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Theoretical basis for the design of Lanthanide-based molecular nanomagnets

Final Report Summary - 4FNANOMAG (Theoretical basis for the design of Lanthanide-based molecular nanomagnets)

The storage density of magnetic hard drives doubles every year. In this race for increasing the capacity of information storage, a logical approach is to decrease the system size of the storage devices. With this respect, the discovery of molecules which retain its magnetisation in the absence of a magnetic field below a blocking temperature, the so-called single molecule magnets (SMM), sets the final limit of the magnetic storage unit miniaturisation at the molecular level. The number of reported polymetallic clusters increases every year.

However, up to now there has not been a relevant increase in the blocking temperatures because spin value and magnetic anisotropy are difficult to be optimised contemporaneously. With this purpose in mind, a recent strategy is the use of lanthanide-containing systems due to the high magnetic moment and the high magnetic anisotropy associated with most lanthanide ions. These are the same reasons because lanthanide ions are widely used in magnet technology.

The aim of the project was to establish a methodology in which the combination of experimental measurements, first-principle electronic calculations and theoretical magnetic models would allow to shed light in the magnetic behaviour, and in particular in the slow relaxation of the magnetisation, of lanthanide-based SMMs.

The principal part of the previous methodology has been the use quantum chemistry methods to compute the electronic structure of the studied molecules. The main issue of such computations was the need to include relativistic effects (the spin-orbit coupling) which are the origin of the magnetic anisotropy. In addition, in order to get accurate results a large number of electronic states must be computed with precision, since they are coupled by the spin-orbit interaction with the ground and the lowest energy states.

Along the two years of the project a number of lanthanide-containing systems have been studied. The main results and conclusions obtained from these studies are enumerated below.

- The confrontation of experimental data and first-principle electronic computations has shown that the employed quantum chemistry method is able to correctly predict both the direction of the magnetic moments of the lanthanide ions and their energy levels structure.
- The ability of the previous method in predicting the direction of the magnetic moments has been essential to theoretically model and fit the magnetic behaviour of polyatomic clusters thanks to the reduction in the number of model parameters.
- In all the studied systems the energy barrier for the relaxation of the magnetisation is of the same order of magnitude than the energy separation between the doublet ground state and the first excited doublet state. This result is the first step in order to understand the mechanism of the slow relaxation of the magnetisation in lanthanide-based SMMs.
- The quantum chemistry calculations have also shown that the energy levels structure of the lanthanide ion depends not only on the ligand molecules in contact with the ions but also on the electrostatic charges close to those ligands.
- The study of compounds in which the lanthanide ions are combined with free radicals have shown that a free radical linking two lanthanide ions enhances the magnetic interaction between them.
- Finally, in one of the studied compounds, a triangular Dy(III) cluster, the non-collinearity of the three magnetic moments results in a non-magnetic ground doublet with a vortex arrangement of the magnetic moments. Such kind of systems have potential application in quantum computation since the nonmagnetic nature of the ground state would reduce the decoherence effects due to the fluctuation of local magnetic fields. At the same time magnetic vortexes are currently investigation as potential memory units as they are less affected by stray magnetic fields.