**New research introduces a non-thermal method for magnetization using circularly polarized XUV light, which induces significant magnetization changes through the inverse Faraday effect, potentially transforming ultrafast data storage and spintronics.**

*Intense laser pulses can be used to manipulate or even switch the magnetization orientation of a material on extremely short time scales. Typically, such effects are thermally induced, as the absorbed laser energy heats up the material very rapidly, causing an ultrafast perturbation of the magnetic order.*

*Scientists from the Max Born Institute (MBI), in collaboration with an international team of researchers, have now demonstrated an effective non-thermal approach of generating large magnetization changes. By exposing a ferrimagnetic iron-gadolinium alloy to circularly polarized pulses of extreme ultraviolet (XUV) radiation, they could reveal a particularly strong magnetic response depending 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, its impact on the magnetization can usually be attributed to the amount of energy introduced into the material when it is absorbed. Microscopically, this corresponds to an optical excitation of electrons, which are rapidly brought into non-equilibrium and start to scatter with each other and other quasiparticles, changing the electron spin and orbital moments and therefore the long-range magnetization.

Although such mechanisms give rise to a variety of fascinating phenomena, including ultrafast demagnetization and laser-induced magnetization switching, they come at the price of a substantial heat load on the material, limiting technological applicability where fast repetition rates are required, e.g., for read/write-operations in future data storage technologies.

#### The Inverse Faraday Effect and Opto-Magnetic Phenomena

An international team of researchers, led by scientists from MBI, has now studied an entirely different, non-thermal pathway of manipulating magnetism by light. Their approach is based on an opto-magnetic phenomenon that does not rely on electronic heating induced by the absorption of the light, but rather on a direct, coherent interaction between the light’s polarization and the electronic spins.

The underlying mechanism is the inverse Faraday effect (IFE), which leads to the generation of magnetic moments in a medium optically excited by circularly polarized radiation, with the direction of the magnetization depending on the left- or right-handedness of the circular polarization, i.e., its helicity. However, as the metallic and highly absorptive properties of most ferro- and antiferromagnetic materials typically suppress the aforementioned non-thermal effects, a special technique had to be developed to generate a sizeable opto-magnetic response.

Using circularly polarized femtosecond pulses of extreme ultraviolet (XUV) radiation, generated at the free-electron laser FERMI, the scientists could demonstrate the generation of a particularly strong IFE-induced magnetization in a metallic, ferrimagnetic iron-gadolinium (FeGd) alloy. This is possible because of the high photon energy of the XUV radiation, allowing resonant excitation of tightly bound core-level electrons, which due to their intrinsic properties (in particular, a strong spin-orbit coupling) facilitate the generation of large opto-magnetic effects.

#### Demonstrating Large Magnetization Changes Using XUV

With this approach, the scientists could show that, for different XUV photon energies around the Fe M_{3,2} core-level resonance, the IFE-induced magnetization can reach up to 20-30% of the ground-state magnetization of the alloy, measured by the difference between the ultrafast demagnetization induced for opposite helicities of the circularly polarized XUV pulses (Figure 1).

Supported by ab initio theory and spin dynamics simulations, it could also be demonstrated that the observed effects are in line with the expected IFE response (Figure 2) and cannot be explained by a purely thermal helicity-dependent mechanism, such as the well-established x-ray magnetic circular dichroism (XMCD).

Providing an efficient method for the non-thermal generation of large magnetization on ultrafast time scales, these findings are expected to be of high relevance for the fields of ultrafast magnetism and spintronics, as well as coherent magnetization control and the science of nonlinear x-ray matter interactions.

Reference: “Ultrafast opto-magnetic effects in the extreme ultraviolet spectral range” by Martin Hennecke, Clemens von Korff Schmising, Kelvin Yao, Emmanuelle Jal, Boris Vodungbo, Valentin Chardonnet, Katherine Légaré, Flavio Capotondi, Denys Naumenko, Emanuele Pedersoli, Ignacio Lopez-Quintas, Ivaylo P. Nikolov, Lorenzo Raimondi, Giovanni De Ninno, Leandro Salemi, Sergiu Ruta, Roy Chantrell, Thomas Ostler, Bastian Pfau, Dieter Engel, Peter M. Oppeneer, Stefan Eisebitt and Ilie Radu, 14 June 2024, *Communications Physics*.

DOI: 10.1038/s42005-024-01686-7

Supported by ab initio theory and spin dynamics simulations, it could also be demonstrated that the observed effects are in line with the expected IFE response (Figure 2) and cannot be explained by a purely thermal helicity-dependent mechanism, such as the well-established x-ray magnetic circular dichroism (XMCD).

Please ask researchers to think deeply:

1. Where does the force of electron spin come from?

2. Is the physical phenomenon observed in the experiment the natural essence of things?

3. Is the spin of electrons related to the spin of topological vortices?

4. Is mathematical imagery related to physical reality?

5. What is the exact moment when the mathematical probability of geometric shapes is transformed into physical reality?

Space has zero viscosity and absolutely incompressible physical properties, which are ideal fluid physical characteristics. Therefore, mathematically, it is not difficult to understand how space forms vortices via topological phase transitions. Once the topological vortex is formed, it occupies space and maintains its existence in time via spin until it changes, cancels out, or annihilates in interaction.

Each topological vortex is an accurate quantum clock. The absoluteness of time lies in the fact that each topological vortex has its own spin period, while the relativity of time lies in the fact that different topological vortices may have different spin periods. As a result, in the spacetime of interaction of topological vortices, time and space are both absolute and relative.

Matter and antimatter are mainly manifested between topological vortices and their twin anti-vortices, rather than between the high-dimensional spacetime matter formed by their interactions.

The study of matter and antimatter should not neglect the material hierarchy and time. Scientific research guided by correct theories can help humanity avoid detours, failures, and pomposity.

If researchers are really interested in science and physics, you can browse https://zhuanlan.zhihu.com/p/701032654.

Symmetry with spin creates the world, symmetry with spin creates all things. The world we see and observe is asymmetric, that is because the synchronization effect of countless topological vortices makes it difficult and impossible for us to see or observe the entire world (including ourselves).