An exciting development in nanoscale magnetic materials occurred in 1993 when [Mn12O12 (O2CMe)16(H2O)4] (hereafter Mn12) was identified as nanoscale magnet, the first to comprise discrete, magnetically non-interacting molecular units rather than a 3D extended lattice (as in metals and metal oxides, for example). The discovery initiated the field of molecular nanomagnetism and such molecules have since been termed single-molecule magnets (SMMs). They derive their special magnetic properties from the combination of a large spin (S) and an Ising (easy-axis) magneto-anisotropy exhibiting a negative zero-field splitting parameter (D), which gives rise to the superparamagnetic-like property of a barrier to magnetization relaxation. This class of compounds does not only display magnetisation hysteresis, but also quantum tunnelling of magnetisation (QTM) and quantum phase interference.
A class of molecules emerged recently where slow relaxation of the magnetic moment arises from a single f-metal ion coordinated by two phthalocyaninato rings (Pc2Ln). These compounds were first described by N. Ishikawa et al. and are formulated in literature as Single-Ion Molecule Magnets (SIMMs); hence they exhibit slow magnetization relaxation as single-molecular property, hysteretic behaviours at low temperatures, and resonant quantum tunnelling of magnetization between hyperfine levels. The high temperature activated spin dynamics of the SIMM [Pc2Tb]-[TBA]+ was determined by solid state 1H NMR spectroscopy showing the implication of phonon-assisted transitions among the crystal field levels with unprecedented activation barriers ranging from 641 cm-1 for the diamagnetically diluted to 584 cm-1 for undiluted samples. Furthermore, the SIMM [Pc2Tb]0 was deposited onto Cu(111) approaching the realisation of single molecular data storage.
Molecular structure (top) of two variations of a SIMM, [TbPc2]- and [TbPc2]0, showing (left) the effective energy barrier Δ for the magnetic moment reversal determined by the ligand field potential (CF) around the f-metal and (right) an STM image of a single SIMM on a Cu(111) surface