Magnetic anisotropy and metal-insulator transition in SrRuO3 thin ...

JOURNAL OF APPLIED PHYSICS 107, 113925 2010
Magnetic anisotropy and metal-insulator transition in SrRuO 3 thin films at
different growth temperatures
X. W. Wang, X. Wang, Y. Q. Zhang, a 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
Received 25 December 2009; accepted 23 April 2010; published online 14 June 2010
Magnetic and transport properties of SrRuO 3 film grown on SrTiO 3 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 OOP 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 or insulator behavior are caused
mainly by strain in the SRO films, with correspondingly larger strain in films grown at lower
temperatures. © 2010 American Institute of Physics. doi:10.1063/1.3431459
I. INTRODUCTION
a Electronic mail: yqzhang@imr.ac.cn.
SrRuO 3 SRO 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 initial 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
SrTiO 3 STO using 90° off-axis sputtering, 7 its Curie
temperature decreases 10 K and saturation magnetization decreases
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 relaxed 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 only a few unit cells thick, 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 ortho-I and ortho-II with different
lattice parameters. When grown at a temperature of
780 °C the film becomes predominantly ortho-I relaxed
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 690 °C with predominantly ortho-II
strained 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 generated
by the different growth temperatures 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 OOP magnetic anisotropy at 750 °C becomes
0021-8979/2010/10711/113925/5/$30.00
107, 113925-1
© 2010 American Institute of Physics
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113925-2 Wang et al. J. Appl. Phys. 107, 113925 2010
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 001 SrTiO 3 substrates by pulsed laser deposition PLD
using a KrF =248 nm excimer laser, with a flux of approximately
2 J/cm 2 and a repetition rate of 2 Hz under a
process pressure of 0.3 mbar of pure O 2 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 XRD, D/max-
2000 Cu K =1.5406 Å and transmission electron microscope
TEM. Bulk SRO crystallizes in an orthorhombic
Pnma 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 AFM, Digital Instruments, Nanoscope IV
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 IP direction by the standard fourterminal
method in the range from 5 to 310 K.
III. RESULTS AND DISCUSSION
Figure 1a shows the -2 XRD patterns for SRO films
grown at temperatures from 600 to 750 °C. Only SRO reflections
close to the STO 001-family are found, but when
SRO films are grown on 001 STO, the film can be grown
epitaxially with its 001, 110, or11¯0 planes parallel to
the STO 001 surface. 10 Also, because of the near degeneracy
of the 002 and 110 planar spacing of SRO, it is
difficult to absolutely determine the film orientations with
the -2 XRD patterns as the only reference. Figure 1b 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 001-oriented and 11¯0-oriented,
as shown in Figs. 1c and 1d, 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 EDP from a single domain can
FIG. 1. Color online a The -2 XRD patterns for SRO films grown at
substrate temperature ranging from 600 to 750 °C. b The low magnification
TEM micrograph for a cross-sectional film grown at 700 °C. c and
d Electron diffraction patters of two different regions of SRO film grown
on at 700 °C. e The growth temperature T g dependence of OOP lattice
constants c.
be obtained. Figure 1c is a superposition of EDPs of 001 f
and 100 s , whereas Fig. 1d is a composite EDP of 11¯0 f
and 100 s . Subscripts s and f denote substrate and film, respectively.
The indexation of SRO is based on an orthorhombic
structure. In Fig. 1c, the growth direction of the 001oriented
domain is along 110; while the growth direction of
the 11¯0-oriented domain is also along 110, as shown in
Fig. 1d. So, the whole SRO film grows along the 110
direction which is very similar to Ref. 11. It is observed in
Fig. 1e that the calculated OOP lattice constant c in
pseudocubic notation 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
shown in Fig. 1, 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 3D AFM surface
morphologies of SRO films grown at different temperatures
and also of the raw STO substrate. The surface of the raw
STO substrate as reference is very smooth with a roughness
of 0.2 nm 55 m 2 . 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 55 m 2 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
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113925-3 Wang et al. J. Appl. Phys. 107, 113925 2010
FIG. 2. Color online 3D AFM images of raw SRO substrate and SRO films grown at different temperatures a 600 °C, b 650 °C, c 700 °C, and d
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 3a–3d 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. 3d shows the
growth temperature dependence of the magnetic moment recorded
at 30 kOe 1 Oe equals about 80 A/m 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
CoFe 2 O 4 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
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113925-4 Wang et al. J. Appl. Phys. 107, 113925 2010
(a)
(c)
(a)
(b)
FIG. 4. Color online a and b Temperature dependence of resistivity of
SRO films grown at different temperatures 600, 650, 700, and 750 °C.
Inset in b: R-T curve of film grown at 650 °C, replotted in the temperature
range from 150 to 300 K.
(b)
FIG. 3. Color online a–d Hysteresis loops of SRO films grown at
different temperatures 600, 650, 700, and 750 °C in the OOP and in the IP
directions at 10 K. Inset in d: the growth temperature T g 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 forces 15 are not very spectacular in themselves
and have been reported in other references. It can be seen
from the inset of Fig. 3d 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 not shown here 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 4a and 4b 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
about 195 cm. 16 A minimum of resistivity appears at
15 K in the film grown at 700 °C. It is commonly thought
(d)
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 600 °C. 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. 17 125
K and our case 158 K for the film grown at lower temperatures
690 and 600 °C 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 from 5 to 300 K 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-
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113925-5 Wang et al. J. Appl. Phys. 107, 113925 2010
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 or insulator
behavior 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.
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