Molecular magnets

Article By:

Holmes, Stephen M. Center for Nanoscience, University of Missouri, St. Louis, Missouri.

Last reviewed:2014

DOI:https://doi.org/10.1036/1097-8542.YB150547

Over the past 2 decades, considerable worldwide attention has focused on the development of super-paramagnetic complexes, known collectively as single-molecule magnets (SMMs). These molecular species belong to a class of magnetic materials that can be easily prepared, tuned, and studied via a range of spectroscopic and magnetic characterization techniques. The most celebrated of these are those containing transition-metal centers linked via oxo- and carboxylate bridges, with {Mn₁₂O₁₂(O₂CMe)₁₆(OH₂)₄} (1) receiving the most attention (Fig. 1). These soluble, single-domain clusters generally display large spin ground states (S_T = 10, 20 unpaired electrons for 1) due to efficient antiferromagnetic interactions between the Mn^IV (S = ³/₂, •) and Mn^III (S = 2, •) spin centers. Owing to a nearly parallel alignment of the Jahn-Teller axes (elongated) at the Mn^III sites, magnetic anisotropy is generated (where D < 0 with small E for the Hamiltonian: H = DS²_T,z), which leads to the observation of slow magnetic relaxation at cryogenic temperatures (for example, below about 10 K; Fig. 1). Under ideal circumstances, these and other magnetic complexes exhibit sizable energy barriers to magnetization reversal, where Δ ∼ {D|S²_T or Δ = |D|(S²_T − ¹/₄) for integer or half-integer S_T values, respectively.

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