Fundamental aspects of all-optical single pulse switching in nanometer-sized magnetic storage media
Final Report Abstract
Our project aimed to elucidate writing mechanisms for digital data storage towards improved energy efficiency. In this context, all-optical helicity-dependent magnetization switching (AOHDS) shows potential to replace conventional data recording technology. Those use small electromagnets hovering directly above the magnetic disk's surface, generating magnetic fields and setting the magnetization of small single-domain particles, thus writing information for storage. New write-head generations already employ integrated lasers to heat the material, reduce the switching fields, and support the electromagnet. After over 40 years of enhancing all technological aspects, this method has reached its optimization limits for a further bit shrinking and speeding up the writing process while reducing energy consumption. At that, AOHDS replaces those electromagnets in write heads with circularly polarized ultrashort laser pulses. These generate two contributions to switching magnetization in FePt granular media. First, the magneto-optical circular dichroism (MCD) enables distinct electron temperatures for opposite spins upon enhanced photon absorption by one spin sort, thus enabling separate spin manipulation and deterministic switching. Second, the inverse Faraday effect (IFE) induces magnetization, improving the probability of switching toward the desired state. We explore the capability of the IFE for the mid-infrared photon energy range in granular FePt media and the interplay between the IFE and the MCD or heat entry after laser pulse absorption. The results show that the switching enhancement from the magnetization induced by IFE unfolds best for wavelengths with a higher absorption. Consequently, wavelengths exhibiting larger intrinsic IFE coefficient values do not benefit from their switching enhancement if their absorption cannot elevate electron temperatures close to the Curie temperature, quenching the magnetization state at low deposited fluences. This condition allows even small IFE contributions to take effect and generate higher switching rates. Energy-efficient data storage is relevant as the annual global energy consumption from data centers, artificial intelligence applications, and cryptocurrency appliances is estimated to exceed 1000 TWh within this decade, e.g., Ireland, a European data center hub, plans to redirect 32% of its electrical power consumption into this sector. All three applications significantly drive growing demand for electric energy and rely on data storage. Their energy demand will likely continue growing in the forthcoming decades. Accordingly, energy-efficient data recording technologies look insignificant, considering single data bits, but amass a significant factor for the overall utilization. Further possible research outcomes are an increased data storage density and recording speed.
Publications
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Exploring the impact of the inverse Faraday-effect on all-optical helicity-dependent magnetization switching. Journal of Applied Physics, 138(4).
Kohlmann, M.; Vollroth, L.; Jäckel, K.; Hovorakova, K.; Schmoranzerova, E.; Carva, K.; Hinzke, D.; Nowak, U.; Münzenberg, M. & Walowski, J.
