- Researchers have discovered an interesting way to create and remove magnets in an alloy of iron and aluminum.
- They’ve used laser pulses of 100 femtoseconds to switch the ferromagnetic ordering reversibly.
- This could have a wide range of application, from optical technology to material processing.
Researchers at the HZB, HZDR and the University of Virginia, have found an astonishing effect – using a laser beam to write and delete magnets in an alloy. Since the process is reversible, it could have a wide range of applications in different fields, including optical technology, material processing and data storage.
Instead of reversibly manipulating the ferromagnetism, most of the techniques used today optically switch the direction of magnetization. In this study, researchers have used laser pulses of 100 femtoseconds (10−15 seconds) to switch the ferromagnetic ordering reversibly in a unique material.
The Special Alloy
They used an alloy made of iron and aluminum – Fe(60)Al(40). The subtle alterations of its atomic arrangement can fully change its magnetic behavior.
How exactly, you asked? Well, the alloy has an ordered structure where layers of aluminum atoms are separated by iron atomic layers. A strong laser beam can destroy this order, forcing the iron atoms to come closer, which makes the overall material behave like a magnet.
The team has made a thin sheet of this alloy on the top of transparent magnesia. When they passed a beam of laser through the magnesia, a ferromagnetic region was formed. The next laser pulses (with less intensity) — shot at the same region — deleted the effect of magnet formed in the alloy.
Laser light writing (left) and erasing (right) information
Nearly half of the previous magnetization level was preserved with one laser pulse (at lower intensity). The magnetization completely vanished with a series of laser pulses.
How This Actually Happens?
Although ion-induced reordering is feasible in some disordered alloys, researchers haven’t demonstrated reversible order-disorder switching. They have used laser pulses to induce amorphous-crystalline transitions through quick quenching, in order to control surface reflectivity of chalcogenide systems reversibly.
According to simulations, when short laser pulses heat up the thin-sheet alloy to its melting level, the ferromagnetic state is formed. Gradually, it cools down and while doing so, it enters a ‘supercooled liquid’ state. Although the temperature drops below melting point, the alloy remains molten.
Lack of nucleation sites is the major reason why the alloy achieves this supercooled liquid state. By nucleation sites, we mean, microscopic regions where atoms could start arranging themselves into a lattice.
Reference: ACS Appl. Mater. Interfaces | doi:10.1021/acsami.8b01190 | HZDR
The temperature-drop continues as the atoms wander around in quest for nucleation sites. Eventually, atoms in the supercooled liquid state produce a solid lattice, where aluminum and iron atoms end up positioning themselves at random locations within the lattice. The overall process takes a very small amount of time (in nanoseconds) and the atoms’ random arrangement forms a magnet.
Then the same beam with less intensity is used to rearrange the atoms as they were before. It only melts the sheet’s thin layers, producing a molten pool that sits on the solid material.
The moment temperature falls below the melting point (within one nanosecond), the solid portion in the alloy sheet beings to reproduce, and atoms quickly rearrange from shaggy structure to solid lattice. When the temperature is still high and lattice is already formed, the atoms have enough energy to diffuse through the lattice and divide into layers of aluminum and iron.
In this experiment, the process of repeatable-laser pulses have been applied up to 10 times at the same region without damaging the alloy sheet. Moreover, repeatability could be limited by the contamination during the laser irradiation process or by the material ablation.
Researchers plan to investigate the same technique in other alloys. They want to study the combined impact of different laser beams. To do this, they may use interference effects to create specific magnetic materials over wide regions.
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This will improve our knowledge of femtosecond laser-induced quick heating and cooling processes in special alloys, in particular, the kinetics and mechanisms of the order-disorder transition which are still not well-explored.
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