Molecular clusters

Molecular magnets consist of several transition metal ions surrounded by organic and inorganic ligands. Their dimensions are a few nanometres, The best-known example is Mn₁₂ acetate ([Mn₁₂O₁₂(CH₃COO)₁₆(H₂O)₄] which is a cluster of twelve manganese ions. Eight are Mn3+ ions and four are Mn⁴⁺ ions. The interaction between Mn³⁺ and Mn⁴⁺ is strongly antiferromagnetic, so the net spin of the molecular cluster is S = [8 × 2 − 4 × (3/2)] = 10, giving a moment of 20 μB per cluster. The overall symmetry of the molecule is tetragonal, and there is strong uniaxial anisotropy due to the crystal field

which produces an overall crystal field splitting of about 300 K. The ±|10〉 doublet is the ground state, and if the material is magnetized at low temperature (∼1 K), it remains in the −|10〉 state because relaxation to the +|10〉 state is extremely slow. In a reverse field, however, it is possible for the molecule to tunnel into an excited state, and a square, staircase hysteresis loop results. Similar magnetization dynamics are observed for other molecular magnets, and even for isolated rare earth ions with large J , the ultimate atomic nanomagnets.