A new class of magnetic molecules could store data at temperatures achievable with cheap liquid nitrogen, replacing the expensive helium cooling that has kept molecular memory devices stuck in the lab for thirty years. Single-molecule magnets (SMMs) are tiny magnetic units that can each hold one bit of information, potentially enabling far denser data storage than today’s silicon chips. But existing SMMs only retain their magnetic memory below about 80 Kelvin – requiring liquid helium, which is costly, energy-intensive, and unsustainable. The researchers have already demonstrated a dysprosium compound that works above 90 Kelvin, and now aim to push past 100 Kelvin by designing linear, two-coordinate lanthanide molecules with rigid structures that maximise magnetic anisotropy and suppress vibrations that destroy memory. If successful, this fundamental science could unlock molecular-scale hard drives that operate with liquid nitrogen cooling – far cheaper and greener than helium. The work also includes depositing these molecules on surfaces, a necessary step toward practical devices. While commercial applications remain years away, the project directly addresses the physical limits of conventional chip miniaturisation by exploring a bottom-up alternative at the atomic scale.
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There is a constant economic drive to make electronic devices smaller, and this miniaturisation has been achieved to date by fitting more transistor nodes onto smaller integrated circuits. This “top-down” approach is soon expected to hit a physical limit, and therefore alternative “bottom-up” approaches are now needed to meet this societal need in future. Single-molecule magnets (SMMs) have emerged as potential candidates to achieve high-density data storage and deliver smaller devices as they can show magnetic hysteresis, which is a memory effect. This property is at the molecular level rather than cohesive bulk magnetic interactions, as is the case for bulk magnets, allowing each SMM to store a bit of information; the energy required to flip magnetic memory in a molecule is much less than that needed to flip a magnetic domain. However, although SMMs have been known for over thirty years they have not yet been used in applied technologies. This is mainly a consequence of them currently only showing long-term magnetic memory using helium cooling, which is energy-demanding, unsustainable, and economically impractical to implement. In this project we will set methods for the routine synthesis of SMMs that operate above 100 Kelvin. This temperature is easily achieved with liquid nitrogen cooling (boiling point 77 Kelvin), which is cheaper and more sustainable than using helium cooling for achieving SMM technologies. Lanthanide SMMs, especially those containing dysprosium and terbium, have shown the greatest potential for high-temperature data storage to date. Highly axial dysprosium compounds with the metal coordinated by planar carbocyclic rings have previously shown magnetic hysteresis up to 80 Kelvin, due to a combination of the large magnetic anisotropy induced by this molecular geometry for these metal ions and the rigidity of the rings, but this approach has appeared to reach a plateau. Axial lanthanide compounds are challenging to isolate as lanthanides are large and show mainly ionic bonding regimes, thus additional molecules will tend to coordinate equatorially and introduce transverse fields, unless suitably bulky rings are used. We have recently discovered magnetic hysteresis above 90 Kelvin for a bent dysprosium compound bound by two nitrogen atoms and a pendant alkene. These preliminary results show a route to achieve the next step change in SMMs. Here we target formally two-coordinate lanthanide compounds with geometries closer to linearity, shorter metal-donor atom distances and more rigid coordination spheres. Ideal linear geometries will maximise the magnetic anisotropy and the purity of magnetic states, and higher molecular rigidity will reduce the effect of molecular vibrations that can be major contributors to magnetic relaxation. As the charge densities of single donor atoms can potentially be far higher than their more charge-diffuse carbocyclic ring counterparts, the target compounds can show greater anisotropy. This work will therefore deliver a new family of high-temperature SMMs with high effective energy barriers to magnetic reversal, and new record magnetic hysteresis temperatures. Once the target compounds have been synthesised, we will perform comprehensive physical and computational characterisation studies to define their electronic structures and SMM parameters. This will provide a deeper understanding of magnetic relaxation mechanisms and their relationship to molecular structure, allowing improved SMMs to be designed and targeted. We will also deposit the novel SMMs on surfaces, and we will characterise their magnetic behaviour in this environment; this is the necessary first step towards SMM devices.
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