Growth and characterisation of metallocene based thin films for spintronic applications

Metallocenes, discovered at the beginning of the 1950s, were very well studied for their diverse properties in redox reactions, chemical catalysis and easy decomposition of ligands. After the theoretical study by Liping Zhou et al. in 2008 that ferrocene wires possess the ability to generate a nearly 100 % spin polarised current, new interest in ferrocene arose as a potential candidate for spintronic applications. The utilisation of ferrocene itself, however, presents big challenges due to its low sublimation temperature and therefore to our knowledge no ferrocene-based thin films without complex connecting ligands have been reported so far. In the present work it is shown that it is possible to produce thin films of molecules containing two linked ferrocene units on a variety of substrates at substrate temperatures of about 300 K. Structure and morphology of the thin films showed a strong substrate dependence. The intrinsic diamagnetic behaviour of the biferrocene molecules was preserved in the thin film. In order to use biferrocene as an active spin manipulation medium for spintronic devices ferromagnetic or antiferromagnetic behaviour is required. Therefore, the magnetic behaviour of single and double oxidized biferrocene thin films was investigated. The biferrocene was single and double oxidized using tetracyanoquinodimethane (TCNQ) and TCNQ-F4, respectively. The effect of the biferrocene:TCNQ and biferrocene:TCNQ-F4 molecular ratios on the oxidation yield was studied. In both cases the highest oxidation yield was achieved for molecular ratios of 1:4. Single oxidized biferrocene showed paramagnetic behaviour whereas double oxidized biferrocene displayed weak intramolecular antiferromagnetic coupling due to ligand-mediated superexchange. With the aim to achieve stronger antiferromagnetic coupling bis(fulvalene)diiron thin films were studied. Due to the reduced iron-iron separation and the two mediating connecting ligands in the bis(fulvalene)diiron in comparison to the biferrocene three times stronger antiferromagnetic coupling was observed for double oxidized bis(fulvalene)diiron. Furthermore, major differences in the morphology and crystal structure in comparison to biferrocene were observed.