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Optical Stimuli Control Magnetism in Novel Materials

Photonics Spectra
Oct 2003
Brent D. Johnson

In 1886, Heinrich Hertz observed that utraviolet light, when directed at a conduction plate, caused an increase in electrical charge. Einstein later articulated this as the photoelectric effect and explained how light waves or photons exist in discrete packets of energy.

Arthur J. Epstein, physics professor and director of the Center for Materials Research at Ohio State University in Columbus, who began his work with organic electronics, has spent the better part of his career investigating how light moves electrons.

Optical Stimuli Controls Magnetism in Novel Materials
Arthur Epstein and graduate student Dusan Pejakovic view an encapsulated sample of manganese tetracyanoethanide illuminated by ~2.0-eV radiation from an argon-ion laser.

In the early 1980s, working with chemist Joel S. Miller, now at the University of Utah, Epstein created some of the first magnets from organic materials. He became interested in how organic magnets might be affected by light. His research led him to investigate molecules with properties that are separate from the solid forms they inhabit, such as doped manganites and spinel ferrite films, which are based on atoms.

Typical ferromagnets, such as iron, cobalt and nickel, have an uncompensated spin, Epstein said. Spin, he explained, is a quantum mechanical effect that is analogous to electrons spinning on their own axis. Each electron has its own spin. Most solids have an equal number of electrons spinning "up" or "down." However, in solids with magnetic properties, an unequal or uncompensated number of electrons are spinning in one particular direction. If the arrangement of these electrons is random, there will be only a weak magnetic effect. But if the unpaired electrons line up "like soldiers in a field," the magnetic effect will be pronounced.

Molecule shaping

By illuminating the materials with both coherent and noncoherent filtered white light from broadband sources, he found that he could change the shape of these molecules, thereby changing the electron organization and, consequently, their physical properties.

Employing an argon laser at 2.7 eV, Epstein exposed an organic Mn[TCNE]2 magnet to blue light, which increased its magnetism. Using green light, he reversed the effect. The demonstration proved, for example, that the technique could be used to write and erase information on a disk drive.

Since the groundbreaking experiment, Epstein and his team have managed to increase the temperature range of the magnets to 75 K. The major challenge will be to improve the temperature performance so that the magnets operate effectively at room temperature. They are continuing to improve higher-temperature organic-based magnets.

Epstein believes that there are opportunities for designing new organic-based materials that could be processed as thin films. These thin films could then be used for magnetic tape or disk drives. The materials could also dramatically reduce the weight of electrical transformers, which operate by guiding magnetic fields from one iron coil to another. He thinks that these coils could be replaced with lightweight polymers. Commercial applications are not expected for several years, he added.


GLOSSARY
argon-ion laser
gas laser using ionized argon as the active medium and applying electronic excitation in order to produce the laser light
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