On-surface magnetism of coordinated spin systems and networks

In this work, the magnetism of two different 3d-metal based ionic coordination complexes and planar coordination networks has been investigated. The focus point of this study has been the high magnitude of the possible magnetization for the former and the geometric frustration of the chemically coupled spin systems for the latter. The samples were characterized by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, (both for the chemical analysis and characterization of the coordination centers) X-ray Magnetic circular dichroism (for the field and temperature dependent magnetic moment), scanning tunneling microscopy (sub-molecular resolution imaging) and SQUID-magnetometry (magnetization).
In the first chapter, an iron-based metal organic network with high spin configuration of Fe(II) has been assembled on diamagnetic Au(111) substrates, and characterized by scanning tunneling microscopy and x-ray photoelectron spectroscopy. A series of low temperature (T = 2.5 K) X-ray absorption spectroscopy and X-ray magnetic circular dichroism experiments show that the system has a very weak magnetic response, much weaker than what is expected for free Fe(II) ions. My sum rule calculations estimate a total magnetic moment per iron atom in the order of a fraction of a Bohr magneton. The temperature-dependent susceptibility data were extracted from the XMCD spectra and imply a negative Weiss temperature. All the evidence together with knowing the exact geometry of the network are implicitly suggesting that the magnetic moment is frustrated in the system.
In the later chapters, a recently synthesized family of rare trigonal prismatic ionic compounds is introduced for the first time. In this coordination cation, 3d metal ions are held in trigonal prismatic geometry in high spin configuration. The electronic and magnetic properties of these novel compounds in polycrystalline form, as monolayers and multilayers were investigated by means of SQUID-magnetometry, X-ray absorption and X-ray magnetic circular dichroism.
At the beginning of Chapter 2 we investigate three members of the family including [FeL6]BPh4, [FeL6]BF4 and [FeL6]O(FeCl3)2 in polycrystalline form to investigate the role of the counter-anion on the magnetic properties of the [FeL6]+ coordination complex. We showed that the unique coordination cage of the cationic part is capable of preserving the high spin configuration of Fe(III) ions with each of the three chosen ligands and in all observed cases of crystal packing. The multiplet calculation and Ligand field-DFT calculations suggest that the electronic configuration of the systems are slightly different compared to ideal trigonal prismatic systems and there is a nearly degenerate state of 6 multiplicity as ground state, with a very small energy splitting due to zero-field splitting.
In the next chapter the on-surface behavior of [MnL7]BPh4 as a Mn(III)-based trigonal prismatic ionic compounds were investigated on two different substrates Au(111) and Cu(111) using scanning tunneling microscopy. A wide variety of different molecular fragments were formed on the metallic surface of Au(111) and Cu(111) which suggest an unprecedented on-surface dissociation process. The dissociation and re-aggregation of the neutralized fragments upon surface adsorption render the investigation of supramolecular interactions more challenging. A wide variety of self-organized features is formed and characterized in our high-resolution STM images. In a complex series of experiments, we have also shown that a concurrent chemical modification occurs in the crucible at the temperature below the sublimation temperature of ion pairs. The cationic-derived fragments form in different lengths.
In the last chapter, the electronic and magnetic properties of [FeL6]BPh4 in monolayer and multilayer regime were investigated on diamagnetic Au(111) substrate using XAS and XMCD. We show that despite having a three dimensional coordination geometry and very complicated on-surface supramolecular interactions, the cation-derived dimers form chains and are able to preserve the high spin configuration of the Fe(III) ions. Our field dependent experiments show that these surface assemblies exhibit a significant easy plane (hard axis) magnetic anisotropy. The molecule-substrate interaction has been investigated by depositing monolayers of [FeL6]BPh4 on a ferromagnetic O terminated Co layer. The XMCD results shows that these molecules partially preserve their magnetic moments and continue to show easy plane magnetic anisotropy.
The presented results demonstrate the significant potential of molecular coordination chemistry by in-vitro and on-surface chemistry to play an important role in the further development of functional material with novel magnetic properties. coordination chemistry provides a unique opportunity for technological developments in the field of spintronics and quantum information technology while at the same time, it creates a unique platform to experimentally investigate and develop theories about the emergence of magnetic behavior from fundamental origins which is often not possible in inorganic magnetic systems.