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

TL;DR

This paper reviews the magnetic properties of supramolecular [N x N] grids, highlighting their potential as models for extended 2D magnetic systems and exploring their connection to 1D rings, with implications for high-temperature superconductors.

Contribution

It provides a comprehensive review of the magnetic behaviors of [2 x 2] and [3 x 3] metal ion grids and discusses their relation to one-dimensional rings, offering new insights into zero-dimensional magnetism.

Findings

Variety in magnetic properties of metal ion grids

Connection between 2D grids and 1D rings

Insights into elementary excitations in antiferromagnetic rings

Abstract

Supramolecular chemistry has allowed the production, by self-assembly, of inorganic complexes with a [N x N] square matrix-like configuration of N^2 metal centers. Interest in these systems is driven by the potential applications in information technology suggested by such a "two-dimensional" (2D), addressable arrangement of metal ions. From the magnetic perspective, [N x N] grids constitute molecular model systems for magnets with extended interactions on a square lattice, which have gained enormous attention in the context of hightemperature superconductors. Numerous [2 x 2] grids as well as a few [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 remarkable variety in their magnetic properties, which will be reviewed in this work with emphasis on the underlying physical concepts. An intriguing issue is the connection of [2 x 2] and [3 x 3] grids with "one-dimensional" (1D) rings, as experimentally realized in the molecular wheels. For a [2 x 2] square of spin centers 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 ring with an additional ion "doped" into its center. Challenging familiar concepts from conventional magnets, the current picture of elementary excitations in antiferromagnetic rings will be discussed, as a prerequisite to understand the complex [3 x 3] grids.