Restoration of Third Law of Thermodynamics in Spin Ice Films Unexpected, Researchers Say

The first thin films of spin ice. The orange colouration is a spin ice film of only a few billionths of a meter thickness.Scientists say that contrary to popular belief, the third law of thermodynamics can be restored in thin film of spin ice.

Spin ice were seen as an exception to the Third Law of Thermodynamics, which states that “the entropy of a perfect crystal is zero when the temperature of the crystal is equal to absolute zero (0 K or minus 273 degrees centigrade.”

The study on spin ice (a type of crystal with ions placed in a tetrahedral shape) was conducted by researchers at the London Centre for Nanotechnology (LCN) in collaboration with scientists from Oxford and Cambridge.

“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,” Prof. Steve Bramwell (UCL Department of Physics and Astronomy), one of the study authors said in a news release.

The study is not only important for theoretical physicists trying to find the “Holy Grail” of physics- the monopole magnet- but also could help other scientists develop technologies that could harness magnetricity, which is the magnetic equivalent of electricity.

What are magnetic monopoles?

Magnetic monopoles- as the name suggests are magnets with a single north or south pole. The idea might seem absurd at first; a common observation is that every magnet has two poles (North and South- cut the magnet in two and you are left with two magnets, each with its own two poles).

However, physicists have long suspected that there are magnets with single poles. Scientists have tried finding this elusive magnet in Antarctic rocks and even moon-dust, but  to no avail. Here’s where “spin ice” comes in to picture.

Spin ice with a twist

The idea of magnetic monopoles has been around since 1930s. In 2008, theoretical physicists tried to explore special crystals on earth to try and find structures that could resemble the rare magnetic monopoles.

And they did. Spin ice has ions in a tetrahedral shape and several tetrahedral clusters of ions form a crystal called pyrochlore. Research has shown that temperature alteration could change the spin of the ions in such a way that some of them behave like magnetic monopole. Note that “spin” refers to a quantum mechanical concept.

A video at the end of the article describes spin ice in detail.

Romancing Randomness

The “spins,” which give the spin ice properties of monopole- magnets, also contribute to the randomness of the crystal.

In the present study, researchers fabricated spin ice films that were a few nanometers thick. At temperature that was half a degree over absolute zero, the entropy (degree of randomness) of the spin ice disappeared, meaning that the Third Law of thermodynamics was restored.

X-ray diffraction showed that the substrate on which the film is grown, strains the spin ice. This strain leads to the loss of entropy.

Applications

Jonathan Morris’s article, which was published in IEEE spectrum in 2013, describes magnetic monopoles and spin ice in detail. According to Morris, magnetic field could be used to harness magnetricity, which is the magnetic equivalent of electricity.

A related study had shown that “magnetic charge” in spin ice (Dy2Ti2O7) behaves like electrical charge.

“This result shows that we can use strain to drastically alter and control the spin ice state” said Dr. Laura Bovo (London Centre for Nanotechnology) the leading author of the team’s paper. “It opens up new possibilities for the control and manipulation of magnetricity and magnetic monopoles in spin ice.”

Another tantalizing idea is that scientists could alter spin ice state using strain to create memory storage devices that have several times more capacity than current ones.

The study is published in the journal Nature Communications.

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