Unpaired Spins and Magnetic Moments

"O body swayed to music, O brightening glance, How can we know the dancer from the dance?"
-- W. B. Yeats, Among School Children

When we think of magnetism, most of us remember iron filings on a piece of thin cardboard. Moving a bar magnet under the paper makes the normally invisible lines of the magnetic field visible. At this point, the inquisitive among us may ask questions: What is a magnetic field? Where does magnetism come from? Think of an attractive dancer circling the outskirts of a crowded dance floor, spinning alone to the music. At a fundamental level, says Arthur Freeman, that's magnetism.

In the three-dimensional dance space occupied by an atom, electrons pair up as they revolve around the nucleus, each spinning on its axis in an opposite direction to its partner -- in effect, canceling each other's spin. Among the outermost electrons of some metal atoms, however -- those most weakly bound to the nucleus, a few unpaired electrons spin in the same direction. It's this net unpaired spin, explains physicist Freeman of Northwestern University, that in these magnetic metals gives rise to a "magnetic moment," creating the pull of a magnetic field.

Relying on powerful computational methods at the Pittsburgh Supercomputing Center to simulate the bewildering complexity of this atomic dance floor, Freeman has revolutionized ideas about magnetism. During the last decade he has shown, contrary to what physicists believed prior to his work, that a surface atomic layer of a metal can have more magnetic moment than the bulk form of the same metal. Research stimulated by this finding has led to increasing magnetic data storage on compact discs by more than 40 times.

Freeman's pioneering work with magnetism along with important computational studies in superconductivity and other solid-state properties has helped to herald a new branch of science, computational materials science. "We are now making materials," says Freeman, "with exotic properties that nature doesn't give us, new materials from old elements. What happens is once you have a new tool -- like Galileo had the telescope -- you make discoveries. We have a new tool -- the computational capability of the computer. So we make new discoveries."

Calculated electron spin density for the surface of iron (left) compared to the free monolayer (right). Dark blue indicates negative spin, and other colors are positive, increasing from light blue through pink. A larger area of positive spin in the free monolayer accounts for a magnetic moment of 3.18 bohrs, compared to 2.98 for the surface and 2.20 for bulk iron.

Researchers: Arthur Freeman, Northwestern University.
Hardware: CRAY C90
Software: FLAPW method
Keywords: Magnet, magnetic field, iron, magnetism, electrons, metal atoms, axis, spin, upaired electrons, magnetic moment, surface atomic layer, superconductivity, solid state properties, computational materials science, thin-layer magnetism, bulk metals, free monolayer, magnesium-oxide, photoemission, magnetic recording industry, compact disc, laser, state-tracking scheme, enhanced storage, electronic structure, quantum-mechanical calculations, full-potential linearized augmented plane wave method (FLAPW), perpendicular magnetism..

Related Material on the Web:
Projects in Scientific Computing, PSC's annual research report.

References, Acknowledgements & Credits