Abstract:
By reshaping and narrowing an ultrafast laser beam, UC San Diego researchers achieved optical switching in a magnetic material composed of nine alternating layers of platinum and cobalt, breaking through the usual limit of material thickness of no more than three layers in previous experiments. The research results were published in "Nature Communications" on September 15.
The digital information in the hard disk is represented by a large number of tiny magnetic areas. Traditional writing methods usually rely on external magnetic fields to flip the state of the magnetic areas, which consumes high energy and limits the data writing speed. The researchers estimate that optical switching, which uses extremely short, focusable light pulses to change magnetic states, may be more than a thousand times faster than methods that rely on external magnetic fields, but has been limited in the past by the structure of the magnetic material and the polarization conditions of the light.
Previous experiments found that when the thickness of the magnetic material exceeds three layers, the optical switching effect will be suppressed, limiting the material thickness and the ability to preserve data for a long time; early plans also required that the light have a specific polarization, that is, the direction of the electric field must meet specific conditions. This time, the team achieved switching in the nine-layer platinum-cobalt structure by adjusting the shape, size and energy distribution of the beam. At the same time, it no longer requires a specific polarization, showing that engineering the beam has the potential to break through the limitations of materials and light.
The magnetization flip occurs gradually over multiple laser pulses. The first pulses concentrate enough heat into a tiny area to invert the magnetic state in that area; subsequent pulses gradually expand the inverted area until it stabilizes. Therefore, controlling where the light energy falls helps determine how magnetized regions form and expand. The research team said that the smaller the beam, the greater the opportunity to achieve smaller and denser optical storage in the same space in the future.
The research was completed by the optical team in collaboration with experts in thin-film magnetic materials. Abdoulaye Endao, senior author of the paper and a professor in the school's Department of Electrical and Computer Engineering, said that the team did not redesign the magnetic material, but redesigned the light itself to observe properties that had not been thought possible before. The first author, Mohammed Walid Khalid, said that the experimental results were novel and unusual, and the team spent a lot of time repeating the experiments to confirm that the findings were not accidental.
The team is currently working on optical structures that can confine light into smaller spaces, with the goal of shrinking the beam to hundreds of nanometers to explore how engineered light can control magnetism at a smaller scale. For use in commercial storage devices, the problem of how to practically generate and deliver the pulses also needs to be solved. The specialized ultrafast laser used in the experiment is currently difficult to integrate into a computer chip. Another possible route is to find magnetic materials that can produce similar effects under the action of lasers that are easier to integrate.
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