Light Polarization Is the Key to Revolutionizing Magnetism: Ultrafast Data Storage and Spintronics

New research introduces a none-thermal route to magnetization using circularly polarized XUV light, which by the Inverse Faraday Effect gives rise to giant magnetization changes that can eventually turn ultrafast data storage and spintronics upside down.

A team of scientists from the Max Born Institute, in collaboration with an international team, demonstrated a most efficient and novel non-thermal path toward generating large magnetization changes. Simple exposure of a ferrimagnetic iron-gadolinium alloy to circularly polarized pulses of extreme ultraviolet radiation was used, allowing for the revelation of an exceptionally strong magnetic response dependent on the handedness of the incoming XUV light burst, left-or right-circular polarization.

The underlying mechanism is based on the inverse Faraday effect, which does not rely on the absorption of the light but provides an efficient interaction between its polarization and the magnetic moments in the material.

Controlling Magnetism With Polarized Light
When an intense laser pulse hits a magnetized medium, the associated modification of magnetization can, in most cases, be referred to as proportional to the energy introduced into the material upon absorption. It provides a microscopic consideration of the optical excitation of electrons that are placed in a nonequilibrium state very fast, and thus start scattering with each other and other quasiparticles, modifying electron spin and orbital moments and eventually the long-range magnetization.

While these mechanisms give rise to a lot of interesting phenomena, such as ultrafast demagnetization and switching of magnetization by laser, they have the drawback of strong heating of the material and, therefore, strongly limit the technological applicability with respect to fast repetition rates that are, e.g., necessary for read/write operations in future data storage technologies.

Magnetization Dynamics Induced by Femtosecond XUV Pulses
Figure 1.: Magnetization dynamics after excitation with identical, circularly polarized () and linearly polarized femtosecond XUV pulses tuned to the Fe M3,2 resonance of FeGd and for two different fluences. For each, a corresponding is shown. The IFE–induced difference of the respective demagnetization amplitudes is denoted by. Credit: MBI/ M. Hennecke

The Inverse Faraday Effect and Opto–Magnetic Phenomena
Now an international team of researchers around scientists from MBI investigated a completely different non-thermal pathway to manipulate magnetism by light. Their approach relies on such an opto-magnetic phenomenon; however, it does not rely on electronic heating caused by the absorption of light or any other indirect effects but rather is connected with a direct, coherent interaction between the light’s polarization and electronic spins.

In the course of optical excitation by the IFE, it generates magnetic moments under the influence of circularly polarized radiation in a medium. Such directed magnetization in the case of radiation will depend on the helicity of the circular polarization: left-handed or right-handed. However, due to the fact that metallic and highly absorptive properties of most ferro- and antiferromagnetic materials typically suppress the above-mentioned non-thermal effects, another special technique had to be developed to generate sizeable opto-magnetic response.

The scientists could demonstrate the generation of particularly strong IFE-induced magnetization in a metal, ferrimagnetic iron-gadolinium FeGd alloy, only using circularly polarized femtosecond pulses of extreme ultraviolet radiation generated at the free-electron laser FERMI. Due to the high photon energy of XUV radiation, it became possible to create resonant excitation of tightly bound core-level electrons. Because of their intrinsic properties, in particular, a strong spin-orbit coupling, these electrons can facilitate the generation of large opto-magnetic effects.

Comparison of the largest experimentally observed helicity-dependent effects.
Figure 2: Comparison of the largest experimentally observed helicity-dependent effects ΔMexp (yellow diamonds, left scale) with the computed IFE response ΔIFE (turquoise line, right scale) as a function of XUV photon energy. ΔMsim (red diamonds, left scale) illustrates the expected contribution of the XMCD (blue line) to the magnetization dynamics, which turns out to be too small to explain the observed effects. Credit: MBI / M. Hennecke

Shows Big Magnetization Changes Realized Using XUV
Using this approach, the authors show that IFE-induced magnetization at various XUV photon energies around the Fe M3,2 core-level resonance can amount to 20-30% of the ground-state magnetization of the alloy, as measured by the difference between ultrafast demagnetization induced for opposite helicities of the circularly polarized XUV pulses (Figure 1).

This could be further confirmed by ab initio theory and spin dynamics simulations since the effects observed are consistent with the expected IFE response, as shown in Figure 2, while they cannot be explained by purely thermal helicity-dependent mechanisms—for instance, by the very well-established x-ray magnetic circular dichroism.

These findings thus provide an efficient route for the generation of large magnetization on ultrafast time scales without thermal means, and therefore will be of high relevance for the fields of ultrafast magnetism and spintronics, coherent magnetization control, and the science of nonlinear x-ray matter interactions.

Potential Applications in Spintronics

This nonthermal magnetization method holds a huge potential for applications in the field of spintronics. Spintronics is one of the rapidly developing fields related to the use of electron spin for information processing and storage. In view of this, it requires efficient ways for the manipulation and control of magnetization at ultrafast timescales. The presently discovered nonthermal, circularly polarized XUV light-induced magnetization change can open completely new routes toward next-generation spin-based devices. These could work much more efficiently and at lower energies than the current technologies, taking computing and data storage capabilities a step further.

Advantages Over Conventional Techniques

Compared with conventional techniques exploiting thermal effects of the intense laser pulses, non-thermal ways opened by the XUV light have a number of crucial advantages. Most importantly, this removes the heat load usually associated with thermal methods and is especially important for many applications that need high repetition rates—a fact demonstrated in magnetic data storage systems. Because the method does not receive thermal perturbations, it enhances the stability and reliability of magnetization control in multicycles and thus enriches the operation lifespan of spintronics devices. More than that, Extreme-precision handling of circularly polarized XUV light now allows such high-spatial-and-temporal-resolution selective control over magnetization orientation, opening new avenues for the control of nanoscale magnetic structures.

Future Directions and Challenges

The remaining challenges in scaling up and integrating this nonthermal magnetization method will be aimed at and mastered in future research. Hence, on the roadmap, an order-of-magnitude challenge is the maximization of efficiency and scalability of the XUV sources driving the production of the circular polarized light in these processes. State-of-the-art technologies today include free electron lasers ; for example, FERMI, as used for these measurements, offers the required photon energies but is often large, difficult, and expensive to be run in high-volume production. More accessible, less-expensive XUV sources delivering high-intensity circular polarization will be important in making a transition from a laboratory discovery to an industrially viable technology.

Evidently, further theoretical investigations and experiments will go on refining the understanding of the underlying opto-magnetic phenomena to ensure strong performance and reliability under different working conditions. Indeed, only through combined efforts by experimentalists, theorists, and industry collaborators can pioneering innovations like these move forward to the realization of full positional control of non-thermal magnetization for practical spintronics and beyond.

Conclusion

This finally shows a breakthrough in the development of a non-thermal pathway to magnetization using circularly polarized XUV light. By making use of the so-called inverse Faraday effect, it has been possible for teams to drive large variations of magnetization in ferromagnetic materials that do not rely on thermal mechanisms. This not only deepens the basic knowledge of light-matter interaction but also shows huge potential to revolutionize spintronics and data storage technologies. With research in this aspect still going on, such methodology is envisioned to foster innovation upon its integration into real devices facilitating next-generation technologies with better performance, efficiency, and reliability.

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