Insight into the exchange coupling between magnetic molecules and the supporting surface : spectromicrocopy correlation including X-ray magnetic circular dichroism

This thesis reports on magnetic properties of organic molecules deposited on ferromagnetic surfaces. Complementary surface science techniques like: scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) or low-energy electron diffraction (LEED) have been employed to study the chemical, electronic and structural properties of the interfaces. X-ray magnetic circular dichroism (XMCD), performed at the synchrotron has given us insights into magnetic interaction of paramagnetic molecules with ferromagnetic substrates. The interpretation of the experimental data has been supported by density functional theory calculations (DFT) performed by a collaborating group from Uppsala University, Sweden.
The manganese tetraphenyl porphyrin chloride (MnTPPCl) molecules are shown to couple magnetically to the supporting cobalt (Co) thin film. The ordering of the molecular spins is parallel with the spin of the substrate, i.e. it is ferromagnetic. It has been experimentally verified that the coupling originates from an indirect exchange interaction between Mn and Co atoms which is mediated by nitrogen atoms present in the molecular macrocycle. MnTPPCl molecules undergo a partial dissociation on the Co surface, leading to the removal of the chlorine (Cl) ligand and to consequent reduction of the Mn ion oxidation state from 3+ to 2+. Both molecular species have been identified in the STM images. Our results also show that Mn in the MnTPP molecule (after dissociation of the Cl) resides in an intermediate spin configuration.
Modification of the substrate by controlled oxygen exposure and the consequent formation of oxide ad-layer, reduces the molecule-substrate interaction, as evidenced by the emergence of molecular self-assembly, which was not present on the atomically clean Co. Also the MnTPPCl molecule itself now remained intact upon deposition onto the O/Co substrate. Notably, the magnetic interaction on an oxidized surface has transferred into a superexchange antiferromagnetic coupling, leading to an antiparallel alignment of the Mn and Co spins.
The last experiment presented in this thesis demonstrates the possibility of magnetic switching of a molecular monolayer. We have been able to switch off the magnetization of MnTPPCl molecules on the surface by supplying nitric oxide (NO) gas as a reactant. NO binds to the Mn in a linear geometry, in which it is formally considered as a three-electron-donor. It could therefore compensate the Mn ion's spin of 3/2 and quench the initially observed XMCD signal. The Mn-NO bond has been shown to resist the thermal treatment and it has not been possible to restore the magnetization of Mn.
Our results have shown that the spectromicroscopy correlation approach, here involving STM, XPS and XMCD, is a powerful tool for the identification of the mechanisms involved in magnetic and structural ordering of the organometallic layers. The combination of such experiments with ab-initio theoretical calculations allows for a detailed description of magnetic processes occurring at the metal-organic interfaces.