By simulating the movements of microscopic bar magnets, a condensed matter physicist at the University of Kentucky is shedding some new light on the nature of a fundamental physical process.

The phrase "loss of order" brings to mind images of a momentous nature. Floods uproot enormous trees. Tornadoes lay waste to entire towns. Fires spectacularly blacken hundreds of acres of dry forest. Disorder goes hand-in-hand with enormous, catastrophic change.

Similarly, it has always been assumed that, on a microscopic level, dramatic change goes hand-in-hand with the disordering of matter--what condensed matter physicists call a "phase transition." During a phase transition, matter, in changing from one state into another, loses its structure. Take, for example, the case of water: after it reaches a temperature of 210 degrees Fahrenheit, it can only be converted to steam, regardless of how high the temperature is raised.

But when and where does this disordering start? Previously, it was believed that a phase transition was always marked by dramatic behavior, such as the transformation of water into steam. In the language of phase transitions, these are called "singularities," and the standard theory assumes that a singularity always marks a change in phase. But Herb Fertig, a condensed matter physicist at the University of Kentucky, argues that researchers may have been, in a sense, looking for a sign where there is not always one to be found.

Using the University of Kentucky's Hewlett-Packard Superdome cluster, an Alliance supercomputing resource, Fertig studies the behavior of vortices in thin film ferromagnets: tiny whirlpools within a layer of magnetic material so flat that it is nearly two-dimensional. These systems are very analogous to many other systems--including superfluids, superconducting materials, and thin crystalline solids, all of which have measurable properties determined by the state of the vortices or similar objects. Thus, the study provides an interesting window into how many other systems work.

The vortices are minuscule. Contrary to the conventional wisdom of condensed matter physics, however, their phase transitions may be nearly imperceptible.

Figure 1. Think of a magnet as a crystal of microscopically small bar magnets. The small magnets are the electrons in the atoms of the materials. Each bar magnet has a north pole (shown in red) and a south pole (blue). When these bar magnets align along a common direction--which happens for ferromagnetic materials when not too hot--the magnetization may be observed macroscopically. The tendency to align has to do with the way neighboring atoms interact and occurs only for certain materials, such as the iron atoms in a refrigerator magnet.

Access Online | Posted 10-23-2003