Traditionally, magnetic materials have been metals or, if inorganic compounds such as oxides, of continuous lattice type. However, in recent years chemists have synthesized increasing numbers of crystalline solids based on molecular building blocks in the form of coordination and organometallic complexes or purely organic molecules, which exhibit spontaneous magnetization. In striking contrast to conventional magnets, these materials are made from solutions close to room temperature rather than by metallurgical or ceramic methods. This book, which originates from contributions to a Discussion Meeting of The Royal Society of London, brings together many of the leading international practitioners in the field, who survey their own recent work and place it in the context of the wider fields of magnetism and supramolecular chemistry. All aspects of molecular-based magnets are addressed, including synthesis, structure-property relations and physical properties. Contents include details of the characterization of the first purely organic ferromagnet, the synthesis of high coercivity materials and a unique description of new materials with Curie temperatures well above ambient. A coherent survey of this rapidly developing field for the more general reader, Metal-Organic and Organic Molecular Magnets will also be welcomed by researchers and lecturers in materials science and inorganic or solid state chemistry.
Metal-Organic and Organic Molecular Magnets
By P. Day, A. E. UnderhillThe Royal Society of Chemistry
Copyright © 1999 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-764-2Contents
Molecular-based magnets: setting the scene P. Day and A. E. Underhill, 1,
p-Nitrophenyl nitronyl nitroxide: the first organic ferromagnet M. Kinoshita, 4,
Discussion: P. Day, 19,
Crystal architectures of organic molecular-based magnets D. B. Amabilino, J. Cirujeda and J. Veciana, 22,
Discussion: P. Day, 40,
Unusual crystal structures and properties of nitronylnitroxide radicals. Possible RVB states in molecule-based magnets K. Awaga, N. Wada, I. Watanabe and T. Inabe, 41,
Discussion: M. Verdaguer, 70,
Muon-spin-rotation studies of organic magnets S. J. Blundell, 71,
Discussion: M. Verdaguer, 84,
High-spin polymeric arylamines R. J. Bushby, D. Gooding and M. E. Vale, 86,
Room-temperature molecule-based magnets M. Verdaguer, A. Bleuzen, C. Train, R. Garde, F. Fabrizi de Biani and C. Desplanches, 105,
Design of novel magnets using Prussian blue analogues K. Hashimoto and S. Ohkoshi, 123,
Magnetic anisotropy in molecule-based magnets O. Kahn, 150,
Multifunctional coordination compounds: design and properties S. Decurtins, 169,
Discussion: M. Verdaguer, 184,
Ferrimagnetic and metamagnetic layered cobalt(II)-hydroxides: first observation of a coercive field greater than 5 T M. Kurmoo, 185,
Discussion: B. J. Bushby, M. Verdaguer, 205,
Towards magnetic liquid crystals K. Binnemans, D. W. Bruce, S. R. Collinson, R. Van Deun, Yu. G. Galyametdinov and F. Martin, 206,
Quantum size effects in molecular magnets D. Gatteschi, 221,
Large metal clusters and lattices with analogues to biology D. J. Price, F. Lionti, R. Ballou, P. T. Wood and A. K. Powell, 240,
New high-spin clusters featuring transition metals E. K. Brechin, A. Graham, P. E. Y. Milne, M. Murrie, S. Parsons and R. E. P. Winpenny, 260,
From ferromagnets to high-spin molecules: the role of the organic ligands T. Mallah, A. Marvilliers and E. Rivière, 279,
Discussion: P. Day, 298,
Molecular-based magnets: an epilogue, 299,
The Bakerian Lecture, 1999,
Lecture held at The Royal Institution 10 February 1999 and University of Oxford 15 February 1999,
The Bakerian Lecture, 1999. The molecular chemistry of magnets and superconductors P. Day, 303,
CHAPTER 1
Molecular-based magnets: setting the scene
By Peter Day and Allan E. Underhill
Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W1X fBS, UK School of Chemistry, University College of North Wales, Bangor, Gwynedd LL57 2DG, UK
In 1839, when Michael Faraday published the first picture of the lines of magnetic flux around a magnet (Faraday 1832), shown in figure 1, the cylinder of material in the centre of the figure could only have been one material, iron. Over the succeeding 160 years, the number of substances showing spontaneous magnetization has increased enormously, while their variety has broadened dramatically. Yet till quite recently, the field of magnetic materials has been traditionally confined to metals, among which the current 'market leaders' such as lanthanide–cobalt and Nd–Fe–B have achieved large technological significance. Among non-metallic phases, transition metal oxides made an early appearance in the years immediately before and after World War II, and the technologically driven need to understand and optimize their properties led to the phenomenological theories of Néel, complemented later by the microscopic models of Mott (1949), Anderson (1963) and Goodenough (1955). The latter, in particular, set out the orbital symmetry rules that brought the subject of cooperative magnetism firmly within the ambit of the solid-state chemist.
Halides, chalcogenides and pnicnides are for the most part continuous lattice compounds, and, apart from isolated instances that could be regarded more or less as curiosities (such as diethyldithiocarbamato–Fe(III) chloride (Wickman et al. 1967a, b) and Mn phthalocyanine (Barraclough et al. 1970)), magnetic solids built up from molecular coordination or organometallic complexes only arrived on the scene quite recently. Early work by the Dutch school (de Jongh & Miedema 1974) had shown that layer perovskite halide salts of Cu(II) were useful models for ferromagnetism in insulating two-dimensional lattices, while we found that by replacing Cu (S = 1/2) with Cr (S = 3/2) organic intercalated insulating ferromagnets could be synthesized with Curie temperatures up to 50 K (Bellitto & Day 1976; for reviews see Day (1986) and Bellitto & Day (1992)). These Cu(II) and Cr(II) salts were also excellent realizations of the ferromagnetic exchange arising from orthogonality between orbitals on neighbouring metal centres containing the unpaired electrons, of the kind pointed out by Goodenough (1963) and Kanamori (1959) for continuous lattice oxides. Organometallic charge transfer salts and one-dimensional ferrimagnetic coordination polymers followed in the 1980s (Miller et al. 1988; Nakatami et al. 1991), while the early 1990s saw the first ferromagnets made from purely organic molecular building blocks, without any metal atoms at all, heralding a new field of p-electron magnetism to complement that of d- and f-electron materials (Tamura et al. 1991).
Given the truly enormous number of magnetic solids prepared, characterized and exploited in the last century or so, it is legitimate to ask what new features, either experimental or theoretical, the molecular-based materials have brought to this field. There are many. From the point of view of synthesis and processing, the contrast with conventional magnetic materials could not be more stark: substances made at (or close to) ambient temperature, usually from solution compared with high temperature metallurgical or ceramic processes. A real chance exists to make a soluble magnet! Correlating magnets with other properties, it should be pointed out that all known molecular-based magnetic compounds are insulators, the precise inverse of the situation for continuous lattice materials. That simple fact has consequences for many of the accompanying properties, of which the most striking (and potentially one of the most useful) is optical. The molecular-based materials are frequently transparent to infrared and visible light. More than 20 years have passed since we first demonstrated the striking colour change occurring in an insulating ferromagnetic transition metal salt on passing through the Curie temperature as a result of coupling excitons to spin waves (Bellitto & Day 1978; Bellitto et al. 1980). Combining magnetism with properties only found in the molecular solid state, such as mesomorphism or chirality, would appear to be another potentially fruitful source of novel physics, as is the construction of unusual lattice topologies, such as the Kagome lattice, with which to test statistical thermodynamic models of critical behaviour. One apparent drawback of molecular-based magnetic materials, especially in the realm of information storage, is that the large majority are very soft magnets, i.e. they have quite low coercivities. However, even this limitation is now being breached, as two contributions to this issue demonstrate (Kahn, this issue; Kurmoo, this issue).
In all, the design, synthesis and study of molecular-based...
