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

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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