Organic semiconductors have attracted worldwide interest for the past two decades. The properties of these molecules can be easily manipulated and exploited, and furthermore benefit from chemical versatility, mechanical flexibility and low cost. This has led to a remarkable success in the field of plastic electronics and molecules have found numerous device applications such as photovoltaic cells (PV), organic light emitting diode (OLED), organic field effect transistor (OFET) and sensors. Organic semiconductors have recently become of considerable interest for spintronic applications, due to the long spin relaxation times and magneto-resistive effects observed in these systems.
In order to fully exploit the advantages of these molecules for spintronic applications, it is essential to explore molecular routes towards all organic spin valves and search for molecule-based magnets as alternatives to conventional spin injector/detector such as La0.67Sr0.33 MnO3 (LMSO) and Co. The scope of this thesis is to investigate the way to control the functional properties and in particular the magnetic interactions and coercivities in molecular thin films, with an emphasis on the charge-transfer salt, [MnTPP][TCNQ], and a ferromagnetic system, FePc (including mixed H2Pc:FePc), respectively, fabricated by organic molecular beam deposition (OMBD). Although the magnetic couplings are currently limited to cryogenic temperature, it is shown that it is possible to engineer exotic physical properties in these mixed films, where the magnetism seen as an intrinsic property to the functional molecules shows a strong dependence on the local chemical structure and spatial displacement for the magnetic ions, which can be manipulated by addition of electron acceptor and non-magnetic substituent. Compared to conventional magnetic semiconductors, this approach is a molecular route towards tuneable magnetic properties, allowing one to directly control the magnetic interactions by varying the film composition via co-deposition, a desirable property that is obtained in the film form and readily exploited in all organic spintronic applications.