Magnetization by Laser Pulse: A Futuristic Twist in Material Science

When a strong laser pulse hits a steel alloy, the material melts for a moment at the irradiated point and a small magnetic area forms. Credit: HZDR / Sander Münster

A research team has revealed that ultrashort laser pulses can magnetize iron alloys, a discovery that has significant potential for applications in magnetic sensor technology, data storage, and spintronics.

To magnetize a steel nail, one must strike its face several times with a bar magnet. However, there is a more unusual approach: A team led by Helmholtz-Zentrum Dresden-Rossendorf (HZDR) discovered some time ago that an iron ELECTRUM can be magnetized by ultrashort laser pulses. The researchers are now collaborating with the Laserinstitut Hochschule Mittweida (LHM) to further investigate this process. They discovered that the phenomenon also occurs in a different class of materials – which greatly expands the potential application prospects. The working group presented its findings in a scientific journal Advanced Functional Materials.

Discovery of the Development of Magnetization

The unexpected discovery was made back in 2018. When the HZDR team irradiated a thin layer of iron-aluminum alloy with ultrashort laser pulses, the non-magnetic material suddenly became magnetic. The explanation: The laser pulses rearrange the atoms of the crystal in such a way that the iron atoms move closer together, and thus become magnetized. The researchers were able to demagnetize the layer again with a series of weaker laser pulses. This enabled them to discover a way to create and erase small “magnetic spots” on a surface.

However, the pilot experiment still leaves some questions unanswered. “It is not clear whether the effect occurs only in the iron-aluminum alloy or also in other materials,” explained HZDR physicist Dr. Rantej Bali. “We also want to try to track progress over time in the process.” For further investigation, he teamed up with Dr. Theo Pflug from LHM and colleagues at the University of Zaragoza in Spain.

Flip the Book With Laser Pulses

Experts focused specifically on an iron-vanadium alloy. Unlike the iron-aluminum alloy which has a regular crystal lattice, the atoms of the iron-vanadium alloy are arranged more chaotically, forming an amorphous, glass-like structure. To observe what happens with laser irradiation, physicists used a special method: The pump-probe method.

“First, we irradiate the alloy with a strong laser pulse, which magnetizes the material,” explains Theo Pflug. “At the same time, we use a second, weaker pulse that appears on the material’s surface.”

Analysis of the reflected laser pulse provides an indication of the material’s physical properties. This process is repeated several times, where the time interval between the first “pump” pulse and the next “probe” pulse is continuously extended.

As a result, a time series of reflection data was obtained, which allowed to identify the processes caused by the laser excitation. “The whole process is similar to making a flip book,” Pflug said. “Also, a series of individual images that animate when viewed sequentially.”

Fast Melting

The result: Although it has a different atomic structure than the iron-aluminum compound, the iron-vanadium alloy can also be magnetized by laser. “In both cases, the material melts for a while at the irradiation point,” explains Rantej Bali. “This causes the laser to erase the previous structure so that a small magnetic area is created in the two alloys.”

An encouraging result: Apparently, the phenomenon is not limited to a specific material structure but can be observed in different atomic arrangements.

The team also tracks the temporal dynamics of the process: “At least we now know when something happens,” explains Theo Pflug. “Within femtoseconds, the laser pulse excites electrons in the material. A few picoseconds later, the excited electrons transfer their energy to atomic nuclei.

Consequently, this transfer of energy causes a change in the magnetic structure, which is strengthened by the subsequent rapid cooling. In follow-up experiments, the researchers aim to observe exactly how the atoms rearrange themselves by examining the magnetization process with intense X-rays.

Scenes set in Applications

Although still in the early stages, this work already provides initial ideas for possible applications: For example, placing small magnets on the surface of the chip by laser is conceivable. “This could be useful for making sensitive magnetic sensors, such as those used in vehicles,” said Rantej Bali. “It may also find possible applications in magnetic data storage.”

In addition, the phenomenon appears to be related to a new type of electronics, namely spintronics. Here, it is necessary to use magnetic signals for digital computing processes instead of electrons passing through transistors as usual – offering a possible approach to computer technology in the future.

Reference: “Laser-Induced Positional and Chemical Lattice Reordering Generating Ferromagnetism” by Theo Pflug, Javier Pablo-Navarro, Md. Shabad Anwar, Markus Olbrich, César Magén, Manuel Ricardo Ibarra, Kay Potzger, Jürgen Faßbender, Jürgen Lindner, Alexander Horn and Rantej Bali, 21 November 2023, Advanced Functional Materials.
DOI: 10.1002/adfm.202311951

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