Molecular Minerals

The synthesis of high nuclearity clusters is a difficult synthetic challenge. However, large clusters of paramagnetic metals display extremely interesting properties and thus are the subject of much current interest. They represent the ultimate small-size magnetic memory devices and promise access to magnetic phenomena such as QTM, which may lead to quantum computation and the development of new mesoscopic magnetic theory. This proposal outlines new design strategies for the manufacture of such clusters and their physical characterisation.

Many present and future specialised applications of magnets require monodisperse, nanoscale magnetic particles, and the discovery that individual molecules can function as nanoscale magnets was thus a significant development. Such a molecule, named a single-molecule magnet (SMM), functions as a single-domain magnetic particle that, below its blocking temperature, exhibits the classical property of a magnet, namely, magnetisation hysteresis. In addition, it straddles the classical/quantum interface in also displaying (QTM) and quantum phase interference (QPI). A SMM derives its unusual properties from a combination of large spin (S) and large, easy-axis-type anisotropy (negative axial zero-field splitting parameter D). These result in a significant barrier to thermally activated magnetisation relaxation (Fig. 3), with upper limits given by S2│D│ or (S2-1/4)│D│ for integer and half-integer spin, respectively.
The potential importance of SMMs can be illustrated by considering the present state of information storage: current disk drives store information at approximately 5 Gbits/in2 while SMMs could theoretically store information at > 40 Tbits/in2. SMMs also have many other advantages over conventionally made nanoscale magnetic particles (usually made from the fragmentation of bulk samples) in that they are (1) of single, sharply defined size (2) amenable to ligand variation – allowing versatile functionalisation (3) soluble, and (4) of sub-nanoscale dimensions. The observation of QTM has also led to speculation that SMMs could be used as Qbits in quantum computing

a) Quantum coherence is at the heart of many fundamental quantum mechanical phenomena, and of possible practical applications of quantum mechanics, such as quantum information processing. There has been great interest in quantum coherence in molecular nanomagnets because of this potential application, but also because of the more fundamental issue of observing quantum coherence in larger and larger (mesoscopic) systems with the aim of exploring the boundary between classical and quantum physics. In spite of this interest, no direct experimental evidence for quantum coherence in molecular nanomagnets has been obtained to date. We have shown direct pulsed electron spin resonance (ESR) evidence for quantum coherence in a [Fe4] molecular nanomagnet. The phase memory time is 300 ns at 4.3 K. Interestingly, coupling of the electron spin to the solvent nuclear spins is observed. The clear observation of Rabi oscillations shows the possibility of coherent spin manipulations. See for example: Phys. Rev. Lett., 2008, 101, 147203.
b) We reported the syntheses and structures of two decametallic mixed-valent Mn supertetrahedra using 2-amino-2-methyl-1,3-propanediol (ampH2); two decametallic mixed-valent Mn planar discs using 2-amino-2-methyl-1,3-propanediol (ampH2) and 2-amino-2-ethyl-1,3-propanediol (aepH2) and a tetradecametallic mixed-valent Mn planar disc, using pentaerythritol (H4peol). The decametallic complexes display dominant ferromagnetic exchange and spin ground states of S = 22 and the tetradecametallic complex dominant antiferromagnetic exchange and a spin ground state of S = 7±1. All display large (the former) and enormous (the latter) magneto-caloric effect (MCE); the former as a result of negligible zero-field splitting of the ground state, the latter as a result of possessing a high spin-degeneracy at finite low temperatures, making them the very best cooling refrigerants for low-temperature applications. See for example: Angew. Chem. Int. Ed. 2007, 46, 4456 and J. Am. Chem. Soc., 2008, 130, 11129.
c) We reported the the syntheses, structures and magnetic properties of a family of highly unusual [Mn16] and [Mn22] wheels which displayed extremely large spin ground states and SMM behaviour. We also reported the syntheses of a family of [Mn6] rod-like complexes which were also SMMs. See for example: Inorg. Chem., 2007, 46, 6968; Dalton Trans., 2007, 532.