Refrigerate Me Magnetically

New findings could help nudge magnetic refrigerators closer to reality by pointing out ways to make them more energy efficient.

Magnets are often thought of as sticking on refrigerators, not behaving as them. But some compounds are magnetic refrigerants, because their atoms' magnetic spins grow orderly in a magnetic field. To counteract this reduction in disorder, the atoms begin to move more disorderly--meaning the refrigerant heats up. When the magnetic field is shut off, the refrigerant cools down, allowing it to absorb unwanted heat. This "magnetocaloric effect" can in principle lead to coolers more efficient than conventional fridges.

In 1997, Karl Gschneidner, a materials scientist at Ames Laboratory, and colleagues reported a giant magnetocaloric effect in the exotic compound gadolinium germanium silicide, making it a magnetic refrigerant that might work at room temperature. However, its spins aligned slowly when magnetic fields were applied or removed--that is, it showed large hysteresis--making it less efficient than it could be as a magnetic refrigerant.

In a report appearing in the June 24 issue of Nature, metallurgist-physicist Robert Shull and colleagues at the National Institute of Standards and Technology in Gaithersburg, Maryland, added a little iron to the compound, reducing hysteresis by more than 90%. Shull explained that the iron suppressed a phase transition--how the atoms rearranged themselves--in a magnetic field. This interference reduced the giant magnetocaloric effect, but also virtually eliminated the large hysteresis. The net effect gives the compound a 15% to 30% improved cooling capacity, Shull says.

Shull says there is certainly potential for commercial magnetic refrigerators in the future, but a crucial disadvantage of gadolinium germanium silicide is that it requires powerful magnetic fields. He hopes his team can devise new refrigerant composites that can operate in 1 to 2 teslas. "That's in the ballpark of feasible magnets." But not everyone agrees the findings are significant, or even new, says physicist Ekkes Bruck of the University of Amsterdam; he's not yet convinced of the improved efficiency.

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Robert Shull
Karl Gschneidner

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