Magnetic properties of transition metal phthalocyanine molecular thin films and nanostructures

Abstract

The unique magnetic properties of transition metal phthalocyanines (MPc) have attracted considerable scientific interest for decades. So far, most of the research was focused on single crystals. In this thesis, MPc materials were grown in thin film form using vacuum-based methods, such as organic vapour phase deposition (OVPD) and organic molecular beam deposition (OMBD). These methods were used to produce a range of molecular film structures and morphologies of MPcs with different spin states. We find that the magnetic properties are superior to single crystals, in terms of Curie-Weiss constant and coercive fields. Understanding the film properties will be essential for future device applications. Firstly, OVPD was used to prepare MnPc thin films that crystallise as the β-polymorph. Superconducting quantum interference device (SQUID) results show that the films behave as canted ferromagnets with a Curie constant of 10.6 K and a coercive field of 9 mT at 2 K. The texture of the MnPc film can be tuned by introducing a molecular layer on to the substrate. Unexpectedly, we find a change of polymorph from β-phase to ε-phase, which was verified by X-ray diffraction (XRD). A strong axial anisotropy of the films was observed from the magnetic measurements. Secondly, α-polymorph FePc films were grown by OMBD with different textures, depending on whether a templating layer was used. Magnetic measurements show that both films behave as canted ferromagnets with strong in-plane anisotropy and have a similar ferromagnetic exchange coupling of 20 K. The coercive fields at 2 K of non-templated and templated film are 85 mT and 35 mT, respectively. Finally, in order to reduce dimensionality and explore a new crystal phase, FePc wires with around 100 nm in width and a few microns in length were prepared by OVPD. XRD indicates that FePc nanowires adopt the η-phase polymorph, identical to what was observed in CuPc nanowires. Both the Curie constant and coercive field are higher than the film form, reaching 40 K and 1 T, respectively.EThOS - Electronic Theses Online ServiceGBUnited Kingdo