行政院國家科學委員會專題研究計畫成果報告
Thickness-dependent magneto-transport properties in
La0.67Sr0.33MnO3 films
計畫編號:Ａ-91-Ｅ-ＦＡ04-1-4
執行時間:94 年 4 月 1 日至 95 年 3 月 31 日
主持人:吳泰伯
國立清華大學材料科學工程學系
計畫參與人員:廖政華
國立清華大學材料科學工程學系
Abstract
加由 105Ｋ增加至 300Ｋ。其導體-絕
Thickness-dependent LSMO films
were grown on SrTiO3(001) substrates
by RF magnetron sputtering. Their
緣體轉換溫度在 30 奈米厚度以上會與
居禮溫度幾乎相同。然而，5 奈米厚度
之 LSMO 卻呈現絕緣的現象。此主要是
structure
and
magneto-transport
properties were measured. The LSMO
films are under biaxial tensile stress on
STO substrates. The Curie temperature
(Tc) of the films increased with the film
thickness from 105 K to 300K. The
metal-insulator transition temperature
因應變使得可流動之電子在部份區域
形成極子，因而使得 LSMO 層的導電度
下降，形成絕緣態。
(TMI) has almost the same value as Tc
for films thicker than 30 nm. However,
for 5 nm film displays insulating over
whole temperature. The strain induced
by the substrate depresses the itinerant
electrons to form “ polaron “ in certain
region. The formation of polaron will
reduce the conductivity of LSMO films,
thus result in the insulating state.
摘 要
我們利用雙射頻磁控濺鍍槍成功
的鍍製出不同厚度的鑭鍶錳氧
（(La,Sr)MnO3）在 SrTiO3 (001)的基
板 上 。其結構與磁傳 導特性 已被測
試。LSMO 層在 STO 基板上受到雙軸張
應力。其居禮溫度(Tc)隨著膜厚的增
Introduction
Over the past few years,
perovskite-type manganites such as
RxA1−xMnO3 (R = rare earth elements
and A = alkaline earth elements) have
been extensively investigated because of
their colossal magnetoresistance (CMR)
properties.1 In these CMR materials, the
magnetic transition is accompanied by a
metal-insulator transition (MIT). At high
temperature, they are paramagnetic
insulators, while they are ferromagnetic
metals at low temperature. The double
exchange mechanism proposed by
Zener2 qualitatively explains the
phenomenon. Recently, these materials
have been explored in the form of thin
films, whose properties are much
different from the bulk ones. Their
physical properties have been attributed
grown by RF magnetron sputtering on
to the structural and magnetic
modifications at the interfaces between
the films and the substrate.3,4 It is well
known that strain due to lattice
mismatch, lattice distortions result from
substrate imperfections or even the
thickness of film can strong affect the
properties of manganite thin films.5 For
example, tensile strain suppresses
ferromagnetism which is generally
SrTiO3 (001) single-crystal substrates.
During deposition, the substrate
temperature was kept at 780 oC and the
gas pressure of deposition was fixed at
10mtorr with an Ar/O2 ratio 3:1. The
deposition rate is 1.56 nm/min. A
thickness-dependent LSMO films were
fabricated in order to study the strain
effect induced by the substrate.
The thickness-dependent LSMO
interpreted by considering a strain
induced distortion of MnO6 octahedra.5,6
Sun et al. reported that the lattice strain
films were characterized by x-ray
diffraction. The chemical composition of
LSMO films were determined from
in La0.67Sr0.33MnO3 is gradually relaxed
with increasing film thickness where the
dead layer is estimated to be 3nm for
films on NdGaO3 and 5nm for films on
LaAlO3.7 Khartsev et al. revealed that
the thickness dependence of MIT
temperature (TMI) in La0.75Sr0.25MnO3 is
inductively
coupled
plasma
spectrometry (ICP) with an accuracy of
±3 at%. The surface morphology of the
films was investigated with an
atomic-force microscope (AFM). The
magnetic properties of the samples were
investigated using a superconducting
due to the stress caused by
film-substrate mismatch.8 These results
suggest that both TMI and Curie
temperature (TC) are decreased with
increasing lattice strain.
In this work, we study the
relationship between the strain effect
induced
by the
substrate
and
magneto-transport
properties
with
quantum interference device (SQUID)
magnetometer and magneto-transport
data were obtained with PPMS, both
from Quantum Design. The diamagnetic
contribution of the substrate was
subtracted from the data. During the
magnetic measurements, the field was
applied along the [100] direction in
order to minimize the demagnetization
different thickness of LSMO films. The
LSMO films ( Cbulk LSMO = 3.889 Å )
on STO ( CSTO = 3.911 Å ) have a tensile
stress, which suppressed both the TC and
TMI.
effects.
Experiments
The (La0.67,Sr0.33)MnO3 films were
Results and discussions
Fig. 1 shows the temperature
dependent magnetization measurements
with different deposition temperature.
The increasing of the deposition
temperature slightly enhances Tc but
improves the remnant magnetization rate
significantly from 25% (700℃) to 75%
(780℃) at T= 200K. For higher
curves of the LSMO films clearly
deposition temperature, the sputtering
atoms have efficient energy to form a
perfect structure so as to diminish the
defect centers, which reduce the
probability of pinned-spin when apply a
reversal magnetic field and hence
increase the remnant magnetization rate.
The surface morphology of the
exhibit a paramagnetic to ferromagnetic
(FM) transition but their shapes and
Curie temperatures are dependent upon
the film thickness. For thickness thicker
than 10 nm, Tc increases lightly from
270K to 300K. However, for those
thinner than 10nm, Tc drops drastically
to 110K for thickness = 5 nm. According
to double exchange (DE) mechanism, Tc
is proportional to the hopping amplitude,
films was given in Fig. 2. The film
thickness = 90 nm and the root mean
square roughness is 0.899nm, which is
to, of eg electron between Mn3+ and Mn4+
through the Mn-O-Mn network. tensile
strain raises the in-plane Mn-O band
quite smooth for sputtering technique.
Figure
3
shows
the
remnant
magnetization ratio, Pr/Ps, measured at
T = 10K and 100K with film thickness =
90nm. The increasing of the temperature
decreases the value of Pr/Ps from 78.5%
to 73.1% and, on the contrary, enhances
length, d, decreases to as to α d-3.5, and
thus Tc.10, 11 For thinner films, the eg
electrons have a tendency to form the
localize charge due to a strong strain
effect, which reduce the probability of
magneto-transport between two Mn
atoms. As the thickness increases, the
the magnetic coercivity (Hc). Normally,
the remnant magnetization ratio
decreases with increasing temperature
due to the thermal-induced spin
perturbation near the surface and/or
interface,9 which would reduce the
exchange coupling and the magnetic
coercivity, respectively. Moreover, the
value of Hc is about 100 Oe and as a
relaxation of strain becomes more
significant and thus enhances the
hopping amplitude, results in the
increasing of Tc. The situation is the
same
for
temperature-dependent
resistivity measurements, as describe
below.
Fig. 5 displays the temperature
dependent resistivity with different film
result the LSMO could be seen as a soft
magnetic phase.
Fig.
4
shows
the
temperature-dependent
magnetization
with different film thickness. The
magnetization data were collected at 200
Oe, which were normalized by the
magnetization value at 5 K. All the M(T)
thickness. As can be clearly seen in Fig.
5, with increasing film thickness the TMI
shifts to higher temperature, which
coincides
with
the
temperature
dependence
of
normalized
magnetization M(T) curves for different
film thickness. The TMI obtained from
the peak of the temperature dependent
ρ(T) curve have almost the same value
We fabricated LSMO films
as Tc obtained from M(T) curve for
films thicker than 30 nm. Approximately,
the resistivity of films increases with
decreasing the film thickness. It is
interesting to notice the thinner films as
5 and 10 nm given in the inset of Fig. 5.
The resistivity of both the films is larger
than the others for an order ~ 2. For 10
nm film shows a TMI at 60 K, which is
lower than Tc obtained from M(T) curve.
with different thickness on SrTiO3 (001)
substrates by rf magnetron sputtering.
The M(T) and ρ(T) curves show the Tc
However, for 5 nm film displays
insulating over whole temperature.
These results reveal that the lattice strain
However, the lattice strain induced by
the substrate plays an important role in
magneto-electric transports more than in
induced by the substrate plays an
important role in magneto-electric
transports more than in magnetic
exchange coupling properties. As the
thickness decreases, the itinerant
electrons become localized due to the
Jahn-Teller distortion which elongates
magnetic exchange coupling properties.
This could be due to the information of
the polaron induced by the strain in
certain regions. However, the value of
the resistivity of all the films is on the
same degree, which shows these films
have the same characteristic regardless
and TMI have different type for thickness
thinner than 10 nm. The measured
values of Tc and TMI of the films reflect
that strain plays a significant
macroscopic factor to influence the
Curie temperature and MIT temperature
in thickness-dependent LSMO films.
the in plane Mn-O-Mn bond length and
of the film thickness.
thus the itinerant electrons tend to form
“ polaron “ in certain region. The
formation of polaron will reduce the
conductivity of LSMO films, thus Reference
increase the resistance.12 When the
1. R. Von Helmolt, J. Wecker, R.
temperature rises, the polaron would be
Holzapfel, L. Schultz, and K. Samwer,
“melt” by the thermal energy and the
Phys. Rev. Lett. 71, 2331 (1993).
resistivity drops drastically. It is
2. C. Zener, Phys. Rev. 81, 440 (1951)
important to note that at temperature ~
330 K, the value of the resistivity of all
the films is in the vicinity of 0.002
Ohm-m, which shows these films have
the same characteristic regardless of the
film thickness.
Conclusions
3. K. S. Takahashi, M. Kawasaki, and Y.
Tokura, Appl. Phys. Lett. 79, 1324
(2001).
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Eom, L. Wu, and F. Tsui, Appl. Phys.
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and C. B. Eom, Appl. Phys. Lett. 74,
3017 (1999).
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Grishin, J. Appl. Phys. 87, 2394
(2000).
9. M. Izumi et al., Phys. Rev. B 64,
064429 (2001).
10. T. Kanki, H. Tanaka, and T. Kawai,
Phys. Rev. B. 64, 224418 (2001)
11. J. Zhang, H. Tanaka, T. Kanki, J-H
Choi, and T Kawai, Phys. Rev. B. 64,
184404 (2001)
12. Kwon, C.; Robson, M.C.; Kim, K.-C.; Gu,
J.Y.;
Lofland,
S.E.;
Bhagat,
S.M.;
Trajanovic, Z.; Rajeswari, M. J. Magn.
Magn. Mater. 172, (1997).
M / M (5K)
1.0
thickness=90nm
10K
100K
o
780 C
o
750 C
o
700 C
0.8
M / Ms 1.0
0.5
0.6
0.0
-1000
0.4
-500
0
500
1000
H (Oe)
-0.5
0.2
0.0
-1.0
0
50
100
150
200
250
300
350
T (K)
Fig. 1, The temperature dependent
magnetization
measurements
different deposition temperature.
with
Fig. 3, the remnant magnetization ratio,
Pr/Ps, measured at T = 10K and 100K
with film thickness = 90nm.
5nm
10nm
30nm
60nm
90nm
M / M (5K)
1.0
0.8
0.6
0.4
0.2
0.0
0
50
100
150
200
250
300
350
T (K)
Fig. 2, The surface morphology of the
films with film thickness = 90 nm. The
root mean square roughness is 0.899nm
Fig. 4, The temperature-dependent
magnetization with different film
thickness. The magnetization data were
collected at 200 Oe, which were
normalized by the magnetization value
at 5 K.
0.0006
5nm
10nm
30nm
60nm
90nm
5nm
10nm
0.08
0.0005
0.0004
R (O m)
Resistivity (Ohm-m)
0.10
0.06
0.04
0.02
0.0003
0.00
0
50
100
150
200
250
300
350
T (K)
0.0002
0.0001
0.0000
0
50
100
150
200
250
300
350
T (K)
Fig. 5, The temperature dependent
resistivity with different film thickness.
The insert shows theρ(T) curve of the
thinner films as 5 and 10 nm.