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Avalanches in a magnetic froth.

Avalanches in a magnetic froth

Thin films of synthetic garnet, an iron-oxide compound, have long played an important role as magnetic bubble memories, in which bits of digital data are stored as compact, circular regions, or domains, magnetized in the opposite direction of the thin magnetic film through which they move. The magnetic garnet films also prove useful for studying the evolution of magnetization patterns, says Kenneth L. Babcock of the University of California, Santa Barbara.

Babcock, working with Robert M. Westervelt of Harvard University, investigated the behavior of thin-film magnetic domains having a disordered cellular pattern resembling a two-dimensional soap froth. They discovered that under the influence of an increasing external magnetic field, magnetized "cells" having fewer than six sides tend to contract and sometimes collapse, while other cells grow to fill the vacated space. That behavior bears a striking resemblance to the evolution of soap froths over time (SN: 7/29/89, p.72).

The researchers have also observed a dramatic "melting" transition, in which a front sweeps through an orderly lattice of magnetized cells, leaving behind a disordered magnetic froth. "These transitions are analogous to the melting of solids induced by changing the pressure while holding the temperature fixed," Babcock says.

Magnetic avalanches can also produce striking patterns, the researchers say. In such avalanches, the elimination of a given cell caused by a small increase in the external magnetic field alters the local pattern sufficiently to trigger the collapse of neighboring cells, producing a wave of destruction.

The researchers find that for certain ranges of external magnetic-field strengths, the magnetic froth shifts into a barely stable state readily susceptible to additional avalanches, even for very small increases in the external magnetic field. In other words, the magnetic froth apparently organizes itself into a precarious state prone to avalanches. That type of behavior seems to meet the requirements of self-organized criticality, a recently introduced theoretical notion proposed as the source of erratic behavior in a variety of computer models and physical systems (SN: 7/15/89, p.40).
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Title Annotation:magnetization patterns
Author:Peterson, Ivars
Publication:Science News
Date:Mar 31, 1990
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