Magnetic molecular wheels and grids - the need for novel concepts in 'zero-dimensional' magnetism

摘要

Supramolecular chemistry has allowed the production, by self-assembly, ofinorganic complexes with a [N x N] square matrix-like configuration of N^2metal centers. Interest in these systems is driven by the potentialapplications in information technology suggested by such a "two-dimensional"(2D), addressable arrangement of metal ions. From the magnetic perspective, [Nx N] grids constitute molecular model systems for magnets with extendedinteractions on a square lattice, which have gained enormous attention in thecontext of hightemperature superconductors. Numerous [2 x 2] grids as well as afew [3 x 3] grids with magnetic metal ions such as Cu(II), Ni(II), Co(II),Fe(II), and Mn(II) have been created. Magnetic studies unraveled a remarkablevariety in their magnetic properties, which will be reviewed in this work withemphasis on the underlying physical concepts. An intriguing issue is theconnection of [2 x 2] and [3 x 3] grids with "one-dimensional" (1D) rings, asexperimentally realized in the molecular wheels. For a [2 x 2] square of spincenters the distinction between a 2D grid and a 1D ring is semantic, but also a[3 x 3] grid retains 1D character: it is best viewed as an octanuclear ringwith an additional ion "doped" into its center. Challenging familiar conceptsfrom conventional magnets, the current picture of elementary excitations inantiferromagnetic rings will be discussed, as a prerequisite to understand thecomplex [3 x 3] grids.