Investigation On The Spin Transport Properties Of Alkane Molecules And Fabrication Of Organic Spin Valve Devices

A new discipline, called spintronics has emerged since the discoveries of giant magnetoresistance and tunnelling magnetoresistance in multilayer structure which revolutionized applications such as magnetic recording and read head in hard disk. Consequently the effect in Co/Cu and Co/Ag granular systems which observed in 1992 widely developed the area of fundamental research. In 2004, Xiong et al. demonstrated a large magnetoresistance (MR) over 40% in ferromagnetic/organic semiconductor/ferromagnetic sandwich structure. Organic spintronics attract much attention due to the weak spin orbit coupling and hyperfine interactions base on the light element (such as C, H, N, and O) which are the main composition of organic materials. The extremely weak spin orbit coupling and hyperfine interactions courses a very long spin relaxation time hence a great potential of stronger magnetoresistance effect or spin relate effect. Beside the low-temperature processing, the tunable electronic properties, and the flexible in mechanics, it makes organic materials great candidate for spintronics applications.This thesis concerns on two aspects. One is the spin transport properties in self-assembled monolayer of alkane molecules on Fe3O4 nanoparticles systems. A few efforts are carried out on the ferromagnetic/organic nanoparticle systems. Such as a low temperature magnetoresistance about 40% is observed under 14 T magnetic field in Fe3O4/Polystyrene system. And in FeCo/Hexadecylamine system, a Coulomb blockade effect is observed. However the distance between the nanoparticles is difficult to be controlled by those approaches. We successfully fabricate the self-assembled monolayers of alkane molecules on Fe3O4 nanoparticles with monolayer thickness on the particle surface, and with this approach we carefully study the length dependent spin transport properties. On the other we developed a reliable technique for fabricating high quality, reproducible organic spin-valve (OSV) devices using indirect deposition (ID) method, which relies on the evaporated metallic atoms scattering with inset gas to reduce energy, to deposit the ferromagnetic electrode Co on top of organic semiconductors. This method significantly suppresses the penetration of Co atoms into organic semiconductor layer during deposition process.The work presented in this dissertation consists of the following parts:Ⅰ. Enhanced magnetoresistance in self-assembled monolayer of oleic acid molecules on Fe3O4 nanoparticlesSpin transport through molecules is investigated using self-assembled monolayers of oleic acid molecules on half metallic Fe3O4 nanoparticles. Fourier transform infrared spectroscopy measurements indicate that one(?)monolayer molecules chemically bond to the Fe3O4 nanoparticles and the physically absorbed molecules do not exist in the samples. The MR of cold-pressed, molecule fully covered nanoparticles is up to 7.3% at room temperature and 17.5% at 115 K under a field of 5.8 kOe. The MR ratio is more than two times larger than that of pure Fe3O4 nanoparticles. This enhanced MR is likely arising from weak spin scattering while carriers hop through the oleic acid molecules.Ⅱ. Investigation on spin transport properties of alkane moleculesOrganic molecular materials are known to have long spin relaxation time owing to the weak spin-orbit and hyperfine interactions; therefore offer unique advantages for potential spintronic applications. Compared with the mature inorganic spintronics which has already found applications in computer memories and read heads, however, organic or molecular spintronics is still at its infancy. For example, robust room-temperature spin-conserving building blocks have yet to be demonstrated with molecular junctions. In this work, we successfully fabricate molecular junction spin valves comprised of superparamagnetic Fe3O4 nanoparticles self-assembled with alkane molecules of different lengths d as the spacer. Although the resistance spans～two decades as the molecular length d varies from 0.7 to 2.5 nm, remarkably, a very large room-temperature tunneling magnetoresistance ratio of～21% stays approximately constant. This molecular length independent spin valve magnetoresistance, originated from the even weaker spin dependent scattering in alkane molecules.Ⅲ. Fabricating organic spin-valve devices using indirect depositionWe report a reliable technique for fabricating high quality, reproducible organic spin valve devices. We use indirect deposition method, which relies on the evaporated metallic atoms scattering with inset gas to reduce energy, to deposit the ferromagnetic electrode Co on top of organic layer Alq3. This method significantly suppresses the penetration of Co atoms into Alq3 layer during deposition process, in comparison with devices fabricated by conventional direct deposition method. The improved Alq3/Co interface is further confirmed by comparing the magnetic moment of depositing Co onto Alq3 and Si substrates by indirect and direct deposition method. And a spin-valve effect with magnetoresistance of 0.07% at room temperature is demonstrated.