Data writing to storage devices based on ultrafast lasers

June 18, 2024

Modern life revolves around data, which means we need new, fast, and energy-efficient ways to read and write data on storage devices.


Optical methods that use laser pulses instead of magnets to write data have received considerable attention in the past decade, with the


development of all-optical switching (AOS) of magnetic materials. Although fast and energy-efficient, AOS has problems with precision.


Researchers at Eindhoven University of Technology in the Netherlands have invented a new method to precisely write data into a cobalt


gadolinium (Co/Gd) layer with laser pulses, using a ferromagnetic material as a reference. Their research was published in Nature



Magnetic materials in hard drives and other devices store data in the form of computer bits. Traditionally, data is read and written to hard drives


by moving a small magnet across the material. However, with the increasing demand for data production, consumption, access, and storage,


there is a considerable demand for faster and more energy-efficient methods of accessing, storing, and recording data.

All-optical switching (AOS) of magnetic materials is a promising approach in terms of speed and energy efficiency. All-optical switching uses


femtosecond laser pulses to change the direction of magnetic spins on a picosecond scale. Two mechanisms can be used to write data: multi-


pulse and single-pulse switching. In multi-pulse switching, the final direction of the spin is deterministic, which means it can be determined in


advance by the polarization of the light. However, this mechanism usually requires multiple lasers, which reduces the speed and efficiency of



On the other hand, single-pulse writing is much faster, but studies on single-pulse all-optical switching have shown that single-pulse switching


is a sliding process. This means that to change the state of a specific magnetic bit, prior knowledge of the bit is required. In other words, the


state of the bit must be read before it can be overwritten, which introduces a reading phase to the writing process, thus limiting the speed.

A better approach is the deterministic single-pulse all-optical switching method, in which the final direction of the bit depends only on the


process used to set and reset the bit. Now, researchers in the Nanostructure Group at the Department of Applied Physics at Eindhoven


University of Technology have developed a new method to achieve deterministic single-pulse writing in magnetic storage materials, making the


writing process more precise.

In their experiments, the TU Eindhoven researchers designed a writing system consisting of three layers: a ferromagnetic reference layer


made of cobalt and nickel, which helps or prevents spin switching in the free layer; a conductive copper (Cu) spacer or gap layer; and an


optically switchable Co/Gd-free layer. The composite layer thickness is less than 15 nm.

Once excited by a femtosecond laser, the reference layer demagnetizes in less than 1 picosecond. Some of the lost angular momentum


associated with the spins in the reference layer is then converted into a spin current carried by the electrons. The spins in the current


aligned with the directions of the spins in the reference layer.

This spin current then moves from the reference layer through the copper spacer (see white arrows in the figure) to the free layer, where it can


help or prevent spin switching in the free layer. This depends on the relative spin orientations of the reference and free layers.

Changing the laser energy results in two states. First, above a threshold, the final spin orientation in the free layer is completely determined by


the reference layer; second, above a higher threshold, switching is observed. The researchers have shown that these two mechanisms can be


used together to precisely write the spin state of the free layer without having to consider its initial state during the writing process. This


discovery provides an important advance in the expansion of our future data storage devices.