Giant Magnetoresistance Inpolyaniline And Its Nanocomposites

The giant magnetoresistance (GMR) has revolutionized magnetic storage industry through the hugely increased recording density of hard drivers by almost four orders of magnitude and has been widely used in the applications including magnetic field sensors, biosensors, magnetic recording and writing heads (hard disc drivers), and magnetic random access memory (MRAM). Recently, the GMR in the organic materials has gained more attentions due to their light-weight, easy processing, low-cost, chemical stability and biocompatibility compared to the traditional metal matrix composites. However, it is still a challenge to obtain the high GMR signal at room temperature in the organic materials because of temperature dependent MR property.In order to obtain the organic materials with high GMR signal at room temperature and explore the GMR effect in the conducting polymer systems, the projects in this dissertation have used the polyaniline (PANI) combined with four different nanofillers including magnetite, silica, silicon, and multi-walled carbon nanotubes (MWNTs) to fabricate the PANI nanocomposites using a facile surface initiated polymerization (SIP) method. The synthesized PANI nanocomposites have been characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Raman, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and transmission electron microscopy (TEM). The MR of these nanocomposites have been mainly investigated.Particularly, the unique and huge GMR effect has been observed in these PANI nanocomposites. Especially, this unique phenomenon is not only observed in the magnetic magnetite/PANI nanocomposites, but also firstly observed in the nonmagnetic silica/PANI, silicon/PANI, and MWNTs/PANI nanocomposites. A room temperature large GMR up to95.0%is observed in the magnetite/PANI nanocomposites with a Fe3O4nanoparticle loading of30wt%. Meanwhile, almost the same positive GMR value (95.5%) is also observed in the silica/PANI nanocomposites with a silica loading of20.0wt%. A MR value up to40.0%is obtained in the silicon/PANI nanocomposites with a silicon loading of20.0wt%. Interestingly, a MR transition from positive to negative at a magnetic field around5.5T is observed in the10.0wt%silicon/PANI nanocomposites. The negative GMR is observed in the MWNTs/PANI nanocomposites oxidized by Cr(VI) and positive GMR is obtained in the MWNTs/PANI nanocomposites oxidized by Ammonium persulfate (APS).Understanding the mechanisms of GMR can help the structure design of theorganic spintronic devices and develop the future applications. This work has explored the electrical transport mechanism in the synthesized polyaniline nanocomposites using the Mott variable range hopping approach through the temperature dependent resistivity property. After confirming the synthesized PANI nanocomposites in the disordered quasi3-dimentional variable range hopping regime, the observed positive MR is well explained by wave-function shrinkage model by calculating the changed localization length (a0, density of states at the Fermi level (N(EF)) and reduced average hopping length (Rhop). The obtained negative MR in the MWNTs/PANI nanocomposites oxidized by Cr(VI) is explored by forward interference model. Meanwhile, the observed MR transition from positive to negative at magneticfield around5.5T in the10.0wt%silicon loading of silicon/polyaniline nanocomposites is theoretically analyzed by both the wave-function shrinkage model and forward interference model. The results revealed that both these two effects contributed to the positive MR and only forward interference effect was responsible for the observed negative MR. The obtained positive MR region is well illustrated by the introduced localization length (a0), density of states at the Fermi level (N(EF)), average hopping length (Rhop) and the negative MR region is interpreted by the quantum interference effect.