Investigation of weakly hybridized magnetic molecules

Abstract

Advances in molecular spintronics rely on the in-depth characterization of the molecular building blocks in terms of their electronic and, more importantly, magnetic properties. In most of the studies with that aim published till date, the molecular orbitals were strongly hybridized with the substrates’ electronic states. Therefore, the investigations did not characterize the molecules but rather the molecule-substrate hybrid system and there have been concerns about how to separate the two contributions to the measured magnetic signal. An efficient approach is the use of an inert substrate for such studies. Its interaction with the adsorbed molecules is usually weak, which is required in order to preserve the molecule’s electronic states. This dissertation deals with the emergence of magnetism in molecules, from understanding of its origin to the description of molecular interactions with a substrate. It is followed by the description of the concept of spin-based logic devices and their experimental realization. The introduction to the measurement technique (scanning tunneling microscopy) is also provided together with its implementation in the experimental setup. In particular the dissertation focuses on the investigations of the magnetic-field response of a single paramagnetic 5,5’-dibromosalophenatocobalt(II) molecule adsorbed on an Fe-intercalated graphene substrate. This substrate is magnetic and interacts only weakly with the adsorbed molecules. The investigations had been performed by means of spin-polarized scanning tunneling microscopy and spectroscopy. The obtained local magnetization curves, spin-dependent tunneling spectra, and spatial maps of magnetic asymmetry for a single 5,5’-dibromosalophenatocobalt(II) molecule reveal its magnetic properties and coupling to the local environment. The molecules are found to be in three magnetic states. Their magnetic moments align parallel or anti-parallel to the magnetic moment of the substrate caused by relatively strong or weak hybridization, respectively. In the latter case, however, the interaction can be weak enough for manipulation of the molecule’s magnetic moment with an external magnetic field. The distinct magnetic behavior of the molecule is found to rely on its position relative to the Fe-intercalated graphene moiré structure. Spatial variations of the electronic properties of the substrate surface in combination with the molecule’s adsorption geometry determine the level of hybridization between the molecular orbitals and the surface π-system.