When water freezes to form ice, the water molecules, with their hydrogen and oxygen atoms, arrange themselves in a complex structure. Water and ice are different phases, and the transformation from water to ice is called a phase transition. In the laboratory, crystals can be produced in which the elementary magnetic moments, the so-called spins, form structures comparable to ice. That is why researchers also refer to these structures as spin ice.
“We have produced artificial spin ice, which essentially consists of nanomagnets that are so small that their orientation can only change as a result of temperature,” explains physicist Kevin Hofhuis, who has just completed his doctoral thesis at PSI.
In the material the researchers used, the nanomagnets are arranged in hexagonal structures – a pattern that is known from the Japanese art of basket weaving under the name kagome.
“Magnetic phase transitions had been theoretically predicted for artificial kagome spin ice, but they have never been observed before,” says Laura Heyderman, the head of the Laboratory for Multiscale Materials Experiments at PSI and a professor at the Mesoscopic Systems lab at ETH Zurich.
“The detection of phase transitions has only been made possible now thanks to the use of state-of-the-art lithography to produce the material in the PSI clean room as well as a special microscopy method at the Swiss Light Source SLS.” The journal
is now publishing the results of these experiments.
The trick: tiny magnetic bridges
For their samples, the researchers used a nickel-iron compound called permalloy, which was coated as a thin film on a silicon substrate. They used a lithography process to repeatedly form a small, hexagonal pattern of nanomagnets, with each nanomagnet being approximately half a micrometre (millionths of a metre) long and one-sixth of a micrometre wide.