From Max Born Institute

Magnetization can be switched with a single 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 storage applications.

Two soft X-ray pulses interfere on the surface of a ferrimagnetic GdFe alloy, leading to a lateral modulation of the electron temperatures, a reduction of the local magnetization and all-optical switching of the magnetization. On the right hand side, the period of grating is reduced to below 25nm. As a result, the temperature profile is washed out before the magnetization is sufficiently reduced and all-optical switching breaks down.
(Picture credits: Moritz Eisebitt)

Researchers at the Max Born Institute in Berlin, Germany, in collaboration with colleagues at the Instituto de Ciencia de Materiales, 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 1Tb 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, all-optical switching (AOS), particularly in terms of faster write/erase cycles and 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, its magnetization can 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.

Fig. 1: Measured diffracted intensity off transient magnetic grating with period of 17 nanometers.
The diffracted intensity decays as the grating washes out because of lateral ultrafast energy diffusion. The model describes our data very well.

An international team of researchers has for the 1st time successfully addressed the question of ‘how small does AOS work’ by combining soft X-ray transient grating spectroscopy with atomistic spin dynamics calculations. They produced an extremely short-lived pattern of dark and bright stripes of laser light by interference at the sample surface of the prototypical magnetic material GdFe. The novelty of the experiment is based on the use of laser light 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 the (electron) temperatures and to a corresponding localized loss of magnetization. Subsequently, the ultrafast spatial evolution of the magnetization grating was probed by diffracting a third soft X-ray pulse with a wavelength of 8.3nm. At this particular wavelength, an electronic resonance at the gadolinium atoms allows the soft X-ray pulse to ‘feel’ the presence of magnetization and thus the change of the magnetization can be detected with femtosecond temporal and sub-nanometer spatial resolution.

Fig 2: The phase diagram for AOS as a function of absorbed energy density and excitation period, suggesting that the minimum size for AOS in GdFe alloys, induced by a nanoscale periodic excitation, is around 25 nm.

Combining the experimental results with state-of-the-art simulations, the researchers could determine the nanoscale and ultrafast energy transport. It turns out that the minimum size for AOS in GdFe alloys, induced by a nanoscale periodic excitation, is around 25nm. This limit is due to ultrafast lateral electron diffusion, which rapidly cools the illuminated regions on these tiny length scales and ultimately prevents AOS. The faster cooling due to electron diffusion can be compensated to some extent by increasing the excitation power, but this approach is ultimately limited by the structural damage caused by the intense laser beam. The researchers expect that the 25nm boundary is rather universal for all metallic magnetic materials.

Article: Exploring the Fundamental Spatial Limits of Magnetic All-Optical Switching

Nano Letters has published an article written by Felix Steinbach, Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born Strasse 2A, 12489 Berlin, Germany, Unai Atxitia, Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain, Kelvin Yao, Martin Borchert, Dieter Engel, Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born Strasse 2A, 12489 Berlin, Germany, Filippo Bencivenga, Laura Foglia, Riccardo Mincigrucci, Emanuele Pedersoli, Dario De Angelis, Matteo Pancaldi, Danny Fainozzi, Jacopo Stefano Pelli Cresi, Ettore Paltanin, Flavio Capotondi, Claudio Masciovecchio, Elettra Sincrotrone Trieste S.C.p.A., 34149 Basovizza, Trieste, Italy, Stefan Eisebitt, Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born Strasse 2A, 12489 Berlin, Germany, and Institut für Optik und Atomare Physik, Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany, and Clemens von Korff Schmising, Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born Strasse 2A, 12489 Berlin, Germany.

Abstract: “All-optical switching (AOS) results in ultrafast and deterministic magnetization reversal upon single laser pulse excitation, potentially supporting faster and more energy-efficient data storage. To explore the fundamental limits of achievable bit densities in AOS, we have used soft X-ray transient grating spectroscopy to study the ultrafast magnetic response of a GdFe alloy after a spatially structured excitation with a periodicity of 17 nm. The ultrafast spatial evolution of the magnetization in combination with atomistic spin dynamics and microscopic temperature model calculations allows us to derive a detailed phase diagram of AOS as a function of both the absorbed energy density and the nanoscale excitation period. Our results suggest that the minimum size for AOS in GdFe alloys, induced by a nanoscale periodic excitation, is around 25 nm and that this limit is governed by ultrafast lateral electron diffusion and by the threshold for optical damage.“