Magnetoresistive and magnetic properties of electrospun La1−xSrxMnO3 nanowires Thomas Karwoth1, XianLin Zeng1, Andrew Kostrubanic2, 1 1 Michael Koblischka , Uwe Hartmann 1 Institute
of Experimental Physics, Saarland University, P.O. Box 151150, D-66041 Saarbrücken, Germany 2 Drexel University, Philadelphia, Pennsylvania 19104, USA
Abstract Nanowires of the material class La1−xSrxMnO3 with different doping levels x = 0.2, 0.3, 0.4 were fabricated employing a sol-gel-process via electrospinning and a subsequent thermal treatment process based on thermal gravity analysis results. Investigations by means of scanning electron microscopy revealed an average diameter of the resulting nanowires of around 220 nm and a length of more than 50 µm. The chemical phases of the samples have been confirmed via X-Ray diffraction. The nanowires are polycrystalline with a grain
size of about 30 nm, which corresponds to the result obtained from transmission electron microscopy. Analyses of the electronic transport properties and of the magnetoresistive effects of the nanowire samples were carried out by a four probe measurement inside a bath cryostat. Of interest are size effects and the dependence of the properties on the stoichiometry. SQUID measurements of M(T) and M(H) at room temperature, 77 K and 4.2 K were carried out as well, revealing the soft magnetic character of the nanowires.
Introduction
Electrospinning method
The La1-xSrxMnO3 is one of the most typical colossal magnetoresitive (CMR) material with high Curie temperature (TC). A number of structural and magnetotransport studies of poly- and single-crystals, as well as thin films of this doped manganite system gained interest in the literature over the past years, including the research of the influence of grain boundaries and spin polarization on the magnetoresistive behavior. In order to have a better comprehension of this relationship, it is a good approach to investigate polycrystalline nanowires with smaller grain size and a high number of grain boundaries.
This method is commonly used for polymer fiber fabrication. Combined with a suitable thermal treatment, it will be a potential method for inorganic nanowire fabrication. The precursor is pushed by a pump, a liquid drop appears at the bottom of a needle, which is connected to a high voltage source. The drop becomes electrostatically charged. When the Coulomb force overcomes the surface tension, thousands of fibers spray out from the drop, which are pulled down to the electrically grounded collection area.
Sample preparation
Magnetic properties
After Electrospinning and subsequent thermal treatment in a box furnace, the sample appears as a black, light ceramic while the randomly oriented nanowire structure remains (lenghts of up to 50 µm and widths of around 300 nm are obtained). The conctacs for the 4 probe resistance measurement are made by application of silver paint (Acheson DAG1415). The grain size is about 30 nm according to the observations by TEM.
Magnetization curves are recorded at 10 K and 300 K for x = 0.2. The saturation magnetization MS at 10 K and 300 K are 1.35×10-4 Am2 and 0.52×10-4 Am2 respectively, which is similar to the thin film sample with comparable grain size[2]. The coercivity field at 300 K is lower than electrospun nanowires prepared with PVP[3] as a polymer, which indicates that the sample has less defects. The susceptibility curve shows that the sample is at ferromagnetic state before room temperature. The Curie temperature is 349 K according to the field applied thermal gravity analysis which is shown in the plot inset.
Resistive and Magnetoresistive properties At a high Sr doping level (x = 0.4) the MR rises with field in the whole temperature range, and we observe a maximum MR at around 50 K, whereas with x = 0.3 the maximum MR shifts to a higher temperature. Here the 2.5 T MR curve shows a crossing to the others at temperatures above 200 K and 250 K. With x = 0.2 we observe a similar behavior.
In comparison to bulk material observations, the metallic behavior of the sample is suppressed, which could be contributed by the small grain size and the high number of grain boundaries, which act as scattering sites for the conduction electrons[1][2].
Further investigations on isothermal MR measurements at different temperatures proves this behavior which indicates, that the spinpolarization tunneling at grain boundaries is dominating the MR behavior[4][5].
After normalization to the room temperature resistivity the zero field cooling resistivity of samples with different doping ratio (x = 0.2, 0.3, 0.4) exhibits three temperature regimes. In the first regime, ranged from room temperature to 125 K, the resistance decreases with higher doping level; the second one, ranging from 125 K to 65 K, the curves for lower doping levels flatten out crossing each other, as for the third regime from 65 K to 4 K the resistance shows smaller values for low doping ratios.
s9thkarw@stud.uni-saarland.de x.zeng@physik.uni-saarland.de
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