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Structure of Mn12-acetate. Color code: Mn = violet, O = red, C = gray, H = light gray. The easy magnetic axis is vertical and normal to the Mn12 disk. Hydrogen atoms of methyl groups are omitted for clarity.
Magnetic hysteresis loop measured at 2.1 K on a single crystal of Mn12-acetate by applying the magnetic field along the easy axis. In zero field, the magnetization can be either "up" or "down" depending on the previous history of the sample. Each molecule in the crystal then displays a magnetic memory effect that can be in principle used to store information.
Single Molecule Magnets
Magnetic materials are key components in today’s information technology. Large amounts of data are stored in magnetic films on computer hard disks, with an ever-increasing demand on storage density, processing speed and device complexity. Researchers have therefore directed their attention toward certain magnetic molecules which act as tiny permanent magnets. Because of their exceedingly small size (a few nanometers), these Single-Molecule Magnets (SMMs) provide the ultimate limit for data storage. As compared with traditional recording media, molecule-based media would afford much greater storage densities, above 10000 Gbit/cm2.
The first-discovered SMM was obtained as the main product of the reaction between Mn(OAc)2·4H2O (containing Mn(II)) and KMnO4 (containing Mn(VII)) in a 60% v/v AcOH-water mixture. It is a crystalline, molecular compound made of dodecamanganese clusters, whose structure is depicted on the right. The manganese ions are in two different oxidation states: 4+ in the central Mn4 tetrahedron and 3+ in the outer Mn8 ring. The resulting [Mn12] core is held together by twelve oxide ions and sixteen bridging acetates, while the four water molecules provide terminal ligation to every second Mn(III) ion. Additional water and acetic acid molecules are also present in the crystal lattice of the compound, whose complete formula is [Mn12O12(OAc)16(H2O)4]·4H2O·2AcOH (Mn12-acetate shortly).
The distinctive property of SMMs is the occurrence of magnetic hysteresis at low temperature, so that each molecule behaves as a tiny magnet. The hysteresis loops of SMMs and ferromagnetic materials are quite similar, but completely different in origin. In fact, no long-range order is established in the crystal lattice of a SMM due to the efficient magnetic shielding provided by the organic ligands which surround the cluster core. Hysteresis in SMMs is a purely molecular property. In fact, it persists also in solution where intermolecular interactions are certainly negligible. It arises from the coexistence of a high-spin ground state (S = 10 in Mn12-acetate) and a strong easy-axis magnetic anisotropy, i.e. a tendency of the magnetic moment to lie along a certain molecular direction (±Z) and much less favourably perpendicular to it. The reversal of the molecular magnetic moment is then subject to an energy barrier U (U/kB ~ 70 K in Mn12-acetate). At temperatures such that kBT << U the magnetic moment is effectively "frozen" along the easy direction. When a SMM is fully magnetized along +Z ("up") or -Z ("down") and the external field is subsequently removed, the molecule retains its magnetization. Hence, the zero-field magnetization can be "up" or "down" depending on the previous "history" of the sample. The molecule effectively behaves as a bistable magnetic unit which can store one bit of information! One additional feature of SMMs is the occurrence of "steps" in their hysteresis loop. Such steps reflect the availability of quantum-mechanical "shortcuts" for the reversal of the magnetic moment. These tunneling mechanisms are among the most actively investigated topics in molecular nanomagnetism.