On-surface magnetochemistry

Abstract

This thesis reports on the on-surface magnetochemistry of square-planar transition-metal complexes adsorbed on ferromagnetic substrates. Specifically, the magnetochemistry of the transition-metal ions (Mn / Fe / Co / Ni) coordinated in square-planar porphyrin / phthalocyanine ligands arranged on native and oxygen-reconstructed ferromagnetic Ni(001) / Co(001) thin-films is studied. The metal-centers in the surface-adsorbed complexes are five-fold coordinated: four coordination-bonds with the square-planar ligand and one bond with the “surface-ligand”. This arrangement leaves the sixth site on-top of the complex open for an additional ligand to bind with the transition-metal center and give the possibility to control the magnetic properties of the on-surface complex. Specifically, nitric oxide (NO) or ammonia (NH3) gas is used to serve as the sixth ligand. The experiments were performed in ultra-high vacuum and the samples were studied by X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD), X-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM). This work is based on the induced magnetic ordering in a monolayer of transition-metal porphyrins adsorbed on ferromagnetic substrates. The effect allows to study the magnetochemistry at ambient / near-ambient temperature in the remanent magnetization of the substrate. The experimental results are complemented by density functional theory with additional Hubbard interactions taken into account (DFT+U) conducted by Kartick Tarafder and Peter Oppeneer from Uppsala (Sweden). Within the scope of this thesis, mechanisms for switching off, tuning and switching on the magnetic moments in the adsorbed complexes are demonstrated and explained. Furthermore, we show that apart from controlling the magnetic moment, the axial-ligand can also be used to control the exchange-interaction with the ferromagnetic substrate. Specifically, we observe that the strength and sign of the exchange-interaction can be controlled. These results clearly illustrate that the coordination-chemistry and magnetochemistry on-surface extends the framework of classical coordination-chemistry, since the interaction with the “surface-ligand” has to be included into the considerations. Furthermore, we show that highly-ordered two-dimensional arrays of molecular spin-systems can be fabricated by chemically directed self-assembly. Specifically, we produce chessboard-like Fe - Mn - Fe spin-arrays by mere co-evaporation of the functionalized molecular building-blocks. In a second step, the magnetic properties of this spin-array can be controlled by ammonia exposure and one half of the chessboard-like spin-array can be selectively and reversibly switched. Also, the on-surface charge-transfer between the strong electron-acceptor TCNQ and alkali-halides (e.g. Na+Cl-) is discussed. The experiments show that the 2D metal-organic layers can be produced by the on-surface reaction of alkali-halides (instead of alkali-metals) and sufficiently strong electron acceptors.