Transport properties and microstructure of La0.7Sr0.3MnO3 nanocrystalline thin films grown by polymer-assisted chemical solution deposition

Perovskite-based materials can be widely used in the aerospace and transportation field. Perovskite manganese oxides La0.7Sr0.3MnO3 (LSMO) thin films were grown on LaAlO3 (100) and Si (100) single crystal substrates by the polymer-assisted chemical solution deposition (PACSD) method. Electronic transport behavior, microstructure, and magnetoresistance (MR) of LSMO thin films on different substrates were investigated. The resistance of LSMO films fabricated on LaAlO3 substrates is smaller than that on the Si substrates. The magnetic field reduces resistance of LSMO films both on Si and LAO in the wide temperature region, when the insulator-metal transition temperature shifts to higher temperature. The low-field magnetoresistance of LSMO films on Si in low temperature range at 1 T is larger than that of LSMO films on LAO. However, the MR of LSMO film on LAO films at room-temperature is about 5.17 %. The thin films are smooth and dense with uniform nanocrystal size grain. These results demonstrate that PACSD is an effective technique for producing high quality LSMO films, which is significant to improve the magnetic properties and the application of automotive sensor.


Introduction
Perovskite-based functional materials have a wide application prospect due to colossal magnetoresistance (CMR) and magnetic refrigeration effects. They can be used to make the magnetic head, magnetic resistance memory, and motor transducer. The transducer can be used to measure and control current, position, angle etc. Moreover, they can be used in aerospace, automotive industry as magnetic refrigeration material. In recent years, as the developments of anisotropic MR effect and CMR in ferromagnetic/nonmagnetic metal multilayer films, the thin film MR sensor has become one of the most active magnetic sensor technologies, which are widely used in mineral exploration, underground drilling, position detection, navigation etc.
Shim et al. [9] reported that the maximum low-field magnetoresistance (LFMR) of LSMO film, was 0.68 %, obtained at 300 K in the 450 Oe field region. Liu et al. [10], also obtained that the LFMR of LSMO film on Si (100) substrate were as high as 2.54 % at H = 0.05 T and 300 K. Different from the intrinsic double-exchange-type CMR, induced by a strong magnetic field in the tesla range, the LFMR effect is attributed to spin dependent tunneling across the grain boundaries (GBs).
Many techniques have been applied to preparing La 0.7 Sr 0.3 MnO 3 epitaxial and polycrystalline films on different substrates including pulsed laser deposition (PLD) [11,12], sputtering [13,14], metal organic chemical vapor deposition (MOCVD) [15,16], molecular beam epitaxy (MBE) [17], chemical solution deposition (CSD) [18] etc. Among these methods, the CSD method has some advantages for the films deposition and growth such as low cost, high stability, and simple process. As for the CSD technique for the preparation of LSMO films, many researches about epitaxy, concentration of precursor solution, annealing, LFMR as well as the GBs on substrates of MgO, LaAlO 3 , Si, SrTiO 3 , and GaAs have been reported [18,19].
In this article, LSMO films were synthesized on LaAlO 3 (100) and Si (100) by polymer-assisted chemical solution deposited (PACSD). The smooth and dense films can be fabricated via this method. Preparation and transport properties on LaAlO 3 (100) and Si (100) were investigated because silicon is a good candidate for spin injection and the lattice mismatch of LAO is small enough to induce epitaxy. The microstructure, the surface, and the electronic transport behavior and magnetoresistance (MR) properties of LSMO thin films were presented.

Experimental
The nanocrystalline thin films of La 0.7 Sr 0.3 MnO 3 were grown by PACSD method on (100) LaAlO 3  Deposition of LSMO was carried out by a spin coater at 500 rpm for 10 s, followed by 4,000 rpm for 50 s. After deposition, the wet films were dried at 80°C for 10 min. Then the films were pyrolized at temperature 200°C for 10 min and 500°C for 20 min. The dried films were finally crystallized at 830-870°C for 1-2 h in air ambience.
The phases were determined by X-ray diffraction using the Philips X-ray diffractometer X'Pert MRD diffractometer (Cu-K a radiation, k = 1.54 Å ), the microstructures of the films were also observed using field emission scanning electron microscope (FESEM, JSM-7001F), and the transport properties and magnetic properties were measured by Physical Property Measurement System (PPMS, Quantum Design 9 T).
3 Results and discussion Figure 1 shows the typical h-2h X-ray diffraction spectra of LSMO film grown on LAO (100) substrates. Only (100) and (200) peaks are observed without any impurity peaks, indicating highly c-axis oriented growth with pseudo-cubic (100) orientation. From the data, the calculated lattice constants are a = 3.876 Ǻ for LSMO and a = 3.796 Ǻ for LAO. The lattice mismatch between LSMO (100) films and LAO (100) substrates is 2.1 %, which are evaluated via the definition of a f À a s a s Â 100 %: XRD pattern of LSMO film on Si (100) substrate is presented in Fig. 2. All the main diffraction peaks belong to LSMO phase, implying that the LSMO film on Si is of single phase with perovskite structure. It is obviously different from the film fabricated on the single crystal substrates of LaAlO 3 , which are advantageous for the growth of (l00) orientation films. The film growth on Si (100) substrate resulted polycrystalline with pseudo-cubic orientation due to the large lattice mismatch degree between LSMO (100) films and Si (100) (a = 0.539 nm) substrates. Nevertheless, the small mismatch between LSMO (110) and Si (100) was reported to be 1.8 % in [10]. Therefore, LSMO grains grow via a self-textured growth mode and are randomly oriented.
In order to evaluate the quality of LSMO thin films from the PACSD method, the microstructures of the films were also examined. Figure 3 shows the FE-SEM images of LSMO films on LAO and LSMO films on Si, which displays that the films are smooth and have high crystal quality. It is clear from Fig. 3b that the LSMO films on Si consist of nanometer polycrystalline. Typical grain sizes are in the range of 50-70 nm with some cavities, which may be related to the volatilization of organic precursors. This is in agreement with the results in [9]. Comparing the morphology of the LSMO films on Si (Fig. 3a) and LAO (Fig. 3b), we find that the latter seems to have denser microstructure and smaller crystallites size of 15 nm or so. It can be seen that LSMO film on LAO is the columnar structure with a thickness of 400 nm. The bulk of film is These results show that substrate has a significant effect on the crystallographic properties of the resulting thin film. Figure 4 shows temperature-dependent electrical resistivity (q -T) for LSMO films on LAO from 360 to 5 K under 0 and 1 T. Note that metal-insulator transition is at 349 K for H = 0 T corresponding to bulk samples temperature around 360 K, which proves a good quality of the film. The film exhibits systematic increase in metal-insulator transition temperature (T MI ) with an increase of applied magnetic field. Due to limitations of the measurement equipment, insulator-metal transition is not observed in the film under applied magnetic field H = 1 T. The resistance is reduced under applied magnetic field 1 T. Within double exchange model, the applied magnetic field can align these disordered spins between parallel arrangements Mn 3? -Mn 4? of LSMO, increase the transition probability of itinerant electrons, and reduce the spin disorder scattering; thereby the resistance is decreased effectively.
The curve of MR versus temperature is shown in Fig. 4, which has an obvious MR peak near T C and the MR fluctuates in whole temperature range. The MR ratio here is illustrated as MR = (q 0 -q H )/q 0 9100 %, where q 0 is the zero field resistivity and q H is the resistivity in the applied field H. With decreasing temperature, MR shows an overall decrease with undulant pattern. The MR of the LSMO films on LAO mainly results from the suppression of spin fluctuation by magnetic field within grains and the contribution of the GBs to MR is inconsiderable [20]. The maximum value of MR is 2.67 % at 0.6 T (not shown here) and 5.17 % at 1 T when T = 300 K.
The temperature dependence of the resistance at different magnetic field (0 and 1 T) from 300 to 5 K and MR curve for LSMO films on Si are shown in Fig. 5. Compared with LSMO films on LAO, the transition temperatures of LSMO films on Si are lowered down to 238 K. Under the applied magnetic fields of 1 T, the resistance is remarkably suppressed in the wide temperature range and the T MI shifts to a higher temperature (246 K), which is dominated by the spin-polarized tunneling at the GBs [21]. The external magnetic increases the carrier capacity and leads to a change in resistivity q. In addition, resistance gradually decreases at lower temperatures for LSMO films on LAO, but rises from 50 to 5 K for LSMO films on Si. The different transport behaviors are assumedly due to the magnetic domain walls at GBs and the reentrant effect in the polycrystalline films [20].
For the LSMO films on Si, the MR increases with decreasing temperature. It is suggested that magnetic external field prevents spin-dependent scattering of polarized electrons at the GBs and results in a large MR.
Obviously, the MR behavior of LSMO films on Si is different from LSMO films on LAO. A similar phenomenon was also observed by Liu et al. [22]. Single crystal and textured films have an MR peak in the vicinity of T MI , while the polycrystalline LSMO films do not show the peak behavior. Thus, the crystalline orientation of the films plays an important role in governing the MR characterization.

Conclusions
Polycrystal LSMO thin films were synthesized on LaAlO 3 (100) and Si (100) single crystal substrates through spincoating method by utilizing PACSD with PVP. XRD data shows evidence that the LSMO films on LAO is a-axis oriented and LSMO films on Si is polycrystalline, implying that the orientation of films depend on lattice mismatch degree between substrates and LSMO films. The resistance of LSMO films fabricated on LaAlO 3 substrates is smaller than that on the Si substrates. The magnetic field leads to reduced resistance of LSMO films both on LAO and Si in a wide temperature region, when the insulator-metal T MI is shifted to a higher temperature. The LFMR of LSMO films on Si in low temperature range at 1 T is larger than that of LSMO films on LAO. The MR of LSMO films on LAO displays large values of MR as high as 5.17 % at room temperature. The MR of LSMO films on Si increases with decreasing temperature. The change in MR is related to GBs effect. Both films are smooth and dense with uniform nanocrystal size grain. The grain size of LSMO films on Si is larger than LSMO films on LAO. From the analysis, it is found that crystalline orientation of substrates plays an important role in affecting grain size, structure, transport properties, and the MR characterization. These results demonstrate that PACSD is a viable method for growing high quality LSMO films.