Organic semiconductor thin films for spintronic applications

Abstract

Organic semiconductors have a major impact on our daily life, as they are increasingly implemented in commercial devices like organic light emitting diodes (OLED), organic field effect transistors (OFET) or solar cells. Recently, this class of materials became of interest for spintronic applications, which make use of the spin in addition to the charge of the electrons. Due to a high spin life-time and an easy manipulation of the chemical and physical properties, organic molecules become a promising alternative to transition metals and metal oxides for both spin transport and polarisation media. In order to achieve the desired device properties like room temperature operation, it is essential to investigate new materials and fabrication methods. This thesis focuses on vacuum processed organic semiconductor thin films, which further exhibit remarkable magnetic properties, making them suitable for spintronic applications. The materials of choice, iron phthalocyanine (FePc) and manganese phthalocyanine (MnPc), show ferromagnetic behaviour with Curie temperatures of up to 40K and the properties can be tuned by blending with other organic semiconductors. In particular, mixing n-type semiconducting materials such as fluorinated cobalt phthalocyanine (F16CoPc) and tetracyanoquinodimethane (TCNQ) with the p-type magnetic metal phthalocyanines has great potential for both molecular electronics and spintronics. In this work, it was found that the paramagnetic F16CoPc becomes ferromagnetic in a blend with FePc and the coercivity of the compound can be tuned by the concentration. Furthermore, ferromagnetic MnPc films with significant coercivity were obtained by annealing of MnPc:TCNQ blends, which exhibit charge transfer from MnPc to TCNQ in the as-deposited state. Finally, the first spin valves based on FePc were produced and exceptionally large magnetoresistive effects were observed.