Femtosecond Coherences in Single-Molecule Magnets

New materials and technologies for data storage are urgently needed to keep up with projected data use in applications of big data and artificial intelligence. More efficient devices will also reduce the energy consumption associated with running data servers worldwide. Magnetic materials have always been used for data storage and are projected to keep their importance for large-scale data storage facilities. The magnetic poles represent binary "one" and "zero", and writing data corresponds to reversing the pole direction. Optical control of the poles is desirable because it will allow for orders of magnitude faster reversal rates using femtosecond lasers, which is a timescale not accessible with electronics.

In response to this growing problem, the field of ultrafast magnetism (i.e. controlled changes in magnetisation occurring on the femtosecond timescale) has developed rapidly since the initial discoveries enabling all-optical magnetisation reversal using femtosecond laser pulses. So far these results have been limited to solid-state magnetic materials. To reduce the size of information centres in hard drives, and therefore increase the data storage density, single-molecule magnets (SMMs) are promising candidates because of their nanometre size. However, to date, the interaction of femtosecond laser pulses with SMMs has not been explored. Here, we will investigate this interaction by building a research programme combining synthesis, ultrafast spectroscopies and advanced computational modelling. Specifically, we will study Mn(III)-based coordination compounds, which are characterised by a partial population of antibonding orbitals. This leads to a geometrical distortion via the Jahn-Teller (JT) effect, which in turn gives a preferred spatial direction of the magnetisation. In a proof-of-principle study [Liedy et al, Nature Chemistry, 12, 452 - 458 (2020)], we showed that by optically redistributing the population of antibonding orbitals, a fast change in the anisotropy of the molecule takes place via the formation of a vibrational wavepacket. Since the geometry is intimately related to the magnetic anisotropy of these molecules, the collective motion associated with the wavepacket opens up possibilities to control magnetisation on the femtosecond timescale. We also found that we could tune the dynamics of the wavepacket by using molecular design, which implies that there is a synthetic route towards achieving fast and efficient magnetisation control in SMMs.

These initial findings are very promising. However, a detailed understanding of the dynamics and the exact nature of the coupling between the electronic and nuclear degrees of freedom remains unclear. The aim of this proposal is to explore new ways to manipulate paramagnetic coordination compounds by creating femtosecond coherent vibrational wavepackets along the JT axis to enable optical control of the magnetic anisotropy. Specifically, we will explore a range of Mn(III)-based complexes by varying the geometry of the JT axis. We will increase the structural complexity of the molecules being studied, from monomeric model systems to exchange-coupled dimers. We will measure the wavepacket motion using transient absorption spectroscopy, ultrafast electron diffraction and X-ray free-electron lasers. Changes to the magnetic anisotropy will be measured using femtosecond magneto-optical spectroscopy.

At the conclusion of the project, we will have developed an understanding of how light can be used to control the magnetisation of Mn coordination compounds and what structural factors are important for achieving efficient changes to the magnetic anisotropy using femtosecond coherent wavepackets. This will enable non-thermal control of the magnetisation, which in turn can lead to the underpinning technology in future low-energy, ultrafast and ultradense magnetic storage devices.