Third Law Of Thermodynamics Discovered In Spin Ice Films

As new materials with novel properties are generated by scientists of diverse stripes, engineering research gains new avenues for development. Researchers from Oxford, Cambridge and the London Centre for Nanotechnology recently created spin ice that exhibits decreasing entropy as the temperature of the material approaches absolute zero, according to a news release.

Spin ice film loses entropy
Spin ice typically has random magnetic moments as it is cooled to temperatures close to absolute zero, distinguishing it from most other substances. However, the team of English scientists found a way to make spin ice that shows entropic behavior comparable to normal materials. In other words, the new material abides by the Third Law of Thermodynamics, which stipulates that zero entropy persists at absolute zero.

"Restoration of the Third Law in spin ice thin films adds an unexpected twist to the story of spin ice," said Steve Bramwell, professor in the University College London Department of Physics and Astronomy. "How the Third Law is first violated and then restored in spin ice is an interesting question of basic physics."

The researchers achieved these results by growing thin ice of exceptional thinness, allowing the substrate to act upon the magnetic properties of the substance. When the nanometers-thick film of spin ice was cooled to half a degree above absolute zero, the team found the entropy of the material vanished.

"This result shows that we can use strain to drastically alter and control the spin ice state," said Laura Bovo of the London Centre for Nanotechnology, leading author of the study. "It opens up new possibilities for the control and manipulation of magnetricity and magnetic monopoles in spin ice."

Restoring the law
Standard spin ice exhibits distinguishing properties, such as magnetricity, due to its seeming transgression of the Third Law of Thermodynamics, the authors explained in an abstract of the study. The spin ice developed by the team loses these characteristics when its entropy drops at extremely low temperatures, though it retains them at temperatures above 2 K.

Using a non-reactive substrate for a foundation, the scientists produced epitaxial films of Dy2Ti2O7 with thicknesses ranging from 5 to 60 monolayers. The reestablishment of Third Law behavior in the spin ice jibed with a predictable, if unusual, strain-prompted arrangement.

This discovery opens the possibility of adjusting the magnetic properties of spin ice using variable temperatures, an option previously unavailable. In time, engineering research and development may find useful applications for this new material.