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Study Sheds Light on Mechanisms Involved in Optical Manipulation of Magnetization

When it comes to studying the underlying issues in magnetism, only a few nanometer-thin films of magnetic materials serve as perfect test objects.

Simple picture of the electron reservoirs of magnetic atoms in a CoPt-alloy.
Simple picture of the electron reservoirs of magnetic atoms in a CoPt-alloy. In Cobalt (Co), the number of spin-down electrons (red) around the Co atoms is significantly lower than that of the spin-up electrons (blue). Consequently, the available space to take up further spin-down electrons is larger. Triggered by the optical excitation, spin-down electrons can be transferred from the Platinum (Pt) to the Co sites (OISTR process), which fills the respective reservoir and leads to demagnetization in Co. At the Pt atoms, because of the high spin-orbit coupling strength, efficient spin-flips can be observed already in the first 10–100 fs after optical excitation, quickly equilibrating the number of spin-down and spin-up electrons. Image Credit: MBI

Moreover, these thin magnetic films have significant technological applications. For instance, they are utilized in magnetic mass data storage devices, such as the magnetic hard drives that are employed in cloud data storage centers.

In the existing technology, magnetic fields are used to manipulate the magnetization in these thin films, but this magnetization can also be influenced using laser pulses.

It was noticed that the magnetization below the laser spot varies when exposed to very short light pulses of just a few tens of femtosecond duration (1 fs is equal to one-millionth of one-billionth of a second).

In basic systems, such a change usually equates to a simple decrease in the magnitude of magnetization. But in more intricate material systems, the magnetization can also be permanently reversed by the light pulse. In such situations, an all-optical magnetization switching, which has an evident scope for application, may offer a solution.

This switching process has excellent speed which is yet to be understood. Due to this reason, researchers worldwide are exploring the microscopic processes underlying “femtomagnetism.”

Now, by integrating theoretical and experimental work, scientists at the Max Planck Institute for Microstructure Physics in Halle and the Max Born Institute in Berlin have currently observed a novel microscopic process “at work,” which was forecast only in the recent past.

Referred to as optical intersite spin transport (OISTR), the process can take place when different types of suitable atoms lie adjacent in a solid. Under appropriate conditions, a pulse of light causes the electrons to move from one atom to its neighbor. Most significantly, this occurs mainly with electrons of a specific spin orientation and hence impacts the local magnetization.

Such a process occurs at the time of optical excitation and does not rely on secondary mechanisms. Hence, it is the fastest process ever imagined and causes a light-induced variation in magnetism.

An atom in a magnetized solid can be visualized as having individual reservoirs of spin-down electrons and spin-up electrons, which are filled to a varying extent. For a Platinum (Pt) atom and a Cobalt (Co) atom, which are both neighbors of one another in a CoPt alloy. This has been outlined in the above image.

The variation in the number of spin-down and spin-up electrons (indicated in blue and red) ascertains the amount of atom’s magnetization. Reduced magnetization means the number of the two spin types needs to equalize.

Spin-flip is a popular process to balance both reservoirs at a single atom. In this process, for example, a spin-down electron changes into a spin-up electron, indicated by a leap from the blue bucket into the red one.

Such spin-flips mainly take place at heavy atoms such as Pt, in which the spin is specifically susceptible to the electron’s movement; physicists talk about a huge spin-orbit coupling. In this spin-flip process, the angular momentum produced is absorbed by the whole array of atoms present in the solid.

In the current research, published in the Nature Communications journal, the scientists have analyzed two model systems—a CoPt alloy and a pure Co layer. The team then tracked the absorption of ultra-short pulses of soft X-rays with regulated polarization and wavelength after the excitation of a laser pulse and made a comparison between their experimental results and theoretical calculations.

In this fashion, the variations in the numbers of electrons with spin-down and spin-up activated by the first laser pulse can be separately analyzed for the Pt and Co atoms.

When the basic system comprising exclusively Co atoms was compared with the alloy comprising both Pt and Co atoms, marked differences were observed in the absorption behavior. These differences are separately predicted by the theoretical calculations.

These variations emerge because in the CoPt alloy, another process can occur where electrons are moved between the different types of adjacent atoms.

Electrons inside the solid are shifted from the Pt atoms to the Co atoms because of the laser pulse. It was observed that these are preferentially spin-down electrons because several vacant states for spin-down electrons exist at the receiving Co site.

Thus, the transferred electrons at the Co atom boost the level of the spin-down electrons, rendering them more analogous to the spin-up reservoir and thus decreasing the Co atom’s magnetic moment. This OISTR process that occurs between Co and Pt is accompanied by a balancing of the electron reservoirs locally at the Pt atoms through spin flips.

This spin-flip efficiently occurs at the heavy Pt atoms displaying large spin-orbit-coupling and only to a relatively minor degree at the lighter Co atoms.

The study’s comprehensive results demonstrate that the potential to optically exploit magnetization through optical intersite spin transport mainly relies on the available states for spin-down electrons and spin-up electrons of the atoms involved.

It is possible to tune these states by bringing the correct types of atoms together in new materials. Thus, the interpretation of the tiny mechanisms involved in the optical exploitation of the magnetization paves the way to a practical design of novel functional magnetic materials, enabling ultrafast control of magnetization through laser pulses.

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