Magnetically Ordered Molecule-Based Materials
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Introduction Extended network-structured inorganic solids have become crucial to our highly technological society by providing the materials base for many advanced technologies. Optical-, electrical-, magnetic-, and mechanical-based areas have all benefited from serendipitous discoveries as well as the deliberate development of improved materials. These important materials are typically composed of atomic as well as ionic components, for example, III–V or II–VI semiconductors or allotropes of elements such as Si. In contrast, the use of molecules in the solid state for technologically important materials is far less developed. Nonetheless, molecule-based materials mimic most inorganic network solids with respect to exhibiting a plethora of technologically important properties.1 Moleculebased materials allow modulation and control of their physical properties by conventional molecule/organic synthetic methodologies as well as facile fabrication and processability. Furthermore, the ultimate miniaturization—molecule-based devices—has stimulated numerous research efforts in nanofabrication and “molecular electronics.” Magnetic materials traditionally are based on metals and metal oxides possessing 3D network structures. Recently, however, magnetically ordered materials based upon organic molecules have been reported. These magnets possess spinbearing organic radicals that contribute to both the magnetization and spin coupling
that leads to magnetic ordering. To date, many different magnetic behaviors have been established for extended networkstructured and molecule-based materials, as summarized in Table I.
Ionic Organic-Based Magnets The first organic-based magnet, [Fe(C5Me5)2]•+[TCNE]•− (where TCNE is tetracyanoethylene; the superscript • refers
to an unpaired electron spin),2 forms a linear chain of alternating S = 1/2 [Fe(C5Me5)2]•+ cations and S = 1/2 [TCNE]•− anions (Chart 1), where S is the total spin quantum number. Despite the lack of extended bonding, as these ions are separated by van der Waals interactions, magnetic ordering occurs at 4.8 K and has a coercivity of 1 kOe and remanent magnetization of 16.0 kemu Oe/mol at 2 K (Figure 1). The magnet’s 16.7 kemu Oe/mol saturation magnetization exceeds that of iron metal by 37% on an iron basis. Hence, this compound is the first organicbased ferromagnet and the first magnet that does not require energy-intensive metallurgical fabrication conditions. In addition, it is a soluble as well as an insulating magnet, possessing properties that do not exist for metal and metal oxide magnets. The replacement of 7,7,8,8-tetracyano-pquinodimethane (TCNQ) (Chart 1) for TCNE forms two polymorphs of composition [Fe(C5Me5)2]•+[TCNQ]•−: a metamagnet (critical temperature Tc = 2.5 K)3,4 and a ferromagnet (Tc = 3.1 K).4,5 Metamagnets have an antiferromagnetic ground state, which in a sufficiently large applied field (1.3 kOe for the metamagnetic polymorph of [Fe(C5Me5)2]•+[TCNQ]•−)4 switches to a ferromagnetic-like state (Figure 2). The compounds [Fe(C5Me5)2]•+[TCNE]•− and [Fe(
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