# New Materials and Effects in Molecular Nanomagnets
## 4 Single-Molecule Magnets  
Probably the most interesting applications of single-molecule magnets are for information storage and spintronic devices. While common magnetic nanoparticles reach the superparamagnetic limit for dimensions below 10-100 nm, single-molecule magnets with transition-metal ions can be used for data storage, while being much smaller [42]. For the first SMM reported, antiferromagnetic interactions between the spins of Mn\({}^{\text{IV}}\) and Mn\({}^{\text{III}}\) ions in the molecule [Mn\({}_{12}\)O\({}_{12}\)(CH\({}_{3}\)COO)\({}_{16}\)(H\({}_{2}\)O)\({}_{4}\)] were found to result in a large spin ground state with the spin quantum number \(S=10\), and in very long magnetization relaxation times, making such a bistable molecule highly interesting for data storage [43, 44]. It should be mentioned that the coercive fields in this system, measured by hysteresis loops, depend not only on the temperature, but also on the field sweep rate [18]. [PERSON] et al. attributed the magnetic uniaxial anisotropy of this system to the magnetic anisotropy of the eight Mn\({}^{+3}\) in the molecule, which show zero-field splitting due to the Jahn-Teller elongation of some bonds and the spin-orbit interactions [45]. In this system, hysteresis loops are also modified in comparison with \"normal\" loops, showing steps at some external magnetic fields due to the resonant tunneling between levels with identical energies in the two energetically favored orientations [46, 47]. Another often investigated complex is Fe\({}_{8}\) in the form of the cluster [Fe\({}_{8}\)O\({}_{2}\)(OH)\({}_{12}\)(tacn)\({}_{6}\)]\({}^{8+}\), with (tacn) indicating a macrocyclic ligand. However, here, the relaxation is temperature-independent below a temperature of 0.36 K or 0.4 K, respectively, as opposed to Mn\({}_{12}\), because tunneling is only possible between the lowest energy levels, i.e., the levels with magnetic quantum numbers \(m_{\text{s}}=\pm 10\)[48, 49]. Similarly, [Mn\({}_{4}\)O\({}_{5}\)Cl(O\({}_{2}\)CCH\({}_{3}\))\({}_{3}\)(dbm)\({}_{3}\)] with the mono-anion of dibenzoylmethane dbm\({}^{-}\) (usually named Mn\({}_{4}\)) with an \(S=9/2\) ground  
Figure 1: Super-exchange coupling between nearest-neighbor 3d orbitals of Ni via the ligands of O\({}^{2-}\) (2p orbitals). (**a**) In the case of a 90\({}^{\circ}\) angle between Ni-O-Ni, super-exchange results in a ferromagnetic coupling between the orthogonal oxygen 2p orbitals; (**b**) for an angle of 180\({}^{\circ}\) between Ni-O-Ni, an antiferromagnetic coupling occurs due to the nickel and oxygen orbitals overlapping; and (**c**) geometry of nickel(II) acetylacetonate (Ni(acac)\({}_{2}\)) molecular magnets. From ref. [39], originally published under a CC-BY license. state shows a temperature-independent relaxation below a temperature of 0.6 K for the same reason [50]. It should be mentioned that, as mentioned in Sections 2 and 3, the magnetic behavior of these clusters based on Mn\({}_{12}\), Mn\({}_{4}\), or Fe\({}_{8}\) is defined by the magnetic exchange, which is necessary for the high ground state spin. However, for lanthanide-based SMMs, magnetic exchange can even be disadvantageous and is not necessary to create an SMM. This should be taken into account for the lanthanide-based SMMs discussed next. One disadvantage of the aforementioned single-molecule magnets is their very small blocking temperature, typically below the temperature of liquid helium. Ideally, magnetic properties should be reached above the temperature of liquid nitrogen, i.e., 77 K. This is one of the challenges diverse research groups have been working on during the last decades [51]. Recently, [PERSON] et al. reported on a relatively large blocking temperature of 9 K found in highly-axial lanthanide SMMs of the form [Dy(C\({}_{3}\)PO)\({}_{2}\)I\({}_{3}\)(CH\({}_{3}\)CN)]. The iodide ions have a large volume and low surface charge density, and serve as weak donors in this six-coordinate neutral molecule.