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 by 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 occur, the magnetic material has to be heated up to high temperatures in order to reduce its magnetization 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 primarily interacts with the electrons, allowing much higher electron temperatures to be reached with very low power levels. However, since hot electrons cool down very rapidly by scattering with the cold atoms, the magnetization must be reduced within the characteristic time scale of such a cooling process, 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 altered when the excitation is confined to the nanoscale: electrons cannot only lose energy by interacting with cold atoms but also by diffusing out of the nanometer-small hot regions. At the nanoscale, all these processes occur on comparable and ultrafast time scales: for instance, the electrons may cool down too quickly, the magnetization is not sufficiently decreased, and AOS breaks down.
An international team of researchers from the Max Born Institute in Berlin, Germany, the Instituto de Ciencia de Materiales in Madrid, Spain, and the free-electron laser facility FERMI in Trieste, Italy, has successfully addressed for the first time the question: how small can AOS work?
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