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

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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 Downloaded 09 Jul 2010 to 210.72.130.85. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp
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 Downloaded 09 Jul 2010 to 210.72.130.85. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp
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