Warner, B; (2014) Engineering the properties of magnetic molecules through the interaction with the surface. Doctoral thesis , UCL (University College London).

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Abstract

The drive to continue Moore’s Law by shrinking electrical components down to the ultimate limit has led to a great deal of interest in atomic and molecular-scale electronics, in which individual atoms and molecules can be used as circuit elements. More recent proposals also seek to exploit the magnetic properties of these nanoscale objects in new applications in information technology and spintronics. In typical device geometries, the magnetic element is coupled to electrical leads, and these interactions can strongly affect the properties of the quantum system. Using scanning tunneling microscopy and spectroscopy, we study the effects of inter- actions between individual magnetic atoms and molecules that are separated from an underlying metallic surface by a thin insulating layer of copper nitride (Cu2N). By utilising the different growth phases of the Cu2N, we show that the position of magnetic molecules can be controlled, and that the properties of a molecule can be controlled through the binding site. For electrical transport through a junction containing an individual iron phthalocya- nine (FePc) molecule on Cu2N, we observe two novel magnetoresistance behaviours that arise from negative differential resistance (NDR) that shifts by unexpectedly large amounts in a magnetic field. Because voltage is dropped asymmetrically in this double barrier junction, the FePc can become transiently charged when its states are aligned with the Fermi energy of the Cu, resulting in the observed NDR effect. Furthermore, the asymmetric coupling magnifies the observed voltage sensitivity of the magnetic field dependence of the NDR, which inherently is on the scale of the Zeeman energy, by almost two orders of magnitude. These findings represent a new basis for making magnetoresistance devices at the single molecule scale. Fur- thermore, the enhancement of the energy scales created by asymmetric coupling of the junction can be used in conjunction with other multi-step tunnelling processes to allow for the investigation of phenomena that would otherwise be difficult to observe. We also show that it is possible to interact with the f-shell magnetic moment when a bis(phthalocyaninato)Dy(III) complex (DyPc2) is strongly coupled to the Cu(001) surface. DyPc2 is a single molecule magnet, a type of molecule which may have applications in both spintronic and quantum computing applications. A Fano lineshape is observed at the Fermi energy, which is caused by the interference between tunnelling into the continuum and into a resonance created by the Kondo effect. By mapping the variance of the amplitude of the Fano line shape we are able to show that the ligand states create the continuum states and the 4f states create the Kondo resonance.

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