Molecular magnets containing lanthanide ions have been receiving an increasing level of attention during the last years. The applicability of molecular magnets on foreseeable devices and new technologies, such as spintronic components, spin valves, or quantum bits, relies on the achievement of magnetic bistability with long relaxation times, such that the molecule allows the designed operation. The use of lanthanides as ingredients in molecular magnetism facilitates the achievement of high-energy barrier for magnetization reversal, a parameter related to bistability, and long relaxation times, thanks to their unquenched orbital momentum which enhances the magnetic anisotropy of the molecule. The understanding and tailoring of the diverse magnetic relaxation mechanisms through adequate chemical design are at the core of this multidisciplinary research field. Nowadays, it is mandatory to use up-to-date ab initio computational tools, providing a new input for the next round of chemical design, which is nowadays involving concepts such as new topologies and interaction dimensionality. In this chapter, we describe the theoretical basis as well as the experimental and computational tools used in the determination of the magnetic relaxation times and the identification of the relevant relaxation processes in lanthanide-based magnetic molecules. We review the recent progress on this class of compounds, from single ion molecules, to homo- and heteronuclear dimers and clusters (both containing only lanthanides and combined with 3d magnetic ions). Recent developments on systems with higher magnetic dimension in the interactions, such as chains, planes, and 3D compounds, are also reviewed. Finally, we focus on the recent progress on lanthanide-based molecules grafted to different substrates and how the relaxation processes are affected by the molecule–surface interaction.

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