Spin ice is one of the few known substances that defies the third law of thermodynamics. However, scientists have found that the third law could be restored in a thin film of spin ice under certain circumstances.
Put simply, the third law of thermodynamics states that the entropy of perfectly crystalline substances at absolute zero temperatures would also be zero. However, a substance known as spin ice is known to be a peculiar exception to this law. Unlike normal substances, spin ice does not have a single minimal-energy state. Entropy can be considered as a measure of disorder or randomness. A substance with zero entropy will have atoms that will have zero randomness.
In order to reduce entropy to zero, it is necessary to cool a substance to the coldest possible temperature: absolute zero. In the case of spin ice however, residual entropy may still be detected even in absolute zero temperatures.
The study conducted by a team of researchers from Cambridge, Oxford and the London Centre for Nanotechnology showed that it is possible to create a type of spin ice that obeys the third law of thermodynamics when nearing absolute zero. The team published its findings in the online journal Nature Communications.
By creating very thin films of spin ice just a few nanometers thick, the team was able to observe that the residual entropy normally exhibited by spin ice disappeared at a temperature just half a degree above absolute zero.
"Restoration of the Third Law in spin ice thin films adds an unexpected twist to the story of spin ice. How the Third Law is first violated and then restored in spin ice is an interesting question of basic physics." said Steve Bramwell, a professor from the University College London's (UCL) Department of Physics and Astronomy.
The researchers were able to observe the peculiar effect using X-ray diffraction. The team found that the "substrate" used to create the thin films of spin ice played a role in the restoration of the third law.
"This result shows that we can use strain to drastically alter and control the spin ice state," said the study's lead author Laura Bovo, a post doctoral researcher from the London Centre for Nanotechnology. "It opens up new possibilities for the control and manipulation of magnetricity and magnetic monopoles in spin ice."
The team's findings has a number of potential applications. Scientists are currently exploring the potential of the discovery for practical application in magnetricity. Moreover, the discovery can also be used to develop new types of data storage devices in the future.
