Molecule-based magnets with a fully interlocked three-dimensional structure

This paper concerns the chemistry and the magnetic properties of a family of compounds with a fully interlocked structure. The general formula is [Etrad](2)[M-2{Cu(opba)}3].S, with M = Mn2+, Co2+, Mg2+, Ni2+ where Etrad stands for a radical cation, and opba stands for ortho-phenylenebis(oxamato). The structure consists of two interpenetrating graphite-like networks with edge sharing hexagons. The magnetic properties of the four compounds have been investigated in detail. The Mg2+ derivative behaves as a paramagnet with weak ferromagnetic interactions between Cu2+ and Etrad(+) ions. The three other compounds are molecule-based magnets with a spontaneous magnetization below 22.8 K for the Mn2+ derivative, 28 K for the Ni2+ derivative and 37 K for the Co2+ derivative. While the Mn2+ derivative is a soft magnet, the Ni2+ and Co2+ derivative display hysteresis loops, with coercive fields higher than 1 kOe depending on the temperature and the size of the particles. The Ni2+ derivative presents also an inversion of the magnetic poles as the temperature varies under a small external field. For higher field, the magnetization reverses two times. These original results can be properly reproduced using the molecular-field theory adapted to the system and the magnetic anisotropy. (C) 2001 Elsevier Science B.V. All rights reserved.