Ultrafast spin dynamics in molecular magnets

Magnetic materials have completely changed how we can access and make use of information during the last century. Digital information is stored in hard-drives in magnetic domains, where the north and south poles represent binary "one" and "zero". How fast data can be recorded is limited to the rate at which the poles of these domains can be reversed. Recent advances using laser pulses as short as a millionth billionth of a second (or femtosecond) have made it possible to overcome this limitation by switching magnetic domains 1000 times faster than what current technology can achieve. Ultrafast magnetism therefore has the potential to drastically increase the rate of writing information to memories by orders of magnitude and is one of the frontiers in current magnetic research. A continued development of new magnetic materials and new ways of controlling them will ensure that we can make the most of large data sets, which in turn will improve many aspects of our lives such as health care, government, logistics and will reduce global energy consumption.
Another development, but hitherto unexplored in the context of ultrafast magnetism, is the study of molecular magnets. These will overcome the problems with reducing the size of data bits in hard drives to that of a few atoms, where the materials currently used have reached their size limit. Besides from reducing the size, molecular magnets also show another advantage for ultrafast magnetism. It has recently been shown that magnetic materials with localised magnetic moments are promising for achieving fast magnetisation reversal. These systems can be switched much faster in a process that generates less heat. Since the magnetic ordering of molecular magnets are from localised magnetic moments, these systems are very promising because their chemical flexibility makes it is possible to tune the interaction between the localised moments, and more importantly, their response to light perturbation. This will allow us to develop nanomaterials that can be switched using ultrashort laser pulses.
In this proposal, we will look at a series of model compounds, where it is possible to systematically change the elemental composition and stoichiometry of the materials to tune their magnetic and optical properties. In particular, the project will be split into two work packages (WPs): spin-flips in Prussian Blue Analogues (WP1) and dynamics of photomagnets (WP2). In WP1, Prussian blue analogues (PBAs) will be studied. It is known that very fast spin-flips can happen in these materials after light excitation. We have recently applied specialised methods to directly observe the spin-flip on a femtosecond timescale. We will extend these methods to a range of PBAs to increase our understanding of how the interaction between the magnetic moments govern the dynamics after the spin-flip on the localised sites. In WP2, we will build on this knowledge and study a similar system based on Fe and Nb. After light excitation, the initially diamagnetic (or "non-magnetic") Fe(II) centres are switched, in a similar process to what was described earlier, but in this case, the spin-excited state is trapped after photoexcitation. This leads to a magnetic interaction between paramagnetic Nb centres and eventually a macroscopic magnetic ordering takes place. It is not known how fast the magnetic ordering process takes place, however, our methods can measure this with unprecedented time resolution. This will allow us to understand the mechanisms for the magnetic switching process, which is necessary for optimising the process to incorporate both the materials and techniques in a future ultrafast and ultradense magneto-optical data storage devices. EPSRC Reference: EP/S018824/1

A successful outcome of the project will lead to a better understanding of how to optically control spins in molecular magnets on ultrafast timescales. This will stimulate further research into materials and technological development, which in the long term will lead to new data storage devices. This will impact on high-tech electronics and chemical industries. Better data storage devices will benefit society on a global scale because the efficient storage and processing of large data sets will have a huge impact on energy, the environment and health care. For example, citizens can reduce energy consumption from the development of large-scale smart grids and demand management. Large-scale car-sharing and self-driving electrical vehicles will also reduce energy consumption and pollution. In fact, it has recently been estimated that the development and growing use of big data can reduce primary energy demands by 20 - 30 % by 2050 (BP technology outlook, 2018). Relying on a high-tech industry, which in turn will produce a highly trained workforce and of course financial gains, will fulfil EPSRC's four Prosperity Outcomes by making sure that the UK is productive, connected, resilient and healthy because big data connects all of these areas. We will work to promote the benefits of continued development of materials and technologies to the general public by creating new public engagement workshop. By working closely with the University of Edinburgh's data science outreach team, we will put our research in context with contemporary research into big data.

Academically, this research bridges several areas in both chemistry and physics. The project will open up new opportunities for ultrafast magnetism, which is traditionally studied by condensed matter physicists, to expand into chemistry. The materials studied will enable critical tests of current models but also provide opportunities to study new physics. The project will also have an impact on inorganic chemistry/functional materials because our findings will guide new synthetic strategies to optimise switching rates in a synergetic way. We will actively attend several conferences in various areas, collaborate with a broad range of groups, and publish in general science journals to ensure the largest impact on a wide range of scientific fields. In addition to the fields described above, understanding photoinduced dynamics of functional molecular materials will also impact on photovoltaics research. Our approach to study spin dynamics will be useful for studying triplet exciton formation in photovoltaic films. Furthermore, films of PBAs are frequently incorporated into new types of batteries and self-powered photovoltaics. We will engage with these communities by attending photochemistry conferences and by continued collaborations with colleagues in the field (N. Robertson, T. Penfold).

New materials are key for future development and it is clear that there is a need for persons trained in both materials chemistry and instrumentation. We will provide this training for the PDRA and several PhD and undergraduate students involved with the research programme. The project will therefore have an impact on the people involved and where they eventually work after completion of the project. The staff and students will be trained in an interdisciplinary environment, where chemical physics will bridge the gap between materials chemistry and condensed matter physics. In the longer term, these persons will have an impact on the UK materials and electronics industries. Working with optics will prepare them for working in industries developing techniques such as self-driving cars, surveillance, and defence.