Chapter 7 - Tunneling in magnetic molecules

Summary

Magnetic molecules represent another interesting area for MQT studies. In this chapter we will describe experiments on spin tunneling in macroscopic crystals of Mn₁₂Ac molecules. A Mn₁₂Ac molecule has spin 10 and is equivalent to a very small single-domain magnetic particle. A crystal consists of a macroscopic number of molecules arranged in a tetragonal lattice. Spins of the molecules interact weakly with each other and are subject to a very strong uniaxial anisotropy. A Mn₁₂Ac crystal is, therefore, equivalent to a large system of identical uniformly oriented magnetic particles, which has been wanted so much by experimentalists. Experiments on Mn₁₂Ac [16] are the first experiments on spin tunneling which have been understood quantitatively without any free parameters. Whether the moment of the molecule, 20μ_B, is macroscopic enough to allow one to talk about MQT is left to the judgement of the reader.

The Mn-12 acetate complex

The chemical formula of the manganese acetate complex discussed below is [Mn₁₂O₁₂(CH₃COO)₁₆(H₂O)₄]·2CH₃COOH·4H₂O. We will denote it by Mn₁₂Ac. This compound was first synthesized by Liz and characterized by Xray diffraction methods [128]. It forms a molecular crystal of tetragonal symmetry with the lattice parameters a = 1.732 nm and b = 1.239 nm. The unit cell contains two Mn₁₂O₁₂ molecules surrounded by four water molecules and two acetic acid molecules. The structure of the Mn₁₂O₁₂ molecule is shown in Fig. 7.1. The molecule possesses S₄ symmetry.