Magnetic anisotropy and metal-insulator transition in SrRuO3 thin films at
different growth temperatures
X. W. Wang, X. Wang, Y. Q. Zhang, Y. L. Zhu, Z. J. Wang et al.
Citation: J. Appl. Phys. 107, 113925 (2010); doi: 10.1063/1.3431459
View online: http://dx.doi.org/10.1063/1.3431459
View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v107/i11
Published by the American Institute of Physics.
Related Articles
Orientation-dependent surface potential behavior in Nb-doped BiFeO3
Appl. Phys. Lett. 100, 172901 (2012)
Laser energy tuning of carrier effective mass and thermopower in epitaxial oxide thin films
Appl. Phys. Lett. 100, 162106 (2012)
Effect of oxygen partial pressure and Fe doping on growth and properties of metallic and insulating molybdenum
oxide thin films
J. Appl. Phys. 111, 083905 (2012)
Laser induced non-thermal deposition of ultrathin graphite
Appl. Phys. Lett. 100, 151606 (2012)
Room temperature deposition of alumina-doped zinc oxide on flexible substrates by direct pulsed laser
recrystallization
Appl. Phys. Lett. 100, 151902 (2012)
Additional information on J. Appl. Phys.
Journal Homepage: http://jap.aip.org/
Journal Information: http://jap.aip.org/about/about_the_journal
Top downloads: http://jap.aip.org/features/most_downloaded
Information for Authors: http://jap.aip.org/authors
Downloaded 23 Apr 2012 to 210.72.130.187. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
JOURNAL OF APPLIED PHYSICS 107, 113925 共2010兲
Magnetic anisotropy and metal-insulator transition in SrRuO3 thin films at
different growth temperatures
X. W. Wang, X. Wang, Y. Q. Zhang,a兲 Y. L. Zhu, Z. J. Wang, and Z. D. Zhang
Shenyang National Laboratory for Materials Science, Institute of Metal Research, and International Center
for Materials Physics, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s
Republic of China
共Received 25 December 2009; accepted 23 April 2010; published online 14 June 2010兲
Magnetic and transport properties of SrRuO3 film grown on SrTiO3 at different substrate
temperatures have been investigated. Metallic behavior over the temperature range from 5 to 300 K
is observed in the film grown at 750 ° C. With a decrease in the growth temperature, a
metal-insulator transition occurs for films grown at 700 and 650 ° C, with transition temperatures of
15 K and 250 K, respectively, and a complete insulator behavior shows up in the film grown at
600 ° C. Correspondingly, out-of–plane 共OOP兲 magnetic anisotropy is gradually weakened, leading
to complete magnetic isotropy in the film grown at 600 ° C. The OOP lattice constant increases from
0.395 nm, for the film grown at 750 ° C, up to 0.403 nm for the film grown at 600 ° C. The
correlation between the magnetic properties, transport properties, and the lattice constants indicates
that the magnetic anisotropy and the metal-insulator transition 共or insulator behavior兲 are caused
mainly by strain in the SRO films, with correspondingly larger strain in films grown at lower
temperatures. © 2010 American Institute of Physics. 关doi:10.1063/1.3431459兴
I. INTRODUCTION
SrRuO3 共SRO兲 film, with a low resistance and high
chemical stability, has been attracting much attention due to
its great potential for applications as oxide electronic devices
based on a heteroepitaxial structure consisting of perovskitebased ferromagnetic, superconducting, and ferroelectric
films.1,2 However, its transport properties, including the magnitude of resistivity, are very sensitive to growth conditions,
such as growth temperature and mode. It was reported earlier
in Ref. 3 that by decreasing the growth temperature from 870
to 700 ° C, SRO film grown on MgO experiences a transition
from metal to insulator without any significant change in
either the Curie temperature or lattice parameters. So, it was
thought that the insulator behavior was caused only by more
disorder produced by lower growth temperatures and/or a
reduced crystalline quality related to the appearance of the
high density of defects such as twins and domain boundaries,
which reflects a decrease in the electronic mean free path.
Recently, it was found that insulator behavior or a metalinsulator transition at low temperature appears in SRO films
with thicknesses less than a critical value, depending on the
degree of disorder during 共initial兲 island growth.4,5 Furthermore, a metal-insulator transition can be induced by ion irradiation on as-grown SRO films on MgO by introducing
disorder.6 In a word, the insulator behavior or metal-insulator
transition at low temperature, reported previously, has been
attributed to disorder. On the other hand, it is well known
that the magnetic and transport properties of SRO films are
strongly dependent on the epitaxial strain from the substrate.
For example, when a strained SRO film is grown on 8° miscut SrTiO3 共STO兲 using 90° off-axis sputtering,7 its Curie
temperature decreases 10 K and saturation magnetization dea兲
Electronic mail: [email protected].
0021-8979/2010/107共11兲/113925/5/$30.00
creases 20%, as compared with bulk material, while its coercive field is more than double that of bulk material. When
SRO films exhibit very high crystallographic quality, thereby
indicating a pure two-dimensional growth mechanism,8 metallic behavior appears in thick films 共relaxed兲 from 10 to
300 K and a ferromagnetic ordering occurs at about 150 K.
With a decrease in film thickness, films which are still metallic in this temperature range do not exhibit ferromagnetic
ordering. In very thin films 共only a few unit cells thick兲, a
semiconducting behavior appears below 30 K. In addition,
the strain induced in the films grown at different substrate
temperatures can also influence the magnitude of resistivity
of the SRO film. It was reported in Ref. 9 that the structure
of all SRO films grown at temperatures ranging between 690
and 810 ° C were found to be a mixture of highly oriented
strained orthorhombic phases 共ortho-I and ortho-II兲 with different lattice parameters. When grown at a temperature of
780 ° C the film becomes predominantly ortho-I 共relaxed兲
and shows a minimum resistivity of 210 ␮⍀ cm at 300 K.
Decreasing the growth temperature increases the resistivity
up to the highest value of 1700 ␮⍀ cm for the lowest
growth temperature 共690 ° C兲 with predominantly ortho-II
共strained兲 phase. As described above, in Ref. 7, the research
on the magnetic properties of strained SRO thin film grown
on 8° miscut STO is mainly compared with bulk properties
and it is found that substrate-induced strain causes a decrease
in the Curie temperature and saturation moment and an increase in the coercive force, in comparison to bulk. In our
paper, we investigate the effect of different strain 共generated
by the different growth temperatures兲 on the magnetic properties of the SRO thin films grown on normal STO substrate.
We noticed a significant effect on the magnetic anisotropy,
and found that with the growth temperature decreasing, the
out-of-plane 共OOP兲 magnetic anisotropy at 750 ° C becomes
107, 113925-1
© 2010 American Institute of Physics
Downloaded 23 Apr 2012 to 210.72.130.187. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
113925-2
Wang et al.
J. Appl. Phys. 107, 113925 共2010兲
gradually weaker resulting finally in magnetic isotropy being
found in films grown at 600 ° C. In addition, we have conducted a detailed research on the effect of different strain on
the metal-insulator transition in SRO thin film at different
growth temperatures.
II. EXPERIMENTAL DETAILS
The SRO films with a thickness of 200 nm were grown
on 共001兲 SrTiO3 substrates by pulsed laser deposition 共PLD兲
using a KrF 共␭ = 248 nm兲 excimer laser, with a flux of approximately 2 J / cm2 and a repetition rate of 2 Hz under a
process pressure of 0.3 mbar of pure O2 at substrate temperatures ranging between 600 and 750 ° C. The films were then
cooled to room temperature at 2 ° C per minute under an
oxygen pressure of 0.4 bar after deposition. Prior to deposition, the PLD chamber was completely cleaned and the other
target in the chamber was completely enclosed leaving only
the SRO target was exposed, thus preventing any crosscontamination of the film from the other target. The substrate
was cleaned in an ultrasonic bath with acetone followed by
ethanol, with no other pretreatment being done. The chamber
was then evacuated using a turbopump down to about 2
⫻ 10−7 mbar to remove any extraneous particles. The structural quality and lattice parameters of the thin films were
investigated using an x-ray diffractometer 共XRD, D/max2000兲 Cu K␣ 共␭ = 1.5406 Å兲 and transmission electron microscope 共TEM兲. Bulk SRO crystallizes in an orthorhombic
共Pnma兲 structure with lattice parameters a = 5.5670 Å, b
= 5.5304 Å, and c = 7.8446 Å. Its lattice parameter is 3.930
Å in pseudocubic notation. The planes and directions of SRO
referred to in this paper are based on the orthorhombic unit
cell. Surface morphology was investigated using atomic
force microscopy 共AFM, Digital Instruments, Nanoscope IV兲
in tapping mode. Magnetic properties were measured by a
superconducting quantum interference device magnetometer
with magnetic fields up to 70 kOe. Transport properties were
measured in the in-plane 共IP兲 direction by the standard fourterminal method in the range from 5 to 310 K.
III. RESULTS AND DISCUSSION
Figure 1共a兲 shows the ␪-2␪ XRD patterns for SRO films
grown at temperatures from 600 to 750 ° C. Only SRO reflections close to the STO 共001兲-family are found, but when
SRO films are grown on 共001兲 STO, the film can be grown
epitaxially with its 共001兲, 共110兲, or 共11̄0兲 planes parallel to
the STO 共001兲 surface.10 Also, because of the near degeneracy of the 共002兲 and 共110兲 planar spacing of SRO, it is
difficult to absolutely determine the film orientations with
the ␪-2␪ XRD patterns as the only reference. Figure 1共b兲 is a
cross-sectional TEM image showing the morphology of the
SRO film grown on STO at 700 ° C. The interface between
the film and the substrate is sharp and flat. Electron diffraction experiments clarify that the as-grown SRO film is composed of domains of both 关001兴-oriented and 关11̄0兴-oriented,
as shown in Figs. 1共c兲 and 1共d兲, taken from the areas including both the film and the substrate. The dimension of each
domain is several hundreds nanometers in length, so that
electron diffraction pattern 共EDP兲 from a single domain can
FIG. 1. 共Color online兲 共a兲 The ␪-2␪ XRD patterns for SRO films grown at
substrate temperature ranging from 600 to 750 ° C. 共b兲 The low magnification TEM micrograph for a cross-sectional film grown at 700 ° C. 关共c兲 and
共d兲兴 Electron diffraction patters of two different regions of SRO film grown
on at 700 ° C. 共e兲 The growth temperature 共Tg兲 dependence of OOP lattice
constants 共c兲.
be obtained. Figure 1共c兲 is a superposition of EDPs of 关001兴f
and 关100兴s, whereas Fig. 1共d兲 is a composite EDP of 关11̄0兴f
and 关100兴s. Subscripts s and f denote substrate and film, respectively. The indexation of SRO is based on an orthorhombic structure. In Fig. 1共c兲, the growth direction of the 关001兴oriented domain is along 关110兴; while the growth direction of
the 关11̄0兴-oriented domain is also along 关110兴, as shown in
Fig. 1共d兲. So, the whole SRO film grows along the 关110兴
direction which is very similar to Ref. 11. It is observed in
Fig. 1共e兲 that the calculated OOP lattice constant 共c兲 共in
pseudocubic notation兲 gradually increases with decreasing
growth temperature. Moreover, a shoulder is observed in the
right side of the SRO peak in films grown at 650 and 700 ° C
共shown in Fig. 1兲, indicating an intermediate evolution of the
OOP lattice constant that is between 0.403 nm found in the
film grown at 600 ° C and 0.395 nm for the one grown at
750 ° C, and corresponds to a state of partial strain relaxation.
Figure 2 shows the three-dimensional 共3D兲 AFM surface
morphologies of SRO films grown at different temperatures
and also of the raw STO substrate. The surface of the raw
STO substrate 共as reference兲 is very smooth with a roughness
of 0.2 nm 共5 ⫻ 5 ␮m2兲. Irregular 3D islands are observed in
the surface of the film grown at 600 ° C with these islands
becoming larger and of a more uniform size as the growth
temperature is increased to 650 ° C. The islands then fade as
the temperature reaches 700 ° C and almost disappear at a
growth temperature of 750 ° C. The root-mean-square roughness of the film surface 共5 ⫻ 5 ␮m2兲 is 3.34 nm and 3.76 nm
for films grown at 600 ° C and 650 ° C, respectively, and
becomes 1.12 and 0.98 nm for films grown at 700 and
Downloaded 23 Apr 2012 to 210.72.130.187. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
113925-3
Wang et al.
J. Appl. Phys. 107, 113925 共2010兲
FIG. 2. 共Color online兲 3D AFM images of raw SRO substrate and SRO films grown at different temperatures 共a兲 600 ° C, 共b兲 650 ° C, 共c兲 700 ° C, and 共d兲
750 ° C.
750 ° C. This indicates that the film surface first becomes
rough and then smooths as increased growth temperature
provides more bond energy.
Figures 3共a兲–3共d兲 show magnetic hysteresis loops of
SRO films grown at different temperatures in the OOP and
the IP directions at 10 K. The inset of Fig. 3共d兲 shows the
growth temperature dependence of the magnetic moment recorded at 30 kOe 共1 Oe equals about 80 A/m兲 in the OOP
direction at 10 K. Magnetic anisotropy is in the OOP direction for the film grown at 750 ° C, in agreement with Refs.
12 and 13. With decreasing growth temperature, the magnetic anisotropy becomes less pronounced and more of the
magnetization rotates into the IP direction. Finally, magnetic
isotropy is found in the film grown at 600 ° C, due to more
induced strain in films grown at lower temperatures. Similar
phenomena was also reported in other systems such as
CoFe2O4 film,14 where magnetic anisotropy is in the IP with
lower temperature growth, and an increase in growth temperature shows more magnetization rotating from IP into
OOP due to strain relaxation. This is in agreement with the
Downloaded 23 Apr 2012 to 210.72.130.187. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
113925-4
(a)
J. Appl. Phys. 107, 113925 共2010兲
Wang et al.
(c)
(a)
(b)
FIG. 4. 共Color online兲 关共a兲 and 共b兲兴 Temperature dependence of resistivity of
SRO films grown at different temperatures 共600, 650, 700, and 750 ° C兲.
Inset in 共b兲: R-T curve of film grown at 650 ° C, replotted in the temperature
range from 150 to 300 K.
(b)
(d)
FIG. 3. 共Color online兲 关共a兲–共d兲兴 Hysteresis loops of SRO films grown at
different temperatures 共600, 650, 700, and 750 ° C兲 in the OOP and in the IP
directions at 10 K. Inset in 共d兲: the growth temperature 共Tg兲 dependence of
magnetic moment recorded at 30 kOe in the OOP direction at 10 K.
theory that the magnetic anisotropy is modified by a magnetoelastic coupling depending on its magnitude and sign. In
addition, Barkhausen jumps were observed in the hysteresis
loop between two regions of opposite magnetic field.
Barkhausen jumps, commonly due to the irreversible motion
of the domain walls between the two regions of opposite
magnetizing forces15 are not very spectacular in themselves
and have been reported in other references. It can be seen
from the inset of Fig. 3共d兲 that the magnetic moment of films
grown at 600 and 650 ° C is almost the same with a value of
about 0.57 ␮B / Ru. at 30 kOe. The magnetic moment increases with increasing growth temperature and to about
1.39 ␮B / Ru. for films grown at 750 ° C, agreeing closely
with the calculated value of 1.45 ␮B / Ru.16 The temperature
dependence of the magnetization 共not shown here兲 was
found to show that the Curie temperature of film grown at
600 ° C is around 158 K, and increases to about 165 K for
films grown at 750 ° C. The decrease in the magnetic moment and the Curie temperature with decreasing growth temperature indicates that these factors are affected inversely by
the greater strain produced at lower temperatures. Namely,
larger induced strain in films at lower growth temperatures
changes the spin-spin coupling through a change in the Ru–
O–Ru interatomic distance or bonding angles, consequently
resulting in changes in the exchange energy among
spins.3,4,16
Figures 4共a兲 and 4共b兲 represent the temperature dependence of the electrical resistivity of SRO films grown at different temperatures. When grown at 750 ° C, the film exhibits a typical metallic behavior in the whole temperature range
from 5 to 310 K. Its resistivity is about 210 ␮⍀ cm at 300
K, which is comparable with that of bulk single crystal SRO
共about 195 ␮⍀ cm兲.16 A minimum of resistivity appears at
15 K in the film grown at 700 ° C. It is commonly thought
that this phenomenon is due to disorder produced during
initial growth. However, the temperature corresponding to
the minimum of its resistivity shifts to 250 K with films
grown at 650 ° C. Moreover, the resistivity behavior is insulator in the whole temperature range from 5 to 310 K when
the film was grown at the lowest temperature 共600 ° C兲. Resistivity behavior in our case is similar to that reported in
Refs. 1 and 17, where SRO films grown on MgO and SRO
films grown on STO experience a transition from metal to
insulator when the growth temperature decreases. However,
as mentioned in the introduction, when SRO films grown on
MgO,3 no significant change in either Curie temperature or
lattice constant is observed with variation in growth temperature. But in our case and Ref. 17, OOP lattice constant increases gradually with decreasing growth temperature, indicating that the film has more OOP tensile strain when grown
at lower temperature possibly indicating a better epitaxial
quality. Furthermore, the Curie temperature in Ref. 17 共125
K兲 and our case 共158 K兲 for the film grown at lower temperatures 共690 and 600 ° C兲 both decrease, compared with
ones grown at a higher temperature. In addition, in our case,
the magnetic moment of the film grown at 600 ° C decreases
slightly more than 60% as compared with the value of the
film grown at 750 ° C. So we argue that metal-insulator transition and insulator behavior in our films grown at lower
temperature are mainly due to larger strain produced at lower
growth temperature. In addition, 3D islands observed in our
films grown at lower temperatures inevitably introduce microstructure disorder, which also contributes to metalinsulator transition or insulator behavior.
IV. CONCLUSIONS
In summary, magnetic and transport properties of SRO
films grown with a variation in growth temperature have
been investigated. With decreasing growth temperature, OOP
lattice constants increase from 0.395 nm at 750 ° C to 0.403
nm at 600 ° C, correspondingly, OOP tensile strain increases.
As a result, metallic behavior appears in the whole temperature range 共from 5 to 300 K兲 for the film grown at 750 ° C
and a metal-insulator transition occurs at 15 K and 250 K for
films grown at 700 ° C and 650 ° C, respectively, and then a
complete insulator behavior appears in whole measured temperature range for film grown at 600 ° C. At the same time,
OOP magnetic anisotropy gradually transforms into mag-
Downloaded 23 Apr 2012 to 210.72.130.187. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
113925-5
Wang et al.
netic isotropy. The Curie temperature of 158 K for the film
grown at 600 ° C is lower than that of 165 K for the film
grown at 750 ° C and the magnetic moment of the former
film is only 40% of the latter one. It is concluded that magnetic anisotropy and metal-insulator transition 共or insulator
behavior兲 are mainly caused by strain in films grown at different temperatures.
ACKNOWLEDGMENTS
This work has been supported by the National Natural
Science Foundation of China under Grant No. 50802098, the
Hundred Talents Program of Chinese Academy of Sciences
and the National Basic Research Program No.
2010CB934603 of China, and the Ministry of Science and
Technology of China.
1
K. S. Takahashi, A. Sawa, Y. Ishii, H. Akoh, M. Kawasaki, and Y. Tokura,
Phys. Rev. B 67, 094413 共2003兲.
2
J. F. Scott, Ferroelectric Memories 共Springer, New York, 2000兲.
3
Z. Sefrioui, D. Arias, M. A. Navacerrada, M. Varela, G. Loos, M. Lucía, J.
Santamaría, F. Sánchez-Quesada, and M. A. López de la Torre, Appl.
Phys. Lett. 73, 3375 共1998兲.
4
D. Toyota, I. Ohkubo, H. Kumigashira, M. Oshima, T. Ohnishi, M. Lipp-
J. Appl. Phys. 107, 113925 共2010兲
maa, M. Takizawa, A. Fujimori, K. Ono, M. Kawasaki, H. Koinuma,
Appl. Phys. Lett. 87, 162508 共2005兲.
5
G. Herranz, B. Martinez, J. Fontcuberta, F. Sanchez, C. Ferrater, M. V.
Garcia-Cuenca, and M. Varela, Phys. Rev. B 67, 174423 共2003兲.
6
Z. Sefrioui, M. A. López de la Torre, D. Arias, M. A. Navacerrada, M.
Varela, M. Lucía, J. Santamaría, and F. Sánchez-Quesada, Physica B 259–
261, 938 共1999兲.
7
Q. Gan, R. A. Rao, C. B. Eom, J. L. Garrett, and M. Lee, Appl. Phys. Lett.
72, 978 共1998兲.
8
P. Orgiani, C. Aruta, G. Balestrino, S. Lavanga, P. G. Medaglia, and A.
Tebano, Eur. Phys. J. B 26, 23 共2002兲.
9
P. Rundqvist, A. Vorobiev, S. Gevorgian, K. Khamchane, and Z. Ivanov, J.
Appl. Phys. 93, 1291 共2003兲.
10
J. C. Jiang, W. Tian, X. Q. Pan, Q. Gan, and C. B. Eom, Appl. Phys. Lett.
72, 2963 共1998兲.
11
J. C. Jiang, X. Q. Pan, and C. L. Chen, Appl. Phys. Lett. 72, 909 共1998兲.
12
L. Klein, J. S. Dodge, T. H. Geballe, A. Kapitulnik, A. F. Marshall, L.
Antognazza, and K. Char, Appl. Phys. Lett. 66, 2427 共1995兲.
13
Q. Gan, R. A. Rao, C. B. Eom, L. Wu, and F. Tsui, J. Appl. Phys. 85, 5297
共1999兲.
14
P. D. Thang, G. Rijnders, and D. H. A. Blank, J. Magn. Magn. Mater. 310,
2621 共2007兲.
15
R. Palai, H. Huhtinen, J. F. Scott, and R. S. Katiyar, Phys. Rev. B 79,
104413 共2009兲.
16
P. B. Allen, H. Berger, O. Chauvet, L. Forro, T. Jarlborg, A. Junod, B.
Revaz, and G. Santi, Phys. Rev. B 53, 4393 共1996兲.
17
K. Khamchane, R. Gunnarsson, and Z. G. Ivanov, Mater. Res. Soc. Symp.
Proc. 751, Z3.20.1 共2003兲.
Downloaded 23 Apr 2012 to 210.72.130.187. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions