Magnetotransport Properties of Compression Molded CrO 2 -Polyimide Composite
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EE13.23.1
Magnetotransport Properties of Compression Molded CrO2-Polyimide Composite Sanjay R. Mishra1, Kartik Ghosh2, Joe Losby1, Ted Kehl2, and Ann Viano3 1. Department of Physics, The University of Memphis, Memphis, TN 2. Department of Physics, Astronomy, and Materials Science, Southwest MissouriState University, Springfield, MO 3. Department of Physics, Rhodes College, Memphis, TN ABSTRACT The conductivity and magnetotransport properties of compression molded halfmetallic CrO2/Polyimide composites over a range of different metallic concentrations have been studied. The conductivity measurements on these composites show negative slope of resistance versus temperature. The magnetoresistance measurement indicates obvious enhancement at low temperatures. The maximum in magnetoresistance (MR) is found to be temperature and metal volume fraction dependent. Significant differences in high and low temperature magnetoresistive behavior in the composite have been observed. The high field, 15 T MR measurements show 23% and 19% MR enhancement at 5K and 75 K, respectively. Thus, it is found that the polymer barrier can contribute to enhancing magnetoresistive properties of the composite.
INTRODUCTION Recently, many studies have focused on the extrinsic magnetotransport effects in ferromagnetic oxides with half metallicity because there is an intensive demand for highly spin-polarized ferromagnetic materials for spintronics devices. The nearly perfect spin polarization of CrO2 suggests that it would be ideal for applications in ferromagnetic tunneling junctions, where a large magnetoresistance (MR) ratio is expected. More recently, therefore, much effort has been made to obtain large extrinsic MR, and understand the physical properties of polycrystalline ceramics, films and pressed powders. The display of low field MR in these samples is believed to be due to a spin polarized tunneling mechanism. Coey et. al reported enhanced MR; ~50% at 5T field at 5K in CrO2/Cr2O3 composites [1]. Since MR in granular composites is dependent on the nature of the barrier, type, height, and thickness, many experimental works on spindependent transport in half-metallic ferromagnetic materials have focused materials with tunneling barriers such as CrO2/Cr2O3, CrO2/TiO2, La2/3Ca1/3MnO3/polymer composites [2,3]. Although low temperature MR enhancement in these half-metal composites has been widely studied, there are few reports on MR enhancement at higher temperature. Thus, the study presented here looks at novel barriers, other than conventional ceramic barriers used in previous studies, for MR enhancement. Herein we report field dependent transport properties of CrO2/polymer composites where the intergranular tunneling is through the polyimide barrier. Considerable enhancement in MR in this composite is observed at low temperature which is found to be metal wt%. fraction dependent. EXPERIMENTAL DETAILS
EE13.23.2
The granular composite of xCrO2/(100-x)polyimide were prepared by compression molding (at 300oC) various weight percent, x, of CrO
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