# New Materials and Effects in Molecular Nanomagnets
## 4 Single-Molecule Magnets
The reduced ligand field strength results in a strongly axial crystal field in this SMM, leading to strong crystal-field splitting and high axial orientation [52]. A recipe for higher blocking temperatures was recently proposed by [PERSON] et al. They used [Dy(C\({}_{5}\)H\({}_{2}\)Bu\({}_{3}\)-1,2,4)\({}_{2}\)][B(C\({}_{6}\)F\({}_{5}\))\({}_{4}\)] as a model complex to investigate the relaxation mechanisms that they found based on Orbach, Raman, and quantum tunneling of the magnetization processes, i.e., some of the special relaxation pathways mentioned in Section 2. Based on these results, they could predict these processes' temperature and field dependencies, showing that suppressing the Raman mechanism is important to increase the blocking temperature. In general, they concluded that a crystal with weakly-interacting rigid molecules was necessary to reach the requirements of the soft acoustic branches and high-energy optical modes [53], which was revealed from the phonon density of states (DOS) computed ab initio. An experimental approach was chosen by [PERSON] et al. They prepared two different disproisacarbonanes, [(C\({}_{2}\)B\({}_{9}\)H\({}_{11}\))\({}_{2}\)Dy(THF)\({}_{2}\)][Na(THF)\({}_{5}\)] (denoted as 1 Dy) and [(THF)\({}_{3}\)(\(\mu\)-H)\({}_{3}\)Li]\({}_{2}\)[\(\{\)\(\)  
SMM DyErScN@_h_-C80 [56]. A much higher blocking temperature of 24 K was observed by [PERSON] et al. in the azafullerene Tb\({}_{2}\)@C79N [57]. Similar blocking temperatures were found by [PERSON] et al. who prepared DyIII single-ion magnets from [Dy(OPCy3)\({}_{2}\)(H\({}_{2}\)O)\({}_{5}\)](CF\({}_{3}\)SO\({}_{3}\))\({}_{3}\):2 OPCy3 (with OPCy3 = tricyclo-hexylphosphine oxide) [58]. Much higher values were observed by [PERSON] et al. who compared the divalent linear metallocenes Ln(Cp\({}^{\text{IP}5}\))\({}_{2}\) (with Ln = Tb, Dy) with the trivalent ones [Ln(Cp\({}^{\text{IP}5}\))\({}_{2}\)][B(C\({}_{6}\)F\({}_{5}\))\({}_{4}\)], and found longer relaxation times in Tb\({}^{\text{II}}\) than in TbIII and longer relaxation times in DyIII than in DyII, and a similar behavior for TbII and DyIII, as could be expected from the identical number of unpaired electrons in the latter. Most importantly, in Tb(Cp\({}^{\text{IP}5}\))\({}_{2}\) they found a high blocking temperature of 52 K [59]. Other high blocking temperatures were found, e.g., in [(Cp\({}^{\text{IP}5}\))Dy(Cp\({}^{\text{IP}5}\))\({}^{+}\) with Cp\({}^{\text{IP}5}\) = penta-iso-propylcyclopentadienyl and Cp\({}^{\star}\) = pentamethylcyclopentadienyl, an SMM for which [PERSON] et al. reported a blocking temperature of 80 K, i.e., even above the temperature of liquid nitrogen [60], for the DyIII metallocenium salt [Dy(Cp\({}^{\text{IP}4}\))\({}_{2}\)][B(C\({}_{6}\)F\({}_{5}\))\({}_{4}\)] for R = Me, i.e., the methyl-substituted complex, with a blocking temperature of 62 K [61], or for the metallocenium cation [(Cp\({}^{\text{tttt}}\))\({}_{2}\)Dy]\({}^{+}\) with Cp\({}^{\text{tttt}}\) = 1,2,4-tri(_tert_-butyl_)cyclopentadienide, where [PERSON] et al. measured a blocking temperature of 60 K [62]. While pure metal SMMs have been investigated in detail [63; 32], metal complexes such as the aforementioned ones are often highly interesting. In particular, 3d-4f metal complexes can add special properties to SMMs, such as a high-spin ground state and a large magnetic anisotropy [64]. The first SMM containing such a 3d-4f metal complex was [Cu\({}^{\text{II}}\)-LT\({}^{\text{III}}\)(hfac)\({}_{2}\)]\({}_{2}\)[65]; other 3d-4f complexes are, e.g., Mn\({}_{3}\)Ln or Fe\({}_{3}\)Ln with the lanthanides Gd or Dy [66]. More generally, typical paramagnetic 3d ions used in such complexes are transition metal ions such as Co\({}^{\text{II}}\), Mn\({}^{\text{II}}\), Mn\({}^{\text{III}}\), Fe\({}^{\text{II}}\), Fe\({}^{\text{II}}\), Ni\({}^{\text{II}}\), and Cu\({}^{\text{II}}\), while 4f ions can be, e.g., lanthanide ions such as DyIII, TbII, Gd\({}^{\text{II}}\), or ErII [67]. Even diamagnetic 3d metal ions can sometimes be used to create SMMs [68]. Recently, [PERSON] et al.