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Journal ArticleDOI

Persistent photoconductivity induced by electric currents in epitaxial thin films of Pr0.7Sr0.3MnO3

17 Mar 2011-Journal of Applied Physics (American Institute of Physics)-Vol. 109, Iss: 7
TL;DR: In this article, the effects of electric currents on the photoconductivity of epitaxial Pr0.7Sr0.3MnO3 films were studied and the observed effects might be related to the coexistence and instability of the multiphases in manganites.
Abstract: The effects of electric currents on the photoconductivity of epitaxial Pr0.7Sr0.3MnO3 films were studied. The as-prepared films showed a transient photoconductivity in a wide temperature range. After the films were processed by a current of density ∼105 A/cm2 for certain duration at room temperature, a highly resistive and metastable state was excited at low temperature. This induced state exhibited significant electroresistance and persistent photoconductivity. The phenomena found in films grown on LaAlO3 were similar to those on SrTiO3. The observed effects might be related to the coexistence and instability of the multiphases in manganites.

Summary (1 min read)

I. INTRODUCTION

  • Doped manganites have attracted great interests for their prominent properties and rich physics. [1] [2] [3] [4] [5].
  • Early studies indicated that in such materials strong coupling between the local t 2g spin and mobile e g electron results in simultaneous conductivity and magnetism, known as double exchange interaction (DE).
  • Actually, different degrees of freedom, including spin, charge, lattice, and orbit, are interconnected in manganites.
  • On the other hand, it may be favorable for practical applications, because the delicate balances between different phases can be shifted by small external stimuli, such as magnetic field, electric current/field, pressure, and light exposure.
  • 13, 14 Although the studies on current-and light-induced effects on manganites are quite active, the exact mechanisms have not been fully understood and there have been scanty explorations on photoresponse of current-induced state.

II. EXPERIMENTAL PROCEDURES

  • After deposition, the films were annealed in situ at grown temperature in 1 bar oxygen for half an hour.
  • As suggested by the x-ray diffraction spectra (h-2h scan and A scan), the obtained films on both substrates have good crystallinity and epitaxy.
  • To get a homogenous illumination and study the current-induced effects, microbridges [$30 lm  100 lm, see Fig. 1 ] were made by using conventional photolithography.
  • Ohmic contacts were achieved by evaporating silver electrodes onto the films and a standard four-probe method was used for electric measurements.
  • A semiconductor laser (wavelength k ¼ 532 nm) with the power density of $2 mW/mm 2 was used to illuminate the sample through the optical window of the cryostat.

III. RESULTS AND DISCUSSIONS

  • Figure 1 depicts the temperature dependence of resistance and magnetoresistance (MR) for a PSMO film on LAO.
  • As shown in Fig. 4 (a) , the resistance decreased upon the irradiation of light and did not increase back to the initial value after the light was turned off.
  • Alternatively, large inhomogeneity appears to be an important feature for visible light-induced PPC.
  • 13, 14 Introduction of appropriate oxygen vacancies to La 2/3 Sr 1/3 MnO 3 caused local lattice distortions and thus electronic imbalance.
  • It seems that the initial state at low temperature was comparably homogenous and dominated by conducting phases.

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Persistent photoconductivity induced by electric currents in epitaxial thin
films of Pr
0.7
Sr
0.3
MnO
3
J. F. Wang and J. Gao
a)
Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong
(Presented 15 November 2010; received 23 September 2010; accepted 22 October 2010; publi shed
online 17 March 2011)
The effects of electric currents on the photoconductivity of epitaxial Pr
0.7
Sr
0.3
MnO
3
films were
studied. The as-prepared films showed a transient photoconductivity in a wide temperature range.
After the films were processed by a current of density 10
5
A/cm
2
for certain duration at room
temperature, a highly resistive and metastable state was excited at low temperature. This induced
state exhibited significant electroresistance and persistent photoconductivity. The phenomena
found in films grown on LaAlO
3
were similar to those on SrTiO
3
. The observed effects might
be related to the coexistence and instability of the multiphases in manganit es.
V
C
2011 American
Institute of Physics. [doi:10.1063/1.3536458]
I. INTRODUCTION
Doped manganites have attracted great interests for their
prominent properties and rich physics.
15
Early studies indi-
cated that in such materials strong coupling between the local
t
2g
spin and mobile e
g
electron results in simultaneous conduc-
tivity and magnetism, known as double exchange interaction
(DE). Late r electron–lattice interaction, namely Jahn–Teller
(JT) distortion, was found to be also significant. Actually, dif-
ferent degrees of freedom, including spin, charge, lattice, and
orbit, are interconnected in manganites. These cross-couplings
give rise to diverse electronic ground states, which are energeti-
cally comparable to each other and constit ute a multivalley
energy landscape. As a consequence, different electronic states
may coexist in a single chemical phase. This characterizes
phase separation (PS) and demonstrates the complexity in man-
ganites. On the other hand, it may be favorable for practical
applications, because the delicate balances between different
phases can be shifted by small external stimuli, such as mag-
netic field, electric current/field, pressure, and light exposure.
The resistance change upon an electric field/current in a
charge-ordered single crystal (Pr
0.7
Ca
0.3
MnO
3
) was as large
as several orders.
6
In materials with other ground states, the
electric field/current-induced effects were also pronounced.
For example, in La
0.8
Ca
0.2
MnO
3
films, asymmetric conduc-
tion and metastable states, which were readily susceptible to
weak currents, could be excited by large currents.
7,8
Light illu-
mination-induced effects in manganites have also gained
considerable attention.
916
Early studies found that the charge-
ordered (CO) state in Pr
0.7
Ca
0.3
MnO
3
single crystals could be
collapsed by light and x ray.
9,10
Other reported photoinduced
phenomena are various, including photoinduced antiferromag-
netic–ferromagnetic phase tansition,
11
photoinduced coherent
magnetization precession,
12
and persistent photoconductivity
(PPC).
13,14
Although the studies on current- and light-induced
effects on manganites are quite active, the exact mechanisms
have not been fully understood and there have been scanty
explorations on photoresponse of current-induced state. In this
paper, we report the current-induced persistent photoconduc-
tivity in epitaxial thin films of Pr
0.7
Sr
0.3
MnO
3
(PSMO).
II. EXPERIMENTAL PROCEDURES
PSMO films were deposited by pulsed laser deposition
(PLD)in70PaO
2
and at 700
C onto (001) SrTiO
3
(STO) and
LaAlO
3
(LAO) single crystalline substrates. After deposition,
the films were annealed in situ at grown temperature in 1 bar
oxygen for half an hour. The thickness of the samples is 300
nm, as controlled by deposition time. As suggested by the x-ray
diffraction spectra (h–2h scan and A scan), the obtained films
on both substrates have good crystallinity and epitaxy. To get a
homogenous illumination and study the current-induced effects,
microbridges [30 lm 100 lm, see Fig. 1(inset)] were made
by using conventional photolithography. Ohmic contacts were
achieved by evaporating silver electrodes onto the films and a
standard four-probe method was used for electric measure-
ments. A semiconductor laser (wavelength k ¼ 532 nm) with
the power density of 2mW/mm
2
was used to illuminate the
sample through the optical window of the cryostat.
FIG. 1. (Color online) Temperature dependence of resistance (l
0
H ¼ 0 and
0.8 T) and MR ratio for the patterned PSMO film on LAO. MR is defined as
MR ¼ [R(H) R(0)]/R(0), where R(H) and R(0) are the resistance with and
without a magnetic field, respectively. (Inset) Schematic picture of the
microbridges.
a)
Author to whom correspondence should be addressed. Electronic mail:
jugao@hku.hk.
0021-8979/2011/109(7)/07D701/3/$30.00
V
C
2011 American Institute of Physics109, 07D701-1
JOURNAL OF APPLIED PHYSICS 109, 07D701 (2011)
Downloaded 28 Nov 2011 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

III. RESULTS AND DISCUSSIONS
Figure 1 depicts the temperature dependence of resist-
ance and magnetoresistance (MR) for a PSMO film on LAO.
As a typical feature of doped manganites, the grown film
shows a metal-to-insulator transition at T
P
260 K, slightly
higher than that of the bulk PSMO (245 K). With the appli-
cation of a magnet ic field, the resistance is reduced and T
P
is
shifted to a higher temperature. The maximum MR ratio is
about 34% at 0.8 T.
Figure 2 shows the photoinduced changes of resistance
for the as-prepared PSMO films on STO. The photoinduced
resistance LR is defined as [R(L) R(0)]/R(0), where R(L)
and R(0) are the resistance with and without light illumina-
tion, respectively. Several characteristics can be noted. First,
under light illumination, the resistance is increased in the
metallic region (dR/dT > 0), while that is decreased in insu-
lating region (dR/dT < 0). Second, after the light was turned
off the resistance recovers to the initial value, indicating a
transient photoconductivity (TPC). Third, LR has a peak
value at a temperature below T
P
. These features are qualita-
tively similar to those reported in hole-doped La
0.67
Ca
0.33
MnO
3
(Ref. 15) and electron-doped La
0.8
Te
0.2
MnO
3,
16
imply-
ing common properties in many doped manganites.
The PSMO/STO films show dramatic changes after being
processed with a large current of 18 mA (2 10
5
A/cm
2
)at
room temperature for 30 min. As can be seen from Fig. 3(a),
for the current-induced state, with decreasing temperature
there is an upturn of resistance below T
P
and the resistance is
greatly increased at low temperatures. Moreover, the low tem-
perature current–voltage characteristic of this new state is
nonlinear. This means a substantial electroresistance (ER)
effect, as displayed in the inset. The great increase of resist-
ance and nonlinearity are not possible to be interfacial effects
since two lead RT curves measured between electrodes 1 and
2 [or between electrodes 3 and 4, see Fig. 1(inset)] prior to
and after current processing are essentially the same.
Figure 3(b) presents the LR for the current-induced state
from room temperature down to 80 K. Like that in the as-pre-
pared film, LR in this temperature range is transient. It is posi-
tive (negative) in the metallic (insulating) region. Intriguing
properties appeared at lower temperatures, i.e., 30 K. As shown
in Fig. 4(a) (inset), the resistance decreased upon the irradiation
of light and did not increase back to the initial value after the
light was turned off. This characterizes the PPC effect. If the
temperature was not raised, the low resistance state could stay
for hours and did not show an observable response when the
light was switched on again. On the other hand, PPC could be
quenched on thermal cycling. If the sample was heated to room
temperature and then cooled in dark, the resistance restored to
the value before illumination and PPC could be generated
again. To further understand this current-induced PPC, IV
curves were measured prior to, under and after light illumina-
tion [Fig. 4(a)]. Whereas distinct differences can be found for
IV curves prior to and during light illumination, there is
almost no change upon the stop of light. With the definition
PPC ¼ [R(PL) R(AL)]/R(PL) , where R(AL) and R(PL) repre-
sent the resistance after and prior to light illumination, the cal-
culated bias-dependent PPC is displayed in Fig. 4(a) (bottom
inset). With the increase of bias voltage, PPC rises from 1%
at V
bias
0Vto5% at V
bias
¼ 3 V, and keeps almost constant
with further increasing of bias voltage.
Similar phenomena were also found in films grown on
LaAlO
3
. Figure 4(b) shows the time dependence of resist-
ance for the current-induced state with light on and off. Like
that in PSMO/STO, the resistance drops quickly at the begin-
ning of light illumination and did not recover upon the
removing of light.
Such a PPC in mangnaites has not been well understood
yet. Cauro et al. reported PPC in oxygen deficient La
2/3
Sr
1/3
MnO
3
and suggested that PPC is associated with oxygen
deficiency.
13
On the other hand, in samples without obvious
oxygen deficiency, such as Pr
0.7
Ca
0.3
MnO
3
(Ref. 9)and
(La
0.3
Nd
0.7
)
2/3
Ca
1/3
MnO
3
,
14
PPC was also observed. This
FIG. 2. (Color online) Temperature dependence of R and LR for an as-
prepared PSMO film on STO. (Insets) Resistance vs illumination time at 283
K (a) and 255 K (b).
FIG. 3. (Color online) (a) RT curves (I ¼ 1 lA) prior to and after current
processing for PSMO on STO: (left inset) two lead R(T) curves (between
electrodes 1 and 2) prior to and after current processing and (right inset) IV
and ER–V curves for the current-induced state (T ¼ 30 K). (b) Temperature
dependence of LR (80 K T 300 K) and resistance vs illumination time
for the current-induced state (T ¼ 80 K, PSMO/STO).
07D701-2 J. F. Wang and J. Gao J. Appl. Phys. 109, 07D701 (2011)
Downloaded 28 Nov 2011 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

implies oxygen deficiency may not be a necessary ingredient
for PPC. Alternatively, large inhomogeneity appears to be an
important feature for visible light-induced PPC.
13,14
Introduc-
tion of appropriate oxygen vacancies to La
2/3
Sr
1/3
MnO
3
caused local lattice distortions and thus electronic imbalance.
Partial substitution of Ca with Nd in La
2/3
Ca
1/3
MnO
3
lead
A-site disordered and could enhance the inhomogeneity. It is
known that manganites are electronically soft.
4
A large elec-
tric field could change the intricate balance between different
phases and drive the system transiting from one state to
another. It seems that the initial state at low temperature was
comparably homogenous and dominated by conducting
phases. With the application of the large electric field/current,
the insulating portion was increased and the inhomogeneity
was enhanced. As a consequence, visible light-induced PPC
appeared. A full understanding of this current-induced PPC
needs further study.
IV. CONCLUSIONS
The effects of current processing on photoconductivity
have been studied in Pr
0.7
Sr
0.3
MnO
3
films grown on SrTiO
3
and LaAlO
3
. The as-grown films show transient photocon-
ductivity, which is negative above T
P
and positive below T
P
.
In contrast, the photoconductivity of the current-induced
state is transient at high temperatures and persistent at low
temperatures. Such a current-induced PPC might be related
to enhanced inhomogeneity.
ACKNOWLEDGMENTS
This work has been supported by a grant of the Research
Grant Council of Hong Kong (Project No. HKU 70240 7P)
and the CRCG of the University of Hong Kong.
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FIG. 4. (Color online) (a) IV curves prior to, under, and after light illumi-
nation for the current-induced state (T ¼ 30 K, PSMO/STO). (Insets) Resist-
ance vs illumination time (upper) and the bias dependence of PPC (bottom).
(b) PPC for the current-induced state of PSMO on LAO (T ¼ 30 K). (Inset)
The RT curve after current processing.
07D701-3 J. F. Wang and J. Gao J. Appl. Phys. 109, 07D701 (2011)
Downloaded 28 Nov 2011 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
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Frequently Asked Questions (1)
Q1. What are the contributions mentioned in the paper "Persistent photoconductivity induced by electric currents in epitaxial thin films of pr0.7sr0.3mno3" ?

3MnO3 films were studied.