1. Introduction

Colossal magnetoresistance (CMR) is a property of some materials, mostly manganese-based perovskite oxides that enables them to dramatically change their electrical resistance in the presence of a magnetic field, i.e., magnetoresistance (MR) [1–3]. To bring the CMR materials toward applications, it is still necessary to further optimize the sample processing to find the optimal microstructure, especially concerning grains in the nanometer range. The low-field

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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

density of about 0.084 g/cm3

2. Experimental procedures

[25–28].

, which is considerably less than the theoretical density of 6.5 g/

http://dx.doi.org/10.5772/intechopen.80451

97

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

cm<sup>3</sup> [23]. Furthermore, there is no sample size limitation imposed by the fabrication technique, as electrospinning may produce very large sample sizes [24]. This makes such fabrics interest-

In order to achieve a better understanding of the transport properties through these nanowire network fabrics, we also performed a thorough microstructure analysis including scanning

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

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.

ing for applications in bulk form, whenever the weight of the sample counts.

electron microscopy (SEM) and transmission electron microscopy (TEM).

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 specific physical properties depending on the chosen preparation route.

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 elements or electrodes [20–22].

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

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 density of about 0.084 g/cm3 , which is considerably less than the theoretical density of 6.5 g/ cm<sup>3</sup> [23]. Furthermore, there is no sample size limitation imposed by the fabrication technique, as electrospinning may produce very large sample sizes [24]. This makes such fabrics interesting for applications in bulk form, whenever the weight of the sample counts.

In order to achieve a better understanding of the transport properties through these nanowire network fabrics, we also performed a thorough microstructure analysis including scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
