Advancement in the field of material science has given us access to a variety of materials at nanoscale with their unique size related features. In the league of such materials, assemblies of nanoparticles (Nps) hold significant importance with promising magnetic, transport and magneto-transport properties. Study of these properties, which is the theme of this thesis, is essential for incorporation of these materials in modern spintronic and electronic devices. We have studied these properties in three different systems each with a different motivation as described below:
(a) Ultra-small Pt Nps (1.5 -3 nm) were chosen for the study of Coulomb blockade and its tuning. Due to their small size, assembly of these particles permits us to observe the Coulomb blockade at room temperature and allows its facile tuning. We shall present our studies showing tunable Coulomb blockade by changing the ligands such as –(CO), -SHPhCOOH,-SHPhOH -SHPhNH2 and various alkyl thiols. Besides, we shall show that dielectric constant of the ligand is another important factor for the tuning of Coulomb blockade in these assemblies of particles.
(b) Secondly, we shall present the magnetic and magneto-transport properties of alloyed FeCo Nps (approximately 10 nm) coated with hexadecyl ammonium chloride. Occurrence of spin dependent tunneling in their assembly renders their resistance sensitive to magnetic field giving rise to the phenomenon of tunnel magnetoresistance (TMR). We have investigated the temperature and voltage dependence of TMR as well as the mechanism of charge transport in the assembly of these particles.
(c) Finally, we shall present magnetic and transport studies on third type of system, known as spin crossover (SCO) complex containing a Fe(II) ion capable of transition between a paramagnetic (High-spin) and a diamagnetic (Low spin) state which can be interconverted by varying the temperature. Magnetic measurement shows a decrease in magnetic moment of the complex in the range of 300 K to 250 K indicating the spin transition. The transition process shows sensitivity towards the presence of solvent and remains absent in completely dried samples. Transport measurements performed to detect the transition shall also be presented. Similar studies have also been performed on the hybrid structure which additionally contains Pt Nps apart from the Fe (II) complex.
To summarize, magnetic and transport studies have been performed on the above mentioned systems. Successful tuning of Coulomb blockade in Pt Nps assemblies, as we shall present in the thesis, can be used in room temperature functional devices. Besides, tunnel magnetoresistance observed in FeCo Nps assemblies show temperature and voltage dependent TMR with some unexpected new features. Such a system could be very useful in magnetic sensors and memory devices. Finally, studies of SCO complexes and their hybrid structure with Pt Nps have shown a possibility of preserving the spin-transition while introducing new components in the system. |