Jun 27, 2024 |
(Nanowerk Information) Magnetization will be switched with a single laser pulse. Nonetheless, it’s not recognized whether or not the underlying microscopic course of is scalable to the nanometer size scale, a prerequisite for making this expertise aggressive for future knowledge storage functions. Researchers on the Max Born Institute in Berlin, Germany, in collaboration with colleagues on the Instituto de Ciencia de Materiales in Madrid, Spain, and the free-electron laser facility FERMI in Trieste, Italy, have decided a elementary spatial restrict for light-driven magnetization reversal.
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They report their finsings in Nano Letters (“Exploring the Fundamental Spatial Limits of Magnetic All-Optical Switching”).
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Trendy magnetic laborious drives can retailer a couple of terabit of knowledge per sq. inch, which implies that the smallest unit of data will be encoded on an space smaller than 25 nanometers by 25 nanometers. In laser-based, all-optical switching (AOS), magnetically encoded bits are switched between their “0” and “1” state with a single ultrashort laser pulse. To appreciate the complete potential of AOS, notably by way of sooner write/erase cycles and improved energy effectivity, we thus want to know whether or not a magnetic bit can nonetheless be all-optically reversed if its dimension is on the nanoscale.
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Two X-ray laser pulses intrude on the floor of a ferrimagnetic GdFe alloy, resulting in a lateral modulation of the electron temperatures, a discount of the native magnetization and all-optical switching of the magnetization. On this vogue, knowledge bits to be saved will be written by purely optical means. On the fitting hand facet, the interval of grating and therefore the scale of a bit is diminished to under 25 nm. Because of this, the temperature profile is washed out earlier than the magnetization is sufficiently diminished and all-optical switching breaks down. (Picture: Moritz Eisebitt)
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For AOS to happen, the magnetic materials needs to be heated as much as very excessive temperatures to ensure that its magnetization to be diminished near zero. Solely then, its magnetization will be reversed. The twist in AOS is that in an effort to mediate magnetic switching, it’s enough to warmth solely the electrons of the fabric whereas leaving the lattice of atomic nuclei chilly. That is precisely what an optical laser pulse does: it interacts solely with the electrons, permitting to achieve a lot increased electron temperatures with very low energy ranges.
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Nonetheless, since scorching electrons cool very quickly by scattering with the chilly atomic nuclei, the magnetization should be diminished sufficiently quick inside this attribute time scale, i.e. AOS depends on a cautious stability between the evolution of the electron temperature and the lack of magnetization. It’s simple to see that this stability is modified when the optical excitation is confined to the nanoscale: now electrons cannot solely lose vitality by “giving atomic nuclei a kick”, however they’ll additionally merely depart the nanometer-small scorching areas by diffusing away. As they solely must traverse a nanometer-small distance so as to take action, this processes additionally occurs on ultrafast time scale, such that the electrons could cool too rapidly, the magnetization just isn’t sufficiently decreased, and AOS breaks down.
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A world workforce of researchers has for the primary time efficiently addressed the query of “how small does AOS work” by combining experiments with gentle x-rays with atomistic spin dynamics calculations. They produced a particularly short-lived sample of darkish and vibrant stripes of laser mild on the pattern floor of the prototypical magnetic materials GdFe, by interference of two gentle X-ray laser pulses with a wavelength of 8.3 nm. This allowed decreasing the space between darkish and vibrant areas to solely 8.7 nm.
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This illumination is just current for about 40 femtoseconds, resulting in a lateral modulation of cold and warm electron temperatures within the GdFe with a corresponding localized lack of magnetization. The scientists may then observe how this sample evolves on the very brief time scales that are of relevance. In direction of this finish, a 3rd gentle X-ray pulse with the identical wavelength of 8.3 nm was diffracted off the transient magnetization sample at completely different time delays from the patter-generating pulses.
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At this explicit wavelength, an digital resonance on the gadolinium atoms permits the gentle X-ray pulse to “feel” the presence of magnetization and thus the change of the magnetization will be detected with femtosecond temporal and sub-nanometer spatial decision.
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Combining the experimental outcomes with state-of-the-art simulations, the researchers may decide the ultrafast vitality transport on the nanometer scale. It seems that the minimal dimension for AOS in GdFe alloys, induced by a nanoscale periodic excitation, is round 25 nm. This restrict is because of ultrafast lateral electron diffusion, which quickly cools the illuminated areas on these tiny size scales and in the end prevents AOS. The sooner cooling resulting from electron diffusion will be compensated to some extent by rising the excitation energy, however this strategy is in the end restricted by the structural harm attributable to the extraordinary laser beam. The researchers anticipate that the 25 nm boundary is reasonably common for all metallic magnetic supplies.
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