Controlling magnetism in molecular thin films

Molecular materials have had an enormous impact on technology due to their unique optical, electronic and mechanical properties. Although it has attracted comparably less attention so far, their spin also offers huge potential in the field of e.g. low energy information technology or controllable optoelectronics. The low atomic number of the carbon-based framework enables extremely long spin lifetimes, which is a unique advantage compared to inorganic materials used traditionally. Our previous work has also shown that the basic building block of a spin valve, magnetic films, can be produced from molecular materials. However, although coupling strengths have increased by one order of magnitude over the last decade, they are still too low for room temperature operation. We recently reported that a change in the co-facial stacking distance in films of cobalt phthalocyanine (CoPc) had a dramatic effect on the magnetic coupling strength, leading to coercivity above liquid nitrogen temperatures. Excitingly, theoretical models based upon this result suggested that increasing the co-facial interaction further could lead to coercivity up to 400 K for fully cofacial stacks of CoPc. This is well above the temperature required for practical applications.
The aims of this project are to develop phthalocyanine analogues that can be deposited in cofacial stacks using highly controlled organic molecular beam deposition (OMBD). The use of OMBD is important because it allows control of the growth conditions, minimising defects and impurities which otherwise have a significant detrimental effect on magnetic properties, and allowing for the growth of the complex architectures required for spintronic devices. Similarly the phthalocyanine scaffold is important to promote axial interactions of the central metal atom, whilst isolating it from adjacent stacks, and offers a unique combination of optical, charge transport and magnetic properties.
Examples of cofacially stacked Pcs are very rare, and unmodified Pcs crystallise without exception in slipped stacks. Cofacial stacking can be promoted, for example by the synthesis of Pc dimers, in which the Pcs are attached to rigid scaffold which force coplanarity. Vacuum deposition of such compounds has so far not been attempted; successful sublimation of double decker phthalocyanine suggests that it could be possible.
This project will utilise non-covalent interactions between individual Pcs to promote the crucial 2D arrangement into sheets, which should then pack cofacially. The relatively weak nature of these interactions should facilitate the sublimination of individual Pcs at elevated temperature, but promote the ordering of the cores as they deposit on the surface. Careful tuning of substrate temperature should give the cores sufficient kinetic energy to reorder as they deposit into ordered 2D sheets. The synthesis of cruciform phthalocyanine analogues will be undertaken. The key structural features are the replacement of the benzenoid outer ring with a 5-membered heterocycle to afford a cruciform shape; the inclusion of templating interactions such as S...F (alternatives to study include S...O & F...H); the inclusion of a heteroatom to promote interaction between sheets (with possible Se/Te inclusion for stronger chalcogen/chalcogen interactions). A higher risk approach will involve the synthesis of cofacial dimers, which will be designed to include two different metals to promote ferrimagnetic interactions. Corresponding films should lead to significantly increased coercivity and coupling strenghs, and generate unique insights into the mechanisms of magnetic interactions in supramolecular materials.