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Data Writing Storage Device Based On Ultrafast Laser

Aug 07, 2020

Modern life revolves around data, which means we need new, fast, and energy-saving ways to read and write data on storage devices. With the development of magnetic material all-optical switching (AOS) technology, the optical method of using laser pulses instead of magnets to write data has received considerable attention in the past decade. Although fast and energy-efficient, AOS technology has problems with accuracy. Researchers at the Eindhoven University of Technology in the Netherlands have invented a new method that uses ferromagnetic materials as a reference to accurately write data into the cobalt-gadolinium (Co/Gd) layer with laser pulses. Their research was published in Nature Communications.

Magnetic materials in hard drives and other devices store data in the form of computer bits. Traditionally, data is read and written to the hard disk by moving a small magnet on the material. However, as the demand for data production, consumption, access and storage continues to increase, 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 method in terms of speed and energy efficiency. The all-optical switch uses femtosecond laser pulses to change the direction of the magnetic spin on the picosecond scale. Two mechanisms can be used to write data: multi-pulse and single-pulse toggle switches. In a multi-pulse switch, 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 writing.

On the other hand, the single-pulse write speed will be much faster, but the research on the single-pulse all-optical switch shows that the single-pulse switching is a sliding process. This means that to change the state of a specific magnetic bit requires prior knowledge of the bit. In other words, the state of the BIT must be read before it can be overwritten, which introduces a read phase to the writing process, thereby limiting the speed.

A better method is the deterministic single-pulse all-optical switching method, where the final direction of the bit depends only on the process used to set and reset the bit. At present, researchers in the Nanostructure Group of the Department of Applied Physics of 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.

 

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Picture source: Eindhoven University of Technology

In their experiment, researchers from Eindhoven University of Technology designed a writing system consisting of three layers-a ferromagnetic reference layer made of cobalt and nickel, which helps or prevents the free layer in the free layer. Rotary switch, a conductive copper (Cu) spacer layer or gap layer, and an optically switchable Co/Gd free layer. The thickness of the composite layer is less than 15 nm.

Once excited by the femtosecond laser, the reference layer is demagnetized in less than 1 picosecond. Some of the lost angular momentum associated with the spin in the reference layer is then converted into a spin current carried by the electron. The spins in the current are in the same direction as the spins in the reference layer.

This spin current then moves from the reference layer through the copper spacer layer (the white arrow 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 direction of the reference layer and the free layer.

Changing the laser energy will cause two states. First, above a threshold, the final spin direction in the free layer is completely determined by the reference layer; second, above a higher threshold, switching is observed. Researchers have shown that these two mechanisms can be used to accurately write the spin state of the free layer without considering its initial state during the writing process. This discovery provides an important development for our future expansion of data storage devices.