Transport and Dielectric Studies of Metallic, Semiconducting, and Magnetic Materials and Devices

Several organic and inorganic systems of importance for fundamental physics and applications have been studied by magnetotransport, dielectric constant, and Raman spectroscopy techniques. At the beginning of my thesis work, I investigated three carbon based organic systems: carbon fibers, pentacene derivatives, and a nanomagnetic material ("V15"). In the latter stages of my dissertation, I used the techniques I had developed to explore the properties of two inorganic systems: NiFe nanopillars in a silicon matrix, and spin systems in multiferroic rare earth-transition metal oxides. The main activities and achievements of my thesis work are as follows: The carbon fibers were characterized by magnetotransport and Raman spectroscopy studies. I found that carbon fibers are promising as wires in molecular electronics and compatible with organic films. Preliminary results on simple films of melted pentacene derivatives connected with carbon fiber wires were a first step in the fabrication and characterization of pentacene field-effect transistors (FET's). The work on the pentacene system resulted in a series of successful logic circuits based on field-effect transistors such as NOT (inverter), NOR, and NAND. The temperature-dependent mobility was described as thermally activated at low gate voltages, but at high gate voltages the mobility was enhanced due to shallow traps. The second system investigated was the organic nanomagnetic material, polyoxovanadate, K6[V15As6O42(H2O)]•8H2O (i.e. V15). The conductivity and the dielectric measurements at high and low temperatures respectively were used to determine electrical properties of this single magnet molecule system. The main accomplishments were the determination of the energy gap (0.2eV) and the identification of multiple dipole relaxation modes. Raman vibrational spectroscopy was used to correlate dielectric relaxation with the Raman intramolecular vibrations. An investigation was then carried out on NiFe nanopillars electrodeposited in nanoporous silicon templates (Si:P), studied with transport and dielectric methods in high magnetic fields. This system exhibited a frequency and temperature dependent dielectric response which followed a Debye relaxation mechanism. It was discovered that in high magnetic fields greater than 10 T, multiple relaxation structures emerged that were magnetic field direction dependent. It was realized that such a phenomena occurs in Si:P, and is not directly related to the NiFe nanostructure. Hence, a new magnetic field induced phenomenon in the dielectric response in Si was observed, which involves the effects of a magnetic field on an electric dipole. Here, the field induces a harmonic oscillator state from the zero field Debye-like relaxation behavior. The final work in the thesis project focused on the inorganic rare-earth transition metal oxide system HoMnO3 and related compounds. Dielectric measurements were used to characterize and map out the magnetic phase transitions in the doped ferroelectric series Ho1−xYxMnO3. The phase transitions involved complex rotations of the Mn spins. I found that the behavior of these spin rotations were highly dependent on magnetic field, magnetic field direction, and the degree of doping with the non-magnetic Y ion. Hence the magnetic field anisotropy study is an important step towards the understanding of magnetic and electric phase competition in the diluted 4 f system by the non- magnetic ion Yttrium (Y).From highly systematic measurements involving these parameters, I mapped out detailed phase diagrams for the Ho1−xYxMnO3 system which will be very useful for future theoretical work to describe the complex spin interactions involved.