FERMI | TIMER Fundamental spatial limits of all-optical magnetization switching Magnetization can be switched with a single, ultrashort laser pulse. However, it is not known whether the underlying microscopic process is scalable to the nanometer length scale, a prerequisite for making this technology competitive for future data storage applications. Researchers at the Max Born Institute in Berlin, Germany, in collaboration with colleagues at the Instituto de Ciencia de Materiales in Madrid, Spain, and the free-electron laser facility FERMI in Trieste, Italy, have determined a fundamental spatial limit for light-driven magnetization reversal. Modern magnetic hard drives can store more than one terabit of data per square inch, which means that the smallest unit of information can be encoded on an area smaller than 25 nanometers x 25 nanometers. Therefore, to realize the full potential of laser-based, alloptical switching (AOS), particularly in terms of faster write/erase cycles and an improved power efficiency, we need to understand whether a nanoscale magnetic bit can still be all-optically reversed. For AOS to take place, the magnetic material has to be heated up to very high temperatures in order for its magnetization to be reduced close to zero. Only then can its magnetization reverse. The twist in AOS is that it is sufficient to heat the electrons of the material while leaving the lattice of atoms cold. This is exactly what an optical laser pulse does; it interacts only with the electrons, allowing to reach much higher (electron) temperatures with very low power levels. However, since hot electrons cool very rapidly by scattering with the cold atoms, the magnetization must be reduced within this characteristic time scale, i.e. AOS relies on a careful balance between the evolution of the (electron) temperature and the loss of magnetization. It is easy to see that this balance is changed when the excitation is confined to the nanoscale: now electrons can not only lose energy to atoms but also by diffusing out of the nanometer-small hot regions. On the nanoscale all these processes happen on comparable ultrafast time scales, such that the electrons may cool too quickly, the magnetization is not sufficiently decreased, and AOS breaks down. The international team of researchers has for the first time successfully addressed the question of “how small does AOS work” by soft x-ray transient grating spectroscopy carried out at the EIS-TIMER beamline at FERMI. The concept of such an experiment is shown schematically in Fig. 1 (a): two femtosecond pulses overlap in space and time on the prototypical magnetic material GdFe, generating an extremely short-lived interference pattern of dark and bright stripes of laser light. The novelty of the experiment is based on the use of laser radiation in the soft x-ray spectral range, so that the distance between dark and bright areas could be reduced to just 8.7 nanometers. This leads to a lateral modulation of the (electron) temperatures and to a corresponding localized loss of magnetization. The ultrafast spatial evolution of the magnetization grating was probed by diffracting a third soft x-ray pulse with a wavelength of 8.3 nm. At this particular wavelength, an electronic resonance at the gadolinium atoms allows the soft x-ray pulse to probe the presence of magnetization and thus the change of the magnetization can be detected with femtosecond temporal and nanometer spatial resolution. Fig. 1 (b) shows the main experimental result: the diffracted intensity first increases within 400 fs, due to the emergence of the spatially modulated demagnetization. The signal then exponentially decays with a time constant of 3 ± 1 ps, which directly encodes the thermalization of neighboring areas into the same magnetic state. One has to emphasize that the decay is dominated by ultrafast energy diffusion and not by remagnetization, as cooling of the sample back to ambient temperatures takes significantly longer times. Stateof-the-art simulations, based on atomistic spin dynamics and microscopic model calculations, with parameters determined by additional all-optical measurements, can very accurately describe the experimental data, cf. solid line in Fig. 1 (b). Based on this, the researchers constructed a phase diagram for AOS both as a function of absorbed energy density and grating period. As shown in Fig. 1 (c), whether AOS (red dots) or only demagnetization (blue dots) occurs depends sensitively on these two parameters. As the grating period decreases, the electrons can cool more and more rapidly due to ultrafast lateral energy transport, requiring stronger excitation. Evidently, on a nanometer spatial scale cooling of electrons due to diffusion approaches the intrinsic time scales of electron cooling due scattering with atoms. While the loss of energy can be compensated for to some extent by increasing the excitation power, this approach is ultimately limited by the sample's damage threshold. The damage threshold of the sample was determined by optical measurements yielding a value of 0.51 GJm−3. This is about 20% above the switching threshold for AOS and is marked by the black dotted line in Fig. 1 (c). This value also corresponds very well to the results from our nondestructive measurements shown in Fig. 1 (b), with an absorbed energy density of a = 0.48 GJm−3. This value was experimentally found to be slightly below the damage threshold (yellow star in the same figure). Hence, the minimum size for AOS in GdFe alloys, induced by a nanoscale periodic excitation, is around 25 nm. As the GdFe sample exhibits an amorphous structure with extremely low electron and lattice diffusivities, the researchers expect that the 25 nm boundary is rather universal for all metallic magnetic materials. Elettra Highlights 2024–2025 88
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