3-octylthiophene) (P3OT) layer was deposited over

the epitaxial LSMO layer by solution dip coating technique and with subsequent deposition of another epitaxial LSMO layer, forming a

LSMO–P3OT–LSMO heterostructure. The effect of P3OT incorporation on magnetotransport properties of this heterostructure has been

examined in the temperature range 77–350 K. Large MR enhancement observed near room temperature in the FM regime is explained in terms of

efficient magnetic field dependent carrier injection at LSMO/P3OT interface.

q 2006 Elsevier Ltd. All rights reserved.

PACS: 75.70.Cn; 75.47.Lx; 72.25.Mk

Keywords: C. LSMO–P3OT–LSMO heterostructure; D. Carrier injection; D. MR enhancement.

In thin films of mixed valence manganites, the occurrence of

appreciable magnetoresistance (MR) around the ferromagnetic

(FM) transition temperature (TC) or metal–insulator transition

temperature (TIM) is one of the prime requirements for its device

application [1]. Efforts have been made to induce MR

enhancement at magnetic fields w1 T or less by fabricating a

variety of heterostructures [2–7]. Of late, the FM–organic

semiconductor–FM heterostructure gained special importance

because of its potential application in spintronics [8,9]. It has been

shown that by adjusting the chemical potential and hence the

conductivity of FM and semiconductor, electrons can be

transferred into the organic semiconductor from the manganite

[6,7]. Recently, a large intriguing MR effect (up to magnetic field

w1 T) has been observed in a sandwich structure having two FM

electrodes, one of a manganite (La2/3Sr1/3MnO3) and another of a

ferromagnetic metal (cobalt, Co) with a p-conjugated polymeric

0038-1098/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ssc.2006.03.024

* Corresponding author. Tel.: C91 11 25742610; fax: C91 11 25726952.

E-mail address: hks65@mail.nplindia.ernet.in (H.K. Singh).

semiconductor [6,7]. Similar effect has been seen [8,9] in

a La2/3Sr1/3MnO3–organic semiconductor–La2/3Sr1/3MnO3 het-

erojunction. But till date no report is available on MR

enhancement in LaSrMnO3-p-conjugated polymer–LaSrMnO3

heterostructure. In this communication, we report an appreciable

MR enhancement in La0.82Sr0.18MnO3-poly(3-octylthiophene)–

La0.82Sr0.18MnO3 (LSMO–P3OT–LSMO) heterostructure. The

bottom manganite layer used in this structure is fully oxygenated

LSMO while a slightly oxygen deficient LSMO acts as the top FM

layer. The sandwiched layer is a p-conjugated polymer; P3OT.

The high carrier mobility along with better thermal and

environmental stability of polythiophenes [10–13] based p-con-

jugated polymer has motivated us to incorporate it as an

intermediate layer in between the two LSMO (FM) layers as an

active semiconducting layer for efficient carrier injection. The

LSMO layers were deposited by DC magnetron sputtering on

SrTiO3 single crystal substrates and the P3OT layer was

deposited by solution dip coating technique.

LSMO thin films having thickness w150 nm were grown by

DC magnetron sputtering. The target having nominal stoichio-

metric composition La0.82Sr0.18MnO3 was prepared by the

polymeric sol–gel process [14]. High purity (w3 N) metal

Solid State Communications 138 (2006) 422–425

www.elsevier.com/locate/ssc

75 100 125 150 175 200 225 250 275 300 325 3500.000

0.004

0.008

0.012

0.016

0.020

ρ (Ω

-cm

)

T (K)

LSMO LSMO-P3OT

Fig. 2. Temperature dependence of resistivity of LSMO and LSMO-P3OT-

LSMO films in the range 77-343 K.

J. Kumar et al. / Solid State Communications 138 (2006) 422–425 423

nitrates, viz., La(NO3)36H2O, Sr(NO3)2 and Mn(NO3)25H2O

were used for the synthesis of LSMO powder for making the

target. The synthesized LSMO powder was pressed in the form

of pellets (2 in. diameter for target and one rectangular pellet

for target characterization) and subsequently fired at w1100 8C

for 24 h. All the films of LSMO were deposited on SrTiO3

(100) single crystal substrate (LZ1.0 cm, WZ0.5 cm and TZ0.05 cm) kept at w750 8C in ArCO2 (80C20%) atmosphere

using DC magnetron sputtering technique and were annealed in

flowing oxygen at w750 8C. The gas pressure during the

sputtering was w200 mTorr. The target to sample distance and

deposition rate were w2 cm and w15 nm/min, respectively. A

p-conjugated organic semiconducting poly(3-octylthiophene)

(P3OT) [11] in its pristine form, synthesized [10] in our

laboratory, was used as an intermediate layer sandwiched

between two FM (LSMO) layers. P3OT thin film (w100 nm) is

deposited on the top of the LSMO thin film by dip coating

method and the thickness is controlled by varying the

concentration of the polymer solution. Once the P3OT layer

was coated, the film was dried at w100 8C for 12 h. The second

top layer of LSMO was deposited at w400 8C (below

degradation temperature of P3OT) keeping other conditions

exactly the same. This multilayer device (LSMO–P3OT–

LSMO) was annealed in-situ in ArCO2 environment at

w450 8C for 2 h. The sandwiched P3OT layer is observed to

be stable up to w470 8C from its thermal gravimetric analysis

(TGA). A schematic view of the LSMO–P3OT–LSMO

heterostructure is shown in Fig. 1.

The epitaxial nature of the LSMO layers were confirmed by

X-ray diffraction by measuring the qK2q and rocking curve

profiles (results not shown). Moreover, a decrease in the

intensity of the diffraction peaks is observed in case of the

LSMO–P3OT–LSMO heterostructure. This may be attributed

to the structural relaxation known to occur at FM–semicon-

ductor polymer interface [15]. It has been shown that in the

interfacial region the end bonds of the manganite structure

expands while the polymer bonds get contracted [15]. The

surface topography of the lower CMR electrode as well as top

one was investigated by atomic force microscopy and is shown

Fig. 1. The surface topography investigated by atomic force microscopy of (a)

the lower LSMO layer, (b) top LSMO layer and (c) schematic view of the

LSMO-P3OT-LSMO heterostructure.

in Fig. 1. The lower electrode, which is fully oxygenated

LSMO (topograph A in Fig. 1) appears highly crystalline and

epitaxial but the top LSMO electrode (topograph in B Fig. 1) is

less crystalline, which is expected in view of the fact that it was

deposited at a lower temperature (w400 8C). The smooth

morphology and the growth feature of the top layer also

confirm that the P3OT layer is intact and is not decomposed

during the deposition process of top LSMO layer.

The FM transition temperature (TC) of the LSMO film is

measured to be w310 K, while the same for the heterostructure

is w302 K. The observed TC depression may be due to the

structural relaxation at the LSMO–P3OT interfaces and oxygen

deficiency of the top LSMO layer. The temperature depen-

dence of resistivity of LSMO and LSMO–P3OT–LSMO films

in the range w77–343 K is shown in Fig. 2. The electrical

transport measurements were done in the four terminal

geometry and schematic of the contact geometry is shown in

Fig. 1. Constant current was applied from the two ends of the

lower LSMO layer (terminals marked as 1 and 2 in Fig. 1) and

the voltage was measured across the lower LSMO layer and the

top LSMO layer (terminals marked as 3 and 4 in Fig. 1). At

343 K, the resistivity of LSMO and LSMO–P3OT–LSMO film

100 150 200 250 300 3500

6

12

18

24

30

MR

(%

)

T(K)

LSMO LSMO-P3OT

Fig. 3. Temperature dependence of magnetoresistance (MR) measured at 10

kOe.

0 3 6 9 120

7

14

21

28

35

MR

(%

)

H (kOe)

LSMO/300K LSMO/280K LSMO-P3OT/300K LSMO-P3OT/270K

Fig. 4. Magnetic field dependence of MR measured in the field range 0-12 kOe.

J. Kumar et al. / Solid State Communications 138 (2006) 422–425424

is w3.5!10K3 and w1.5!10K2 U cm, respectively. Both

these films exhibit a semiconducting behaviour in the

higher temperature regime and undergo a semiconductor to

metal (S–M) transition as the temperature is lowered. These

S–M transitions occur at w312 K (rw3.74!10K3 U cm) and

at w293 K (rw1.73!10K2 U cm), respectively, in the

virgin and heterostructure film. The resistivity of the LSMO–

P3OT–LSMO film increases by more than one order of

magnitude in the whole temperature range studied. The slope

(dr/dT!0) of the r–T curve above the S–M transition is

stronger in case of the heterostructure film. The observed

increase in the resistivity may be due to the spin-polarized

carrier injection from the LSMO–FM layer into the P3OT layer

[6,15,16].

Fig. 3 shows the temperature dependence of magnetoresis-

tance (MR) measured at 10 kOe. LSMO film shows a magnetic

field independent peak in the MR–T curve at TmaxZ280 K and

the corresponding MR is w15%. Above and below the peak

temperature (Tmax) MR drops rather sharply in the virgin film.

In contrast, the LSMO–P3OT–LSMO heterostructure exhibits

peak in the MR–T curve at a slightly lower temperature at Tmax

Z270 K. However, large enhancement in MR is observed in this

heterostructure and at HZ10 kOe the maximum measured MR

is w28%. In the lower temperature range (T!200 K) the MR of

the pure LSMO film is very small, typicallyw1% at HZ1 T

and the same is the case for the LSMO-P3OT-LSMO

heterostructure film. However, in the intermediate temperature

range (320 K!T!210 K), LSMO–P3OT–LSMO film exhibits

larger MR than the LSMO film. It is well established that in the

FM metallic regime of the Sr doped manganites, the MR effect at

T!TC originates due to the suppression of spin fluctuations by

the applied magnetic field [17]. However, in case of FM–organic

semiconductor heterojunctions, the MR is affected mainly

through field induced enhancement in spin-polarized carrier

injection [6–9,15,16]. Thus, the observed MR enhancement in

the vicinity of Tmax of LSMO–P3OT–LSMO film is due to the

magnetic field induced effective spin-polarized carrier injection

from the LSMO–FM layer into the p-conjugated polymer P3OT

layer.

The magnetic field dependence of MR was measured at two

temperatures up to the magnetic field of 12 kOe. Fig. 4 shows the

MR-H correlation measured at 300 K and the temperatures

(w280 K in case of LSMO and w270 K for LSMO–P3OT–

LSMO) where maximum MR was observed. At peak MR

temperatures, the MR of the LSMO–P3OT–LSMO film rises

rapidly as compared to the LSMO film when the magnetic field

HO1.6 kOe. At TZ280 K, the LSMO film exhibits MRw6% at

Hw1.6 kOe, which slowly increases to 17% at 12 kOe. Such

behaviour is typical of the Sr doped manganites in the FM regime

and has its origin in the suppression of the spin fluctuations by the

applied magnetic field [17]. In comparison to the LSMO film,

LSMO–P3OT–LSMO heterostructure shows a large enhance-

ment in MR. For example, at Tmaxw270 K and at lower fields

such as Hw1.6 kOe, the MR of LSMO–P3OT–LSMO film is

small (w6%) but it increases rapidly to w32% when the

magnetic field is increased to 12 kOe. Similar behaviour has been

observed in La2/3Sr1/3MnO3–p-conjugated organic semiconduc-

tor heterojunction by Wu et al. [6] and has been explained in terms

of magnetic field enhanced spin-polarized carrier injection from

FM into the semiconductor layer. This field enhanced carrier

injection is explained in terms of an anomalous field induced

Fermi level shift [6]. The MR–H plot (Fig. 4) also shows that the

MR enhancement is significant only around the peak in MR–T

curve (Fig. 3). This suggests that the field induced carrier

injection is temperature dependent and dominant only in the

vicinity of temperature where the MR–T peak is observed.

Further studies are in progress to explore the MR enhancement in

other variants of manganite–p-conjugated semiconducting

polymer multilayer heterostructure.

In conclusion, we have fabricated manganite–p-conjugated

semiconducting polymer–manganite thin films and shown that

a large enhancement in MR near room temperature can be

achieved. This observed enhancement originates from the

magnetic field induced spin-polarized carrier injection from the

manganite to the semiconducting polymer layer and is

observed to be temperature dependent.

Acknowledgements

Authors are grateful to Prof. Vikram Kumar, director NPL,

India for his support and encouragement. We are also grateful

to Dr S. Majumdar, Department of Chemistry, University of

Delhi, India for providing AFM facility. Financial support from

CSIR and MNES, New Delhi is thankfully acknowledged.

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