2. Experimental procedures

magnetoresistance response (LFMR) of such manganese perovskite oxides like La1<sup>x</sup>SrxMnO3 (LSMO) with different doping levels x is closely connected to the existence of interfaces and grain boundaries (GBs) within the samples [4–7], and the high-field magnetoresistance (HFMR) was found to increase progressively on decreasing the grain size [8]. Therefore, the measurements of nanostructured or nano-sized samples are an important issue to provide a deeper understanding and further improvement of the MR effect, especially for the improve-

Commonly, the manganese perovskite samples studied in the literature are prepared as bulks or as thin films, mainly on SrTiO3 substrates. Nanometer-sized bridges are then prepared using lithography techniques or the focused ion-beam technique [9, 10]. The use of a substrate in the case of thin-film samples is always causing strain effects within the functional layer due to the lattice mismatch, which may play an important influence on the resulting magnetic properties [11]. The situation may be considerably different in nanostructures without substrate, being recently investigated in several types of nanoscale composites [12–14]. These nanostructures include nanorods, nanowires, nanotubes, and nanobelts; all of them having

In the present contribution, we have fabricated nanowire network fabrics of La1<sup>x</sup>SrxMnO3 (LSMO) with different doping levels x by means of the electrospinning technique [15–17]. This technique is common for the fabrication of organic polymer nanostructures but can be modified by employing different precursors to deliver inorganic compounds. Up to now, only a small number of reports are dealing with magnetic nanostructures prepared in this way [17– 19]. In the case of LSMO, exhibiting the CMR effect, the resulting nanowires are of polycrystalline nature with a high-aspect ratio (length up to 100 μm and diameters of about 230 nm) and show a large number of grain boundaries (GBs) within each individual nanowire. In Ref. [19], individual nanowires were separated from the as-spun networks and placed on a prepatterned substrate. Electric contacts were prepared using Ti/Cu electrodes, and the nanowires could be measured individually, allowing to observe a dependence of the MR on the nanowire diameter. Single nanowires of CMR materials may be employed as sensitive gas sensor ele-

However, the as-spun nanowires form a nonwoven fabric-like network, where numerous interconnects between the individual nanowires are formed in the final heat treatment step. As a result, the current flow through such a nanowire network fabric shows percolative character, and several sub-loops can be formed. The interconnects between the individual nanowires add additional crossover points for the currents and can enhance the tunneling transport across the interfaces, together with the GBs. This additional scattering of the electrons at the interfaces provided by the interconnects is lost when measuring only extracted parts of the nanowires as done in Ref. [19]. Furthermore, no information on the LSMO grain size of their nanowires was presented. An analysis of the grain sizes within our nanowires showed values ranging between 10 and 32 nm. Therefore, it is obvious that the LSMO grains

Therefore, we may expect interesting new properties of this new class of magnetic material. Furthermore, the nanowire network fabrics are an extremely lightweight material with a

ment of the behavior of devices based on the MR effect in reduced dimensions.

specific physical properties depending on the chosen preparation route.

within the present nanowires are smaller as compared to, e.g., Ref. [8].

ments or electrodes [20–22].

96 Nanowires - Synthesis, Properties and Applications

The electrospinning precursor is prepared by dissolving La, Sr, and Mn acetates in PVA (highmolecular-weight polyvinyl alcohol). The PVA is slowly added to the acetate solution with a mass ratio of 2.5:1.5. This solution is stirred at 80C for 2 h and then spun into cohering nanofibers by electrospinning. To remove the organic compounds and to form the desired LSMO phase, the sample is subsequently heat treated in a lab furnace. An additional oxygenation process is required to obtain the correct phase composition. The constituent phase was checked by means of X-ray diffraction (XRD) and EDX analysis. Further details about the electrospinning process of ferromagnetic and superconducting nanowires are given elsewhere [25–28].

Figure 1. Images of the electrospun samples fabricated from the La0.8Sr0.2MnO3 precursor. Images (a) and (b) present a view of an as-prepared sample before thermal treatment, whereas images (c) and (d) give a La0.8Sr0.2MnO3 sample after the whole annealing process applied. As can be directly seen from the images, the size (area) of the sample shrinks to one sixth as compared to the original one after the thermal treatment.

Figure 1(a, b) presents photographs of an as-grown La0.8Sr0.2MnO3 nanowire fabric. The nanowire fabric consists of polymer nanowires containing the ceramic precursor material. The as-grown fabric has a white color, and the entire fabric sample is fully flexible. Figure 1(c, d) finally presents the fully reacted sample after having received the full thermal treatment. The reacted sample shows a fully black color, indicating the completed chemical reaction. As a result, the final nanowire network fabric is extremely thin and brittle. Here, it is important to note that the sample size shrunk to about one sixth of its original size. This shrinkage has to be considered for the application of such fabric-like materials. In the thermal treatment, numerous interconnects between the individual nanowires are formed, which are essential for the resulting current flow through the sample.

The entire nanowire network was electrically connected by means of silver paint and Cu wires (50 μm diameter) to the sample holder. Due to the high fragility of the ceramic sheet, a pseudo four-point configuration is realized where the current and voltage links connect immediately on the sample contacts. This arrangement is presented in Figure 2. The magnetoresistance is measured in a 10/12 T bath cryostat (Oxford Instruments Teslatron) with a Keithley source meter (model 2400) as a current source, and the voltage is recorded using a Keithley 2001 voltmeter.

> The constituent phases of the samples were determined by means of a high-resolution automated RINT2200 X-ray powder diffractometer using Cu-K<sup>α</sup> radiation (40 kV, 40 mA) Figure 3. SEM imaging was performed using a Hitachi S800 scanning electron microscope operating at a voltage of 10 kV, and the TEM analysis was performed by a JEOL JSM-7000F transmission electron microscope (200 kV, LaB6 cathode). For TEM imaging, pieces of the nanowire network fabrics were deposited on carbon-coated TEM grids. High-resolution TEM and EBSD were performed on selected nanowire sections being thin enough for electron transmission (Figure 6). The magnetization of the nanowire networks was measured using a SQUID magnetometer (Quantum Design MPMS3) with 7 T magnetic field applied perpendicular to the sample

Magnetoresistance and Structural Characterization of Electrospun La1−*x*Sr*x*MnO3 Nanowire Networks

Scanning electron microscopy revealed an average diameter of the resulting nanowires of around 220 nm and a length of more than 100 μm. Fabric-like nanowire networks with numerous interconnects are formed after the heat treatment. The individual nanowires are polycrystalline with a grain size of about 10–30 nm, which corresponds to the dimensions obtained via transmission electron microscopy and electron backscatter diffraction (EBSD)

This is presented in Figure 4 giving SEM images of the nanowire network fabrics at 5000 magnification (first column) and at higher magnification (10,000, second column) for all LSMO samples studied here. Figure 4(a) and (b) shows the sample x = 0.2, (c) and (d) the

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surface, using a piece of the nanowire network fabric with a size of 14.86 mm2

Figure 3. XRD measurements on all three types of LSMO nanowire fabric samples.

3. Results and discussion

3.1. Microstructure

analysis.

Figure 2. Nanowire network sample with electrical contacts for the quasi four-point measurement.

Magnetoresistance and Structural Characterization of Electrospun La1−*x*Sr*x*MnO3 Nanowire Networks http://dx.doi.org/10.5772/intechopen.80451 99

Figure 3. XRD measurements on all three types of LSMO nanowire fabric samples.

The constituent phases of the samples were determined by means of a high-resolution automated RINT2200 X-ray powder diffractometer using Cu-K<sup>α</sup> radiation (40 kV, 40 mA) Figure 3. SEM imaging was performed using a Hitachi S800 scanning electron microscope operating at a voltage of 10 kV, and the TEM analysis was performed by a JEOL JSM-7000F transmission electron microscope (200 kV, LaB6 cathode). For TEM imaging, pieces of the nanowire network fabrics were deposited on carbon-coated TEM grids. High-resolution TEM and EBSD were performed on selected nanowire sections being thin enough for electron transmission (Figure 6).

The magnetization of the nanowire networks was measured using a SQUID magnetometer (Quantum Design MPMS3) with 7 T magnetic field applied perpendicular to the sample surface, using a piece of the nanowire network fabric with a size of 14.86 mm2 .
