Researchers have for the first time used lasers to continuously tune 2D materials’ magnetic properties, enabling new data storage and processing applications.
(Nanowerk News) Sometimes using conventional tools in a novel way produces astounding results. That’s what happened when researchers used the high-tech laser equipment in PSI’s cleanroom for something it was not intended to do. It was originally purchased for photolithography – a process for producing tiny 2D structures.
Normally, the laser irradiates a photoresist with light of different intensities, thus creating different exposure levels. This so-called grayscale lithography produces a three-dimensional relief that can then be transferred to the desired material. One important application area for this technology is modern microoptics; it can be used, for example, to fabricate lenses for smartphones.
“We’re using this tool for something other than its original purpose,” explains Aleš Hrabec: “We are using it to create two-dimensional continuous changes in magnetic properties in materials that are important for a variety of applications.” Hrabec is a scientist in the Mesoscopic Systems research group, which is led by Laura Heyderman and belongs to both PSI and ETH Zurich. What the researchers mean by the term mesoscopic is systems with dimensions on the scale of a few micrometres.
A crazy idea that works
If you want to change the properties of a magnetic material, you can, for example, heat it in an oven. But this changes the entire sample. In search of a method for locally constrained changes, the PSI researchers came up with the idea of putting a thin film of a magnetic material, without photoresist, into the existing lithography device.
“It was a crazy idea, so I was very surprised that it worked right away,” says Lauren Riddiford, a postdoc in the Mesoscopic Systems group: “When we looked at the magnetic contrast under a special microscope, we could immediately see the continuous changes in the magnetic properties.”
The laser essentially acts like an oven, but its effect changes the magnetic properties with pinpoint accuracy. The laser is used to scan the surface of the material sample, modulating the light intensity as desired. This heats very small areas only 150 nanometres in size. The process is called direct-write laser annealing, or DWLA for short. Such targeted heating causes local changes in a material – it oxidises, crystallises, or alloys two metals. This can change the strength or direction of the magnetisation and influence interactions at the interface between two materials.
The local, gradual approach has a unique ability to create gradients of magnetic properties that can take on arbitrary shapes. Until now, it was only possible to produce lateral, one-dimensional gradients of such material properties. Now circles, spirals, or even more complex shapes are possible, as Riddiford demonstrates in a video showing the creation of a magnetic structure in the shape of a snowflake.
“When we apply a field to the processed sample, the magnetisation in the center first changes direction from upwards to downwards. When the field becomes stronger, this switching spreads radially,” the researcher explains. In the areas around the snowflake, the material was heated sufficiently with the laser to ensure that it is no longer magnetic.
From laser light to snowflake: This magnetic structure was created at PSI using an industrial laser – not for decoration, but as a demonstration of precise material modification with potential for data storage, AI, and photonics. (Image: Paul Scherrer Institute PSI / Aleš Hrabec, Lauren Riddiford, Jeffrey Brock)
The researchers’ aim is not to make pretty pictures but rather to enable concrete applications, for example in data storage technology. Small magnets have long been used to save data on computer hard drives. Depending on which direction the pole of a magnet is pointing, this corresponds to a one or a zero, that is, the value of a bit. Above the rotating hard drive is a coil that reads and writes information using a magnetic field.
“With our technique, we want to find out which magnetic materials and properties are best suited to producing storage devices that no longer have moving parts and do not require the use of magnetic fields,” says Jeffrey Brock, another postdoc in the Mesoscopic Systems group.
Because of the continuous changes in the magnetic properties in the storage medium, no magnetic field is needed to change the magnetisation of the bits. An electric current can be used to write and read the information. Such storage elements already exist.
“However, we believe that our approach to locally altering material properties is much simpler and faster than the technologies currently used to create such patterns,” Brock says.
Data storage devices switched with electricity are faster, and more data can be stored in a smaller space. The researchers also want to apply this to a special class of materials called synthetic antiferromagnets. This would make data storage more permanent and secure, as this material is immune to an external magnetic field.
Computing and storage on the same chip
One other possible application is so-called in-memory computing – in which data processing and storage take place on the same chip. In today’s electronic devices, data is constantly transported back and forth between the fast processor and the much slower storage unit, which costs a lot of time and energy. Using a single chip would enable an extreme increase in speed.
Four years ago, a research collaboration between PSI and ETH Zurich succeeded for the first time in carrying out logical operations in a magnetic material in which simultaneous data storage is also possible – an invention that was patented. But the material used to date is not suitable for the manufacturing processes that today’s semiconductor industry relies on.
“We’re hoping we can use the laser technique to produce a magnetic material that is compatible with these standard processes,” Hrabec says.
Another new research field of interest is so-called neuromorphic computing – an approach to data processing inspired by the brain and the network of nerve cells, that is, neurons. Here, for example, tiny magnets in different configurations are intended to interact with each other like neurons in biological networks.
“But the brain does not consist of one simple material,” Hrabec says. “Therefore you can’t just use a thin layer of a single magnetic material such as cobalt for this purpose; you need something more complex.” An ideal task for the new laser technology, which can be used to create arbitrary magnetic landscapes.
Hrabec is convinced that the research team’s work will open up many other applications, for example in the fields of sensor technology and photonics, in which light is used to transmit information. Laser heating and crystallisation in a material can change its refractive index and thus its optical properties. The big advantage of laser annealing is that the equipment used is a commercial device that is already available in many laboratories around the world. It does not require a vacuum or other special conditions.
Also, it can achieve in a matter of seconds what would take hours in an oven. Hrabec sums it up in this way: “The great strength of this technology is that it is relatively inexpensive, very fast, and easily available.”
