When water freezes to form ice, the water molecules with their hydrogen and oxygen atoms organize themselves into a complex structure. Water and ice are different phases, and the change from water to ice is called a phase transition. In the laboratory, crystals can be produced in which the primary magnetic moment, the so-called spin, creates structures comparable to those of ice. That’s why researchers also call these structures spin ices.
“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 just completed his doctoral thesis in PSI. is completed.
In the material used by the researchers, the nanomagnets are arranged in hexagonal structures – a pattern known from the Japanese art of basket weaving known as kagome.
“Magnetic phase transitions were theoretically predicted for artificial Kagome spin ice, but they have never been observed before,” said Laura Hedermann, Head of the Laboratory for Multiscale Materials Experiments at PSI and Professor in the Mesoscopic Systems Lab at ETH Zurich. it is said.
“Phase transition detection is now only made possible thanks to the use of state-of-the-art lithography to produce the material in a PSI clean room, as well as a specialized microscopy method at the Swiss Light Source SLS.” Magazine nature physics Now publishing the results of these experiments.
The trick: Small magnetic bridge
For their samples, the researchers used a nickel-iron compound called permaloy, which was coated as a thin film on a silicon substrate. They used a lithography process to repeatedly create a tiny, hexagonal pattern of nanomagnets, with each nanomagnet about half a micrometer (one-tenth of a meter) long and one-sixth of a micrometer wide.
but that’s not all. “The trick was that we connected the nanomagnets with tiny magnetic bridges,” Hofhuis says. “This caused small changes in the system to make it possible for us to tune the phase transition in such a way that we could observe it. However, these bridges had to be really small, as we did not want to change the system too much.”
The physicist is still baffled that the undertaking was actually successful. With the creation of nano bridges, he was pushing against the limitations of the technically possible spatial resolution of today’s lithography methods. Some bridges are only ten nanometers (billionths of a meter) across.
Microscopy and Principles
At SLS’s SIM beamline, the team used a special method called photoemission electron microscopy that made it possible to observe the magnetic state of each individual nanomagnet in the array.
“We were able to record a video that shows how the nanomagnets interact with each other as we change the temperature,” summarizes Hofhuis. The original images only have black and white contrast which changes from time to time. From this, the researchers were able to extract the configuration of the spins, that is, the alignment of the magnetic moments.
“If you watch a video like this, you don’t know what stage you’re in,” explains Hofhuis. This demanded theoretical consideration, which was contributed by Peter Derlett, PSI physicist and assistant professor at ETH Zurich. Their simulations showed what should theoretically happen in the phase transition. Comparison of only the recorded images with these simulations proved that the processes observed under the microscope are indeed phase transitions.
phase transition manipulation
The new study is another achievement in the investigation of artificial spin ice that Laura Hederman’s group has been doing for more than a decade. “The great thing about these materials is that we can tailor them and directly observe what’s going on inside them,” says the physicist.
“We can observe all kinds of attractive behavior, including phase transitions and orders that depend on the layout of the nanomagnet. This is not possible with spin systems in conventional crystals.” Although these investigations are pure fundamental research at the moment, researchers are already thinking of potential applications.
It may be interesting to control the different magnetic phases for different types of data processing. Researchers at PSI and elsewhere are investigating how the complexity of artificial spin ice could be harnessed for novel high-speed computers with low power consumption. “The process is based on information processing in the brain and takes advantage of how artificial spin ice responds to stimuli such as magnetic fields or electric current,” Hederman explains.
Source: ETH Zurich