Imitating Iron's Magnetism; Researchers Report the First Steps on the Road to Plastic Magnets
Peterson, Ivars, Science News
Imitating Iron's Magnetism
Researchers report the first steps on the road to plastic magnets
Everyday experience teaches us whatto expect when we use or handle common materials. Thus, for anyone who has played with a bar magnet, finding a chunk of iron that attracts a fringe of iron filings or picks up a string of paper clips is hardly surprising. Iron is known to be one of a few metals, termed ferromagnetic materials, that can be magnetized. On the other hand, it would be astonishing to see a lump of plastic acting like a permanent magnet. Nevertheless, researchers are now on the track of polymers and molecular solids that could readily pass for metallic ferromagnets.
Recently, three different researchgroups announced varying degrees of success in synthesizing organic ferromagnetic materials, consisting of compounds largely made up of carbon, hydrogen and nitrogen atoms. Two of the products are polymers; the third is a type of crystalline solid known as a charge-transfer salt.
This achievement parallels the unexpecteddiscovery during the last decade of organic materials with a range of electrical properties. Whereas scientists once considered organic materials, such as polymers, to be insulators, now they can turn these materials into electrical conductors. Previously, only metals and some inorganic substances were known to conduct electricity.
Like the discovery of conducting polymers,the demonstration of the existence of organic or molecular ferromagnets opens up a new field of study. Although the road to plastic magnets, novel coatings for magnetic recording tape and other potential applications is likely to be a long one, the first steps have been taken.
Magnetic materials owe their magnetismmainly to the spin of their electrons. Each electron can be thought of as a tiny magnet pointing up or pointing down. Often, these electrons occur as pairs, each pair consisting of electrons with opposite spin. An atom or molecule with paired electrons has no net spin and exhibits only mild, subtle magnetic effects.
Iron atoms happen to have unpairedelectrons. As a result, these atoms have a net magnetic moment. When iron atoms cluster, as they do when iron crystallizes, the unpaired electrons tend to align their individual spins so that electrons in large regions of the material have the same spin. These regions of common spin are called domains. A weak, externally applied magnetic field aligns all the domains so that the whole material behaves in a coordinated fashion to create a permanent magnet. This type of magnetic effect is called ferromagnetism. Because ferromagnetism is apparent only if a sufficiently large number of atoms cooperate, it's considered to be a "bulk' property of a material rather than a property of the atoms themselves.
In general, individual molecules havean even number of electrons. These are paired so that the material shows no net magnetism. The trick to creating a molecular rather than an atomic ferromagnet is to build molecules with an odd number of electrons so that at least one electron on each molecule is unpaired. A ferromagnet will result if the spins of neighboring molecules are somewhow lined up so that all the spins are in the same direction.
That's much easier said than done. Forone thing, molecules with unpaired electrons, also known as radicals, are often highly reactive. Moreover, just as opposite magnetic poles attract, spins on adjacent molecules are more likely to be in opposite directions rather than in the same direction.
"There are a large number of peoplewho will tell you [that synthesizing an organic ferromagnet] is impossible,' says Jerry B. Torrance of the IBM Almaden Research Center in San Jose, Calif., "yet a lot of us feel it is possible, and we think we can see how to do it.' Nevertheless, he adds, "most people are in this for the challenge. The difficulty is beyond belief. …