Laser Reveals Surprising Optical Effect on Ferromagnets

Investigations of magnetism using a free-electron laser unveiled a new mechanism that causes ferromagnetic materials to demagnetize. The finding — previously theorized, but never before observed &dmash; could play a role in reducing the size of magnetic memories for faster storage.

For 20 years scientists have studied the phenomenon that ferromagnets can be demagnetized extremely quickly when they are irradiated with laser light pulses. In a few hundred femtoseconds, magnetism breaks down, and the ferromagnet loses its magnetic properties. After a while, magnetization builds up again.

When investigating the process with DESY’s x-ray laser FLASH, scientists from the Technical University of Berlin, the universities of Hamburg and Paris, Helmholtz-Zentrum Berlin, DESY and six additional research centers detected another mechanism that causes demagnetization: when the material is divided into magnetic domains—a common occurrence.

The picture of a 10 x 10-µm sample made with a magnetic force microscope shows the labyrinth-like structure of the magnetic domains. Courtesy of Bastian Pfau.

“When bombarded with laser light, released electrons that speed through the material will break through the domain walls, thereby virtually changing sides,” said Bastian Pfau, a junior scientist at TU Berlin. “This way, they go from one domain into another domain with a different magnetic polarization, causing the destruction of the local magnetization. Materials with nanometer-sized domains can this way develop another possibility of demagnetization as soon as electrons become more mobile through the laser bombardment.”

Such an interaction between two magnetic domains was previously theorized, but was never before observed.

The prerequisite for this type of demagnetization is that the basic material be divided into domains. Within these areas, the magnetization has the same direction, but there is a random magnetization of these miniature areas toward each other. When the bar is magnetized by an outside magnetic field, the magnetic field of the individual domains aligns in parallel, and the whole bar becomes magnetic.

The scientists investigated samples of a cobalt-platinum layer system containing tiny labyrinth-like magnetic domains. They induced demagnetization of the sample with an extremely short infrared laser pulse, and after 100 fs, were able to simultaneously measure the smallest modifications in size and properties of the sample with FLASH.

With the irradiation of a laser pulse, the excited electrons (violet and pink with different spin orientations) will change to a different magnetization (green-red), contributing to the demagnetization of the material. Also, electron-electron collisions increase the number of the so-called “hot” electrons. Courtesy of DESY.

The initially sharp walls between the different domains in the investigation became blurry shortly after the infrared laser pulse — the limits became wider because the electrons broke through the domain walls. Shortly after pumping, this became visible when the FLASH accelerator’s probe x-ray pulses hitting the sample met a weaker deflection of their flight direction than before.

“Our research shows that location and density of magnetic domain walls may influence the demagnetization behavior,” said Dr. Stefan Eisebitt of TU Berlin. “This provides a new approach for the future to develop faster and smaller magnetic memories with a specific arrangement of magnetic nanostructures.”

The research was published in Nature Communications (doi: 10.1038/ncomms2108).

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