Ultrafast spin dynamics in molecular magnets

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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