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Magnetization manipulation is an indispensable tool for both basic and applied research [1]. The dynamics of the response depends on the energy transfer from the laser excited electrons to the spins within the first femtoseconds. This determines the speed of the ultrafast magnetization. A special material of interest for magnetic storage development is FePt. In a seminal experiment all optical writing had been demonstrated for FePt nanoparticle of a magnetic hard disc media, completely surprisingly, by Lambert et al. in Science 2014. But, the mechanism remained unclear and it opened many questions about the extension of possibilities of all optical writing as a general mechanism. Meanwhile writing experiments by single laser spots point to an asymmetric writing per each shot. This is consistently observed by different groups. These effects can be described within different rate models. I will review the current understanding of the interaction of ultrafast excitation and heating, influence of
magnetic dichroism and the presence of the inverse Faraday effect and attempts of understanding of these processes so far. From the experimental side in especial single shot writing experiments, that show a kind of accumulation effects of the writing, allow to pinpoint the underlying mechanism of writing in these media. Together, ab-initio calculations of the optical effects (inverse Faraday effect and magnetic dichroism induced heating) and the thermal modeling, allow to calculate the switching rates of the individual FePt nanoparticles. The latter then provides a rate of switching of the ensemble. A careful experimental determination of the absorbed fluence in the spherical geometry of the nanoparticles gives us a complete picture of the competing effects, of heating and writing asymmetries, and we can trace the different processes from the beginning of the laser pulse impact. In addition, this theoretical description allows us to optimize the number of shots needed to write the magnetization of the FePt nanoparticles and to pinpoint how to optimize the all optical writing. In my talk, I will review these recent developments that may lead to address an individual nanosized magnetic element in the far future all optically, for writing magnetic memory and memory storage.
[1] J. Walowski and M. Münzenberg, Perspective: Ultrafast magnetism and THz spintronics, J. Appl. Phys. 120, 140901 (2016).
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