Organic-Based Magnets: Opportunities in Photoinduced Magnetism, Spintronics, Fractal Magnetism, and Beyond
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Organic-Based
Magnets: Opportunities in Photoinduced Magnetism, Spintronics, Fractal Magnetism, and Beyond Arthur J. Epstein
Abstract This article is based on a presentation on organic-based magnets given as part of Symposium X—Frontiers of Materials Research on December 4, 2002, at the 2002 Materials Research Society Fall Meeting in Boston. The advent of organicbased magnets opened the opportunity for tuning magnetic properties by molecular design and the discovery of new phenomena that rely on the internal structure of the molecules that make up these magnets. In the past 18 years, numerous classes of organic-based ferromagnets, ferrimagnets, and spin glasses (spins essentially frozen in place without long-range order) have been reported. These materials have magnetic ordering temperatures ranging from 1 K to above room temperature and demonstrate many of the magnetic properties associated with conventional magnets. This article concentrates on new phenomena that are unique to organic-based magnets. Three of these effects—“high-temperature” light-induced magnetism, spin-polarized magnetic organic semiconductors with the potential for spintronics, and the development of fractal magnetic order—are discussed to illustrate the richness of opportunity in organic-based magnets. Keywords: fractal magnetism, light-induced magnetism, molecule-based magnets, organic-based magnets, spintronics, tetracyanoethylene (TCNE).
Introduction Magnets are an integral part of our society. Common uses range from the huge electromagnets in local recycling centers and scrap yards that pick up and separate ferrous metals, to the magnets used in
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electric generators and motors, to the magnetic materials that make up computer memories. Developing new types of magnets for conventional and new applications is of increasing interest and importance.
The earliest known magnet, lodestone, is based on magnetite, a mineral that is primarily naturally occurring Fe3O4. Commercial magnets are materials based on transition-metal and rare-earth elements and their oxides in which there are electrons with unpaired spins. It was not until the mid-1980s that magnets were reported based on unpaired electrons in the p, , or s orbitals of organic molecules.1 The advent of organic-based magnets opened the opportunity for tuning magnetic properties by molecular design and the discovery of new phenomena that rely on the internal structure of the molecules that make up these magnets. Organic-based magnets may be lightweight, biocompatible, inexpensive, and even soluble in common solvents.2,3
The Origins of Magnetism Atoms and molecules have electrons in atomic and molecular orbitals, respectively. When exposed to a magnetic field (e.g., produced by a permanent magnet or electromagnet), electrons change their orbital velocity around nuclei to partially screen the applied magnetic field, as described by Lenz’s law. This effect results in diamagnetism, that is, the induced magnetization of a material (represented by the vector M) pointing in the opposite
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