Basic spatial limits of all-optical magnetization switching – Uplaza

Magnetization might be switched with a single laser pulse. Nevertheless, it’s not identified whether or not the underlying microscopic course of is scalable to the nanometer size scale, a prerequisite for making this know-how aggressive for future knowledge storage purposes.

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. The paper is revealed within the journal Nano Letters.

Fashionable magnetic arduous drives can retailer a couple of terabit of information per sq. inch, which signifies that the smallest unit of knowledge might 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 understand the complete potential of AOS, notably by way of quicker write/erase cycles and improved energy effectivity, whether or not a magnetic bit can nonetheless be all-optically reversed if its measurement is on the nanometer-scale must be understood.

For AOS to happen, the magnetic materials must be heated as much as very excessive temperatures to ensure that its magnetization to be decreased near zero. Solely then, its magnetization might be reversed. The twist in AOS is that as a way to mediate magnetic switching, it’s ample 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.

Nevertheless, since sizzling electrons cool very quickly by scattering with the chilly atomic nuclei, the magnetization should be decreased 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 straightforward to see that this stability is modified when the optical excitation is confined to the nanoscale: now electrons can’t solely lose power by “giving atomic nuclei a kick,” however they will additionally merely go away the nanometer-small sizzling areas by diffusing away.

As they solely should traverse a nanometer-small distance so as to take action, this processes additionally occurs on ultrafast time scale, such that the electrons might cool too rapidly, the magnetization is just not sufficiently decreased, and AOS breaks down.

A global workforce of researchers has for the primary time efficiently addressed the query of “how small does AOS work” by combining experiments with mushy X-rays with atomistic spin dynamics calculations. They produced a particularly short-lived sample of darkish and shiny stripes of laser mild on the pattern floor of the prototypical magnetic materials GdFe, by interference of two mushy X-ray laser pulses with a wavelength of 8.3 nm.

This allowed lowering the space between darkish and shiny areas to solely 8.7 nm. This illumination is barely current for about 40 femtoseconds, resulting in a lateral modulation of cold and hot electron temperatures within the GdFe with a corresponding localized lack of magnetization.

The scientists may then comply with how this sample evolves on the very brief time scales that are of relevance. In direction of this finish, a 3rd mushy 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.

At this explicit wavelength, an digital resonance on the gadolinium atoms permits the mushy X-ray pulse to “feel” the presence of magnetization and thus the change of the magnetization might be detected with femtosecond temporal and sub-nanometer spatial decision. Combining the experimental outcomes with state-of-the-art simulations, the researchers may decide the ultrafast power transport on the nanometer scale.

It seems that the minimal measurement 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 finally prevents AOS.

The quicker cooling attributable to electron diffusion might be compensated to some extent by rising the excitation energy, however this strategy is finally restricted by the structural injury brought on by the extraordinary laser beam. The researchers anticipate that the 25 nm boundary is slightly common for all metallic magnetic supplies.