Academic Awards 2023 booklet

71 Quick as a flick: All-optical control over ultrafast magnetization writing and spin transport In the modern information age, massive amounts of data are being generated and transported every single second. Much of this data is stored in datacenters on magnetic data carriers, where writing is done using locally applied magnetic fields. This constitutes a large portion of the energy consumption in datacenters. An attractive candidate for increasing both energy efficiency and speed of data storage is the use of ultrashort (femtosecond) laser pulses to switch magnetic bits. Although this technique, known as all-optical switching, has been widely demonstrated in laboratory settings, several obstacles remain that hinder implementation in real-world devices. The work in this thesis moves us towards removing some of these obstacles by investigating several aspects of all-optical switching. Firstly, we were able to determine and significantly increase the speed with which subsequent bit writing events can occur, where we highlighted that efficient heat dissipation is essential for consistent ultra-fast device operation. Secondly, we have shown that mobile electrons that carry magnetic information (spin currents) are key components in a full understanding of the all-optical switching process. We also show that these spin currents can be used to achieve novel functionality, making the technique more attractive for use in future energy-efficient data storage devices. 2 1 240 ps 50 ps 10 ps fs laser Δt Pulse 1 Pulse 2 a b = N S = 0 = S N = 1 Initial Toggle switching Deterministic writing 0 1 1 0 0 0 Figure 1: Experiment to determine the minimum time between two all-optical switching events in a thin magnetic film. (a) Sketch of the experiment. (b) Microscope images with magnetic contrast for different time delays. Returning to ‘0’ in the overlapping region is the desired result. Scale bar represents 50 µm. Figure 2: Demonstration of novel functionality in all-optical switching. By carefully choosing the magnetic layers we can transition from ‘conventional’ toggle switching, where each laser pulse flips the magnetization, to deterministic writing of a desired bit state. Scale bar represents 50 µm.