Ultrafast all-optical manipulation of magnetization in hybrid metal-ferromagnet thin films

Abstract

The interaction between ultrafast lasers and magnetic materials have triggered extensive research interest over the past decades. One significant discovery in this field is all-optical helicity-dependent switching (AO-HDS), in which circularly polarized femtosecond laser pulses can deterministically switch the magnetization without the need of external magnetic fields. Furthermore, as the laser pulses are the shortest known stimulus in experimental condensed matter physics, the laser-induced magnetization switching can be extremely fast. Therefore, the phenomenon of AO-HDS makes it possible to realize all-optical magnetic data storage with a speed much faster than the current technologies. Since the experimental demonstration of AO-HDS in 2007, researchers have explored different magnetic materials and laser parameters that enable AO-HDS. In particular, AO-HDS in the technologically important class of materials, such as ferromagnetic thin films, was reported in 2014. Besides the discovery of AO-HDS, another important breakthrough is heat-assisted magnetic recording (HAMR). In HAMR, plasmonic nanostructures are utilized to locally enhance the light field intensity within one data bit, rendering it possible to record data bit at the nanometer scale with reduced power. The proof-of-concept experiment was demonstrated in 2009, which achieved data recording at an areal density of ~375 Tb m-2. The central goal of my thesis work is to realize future all-optical, low-energy, and high-density information recording devices by combining the research fields of AO-HDS and plasmonics. In this dissertation, I will first discuss robust AO-HDS in hybrid metal-ferromagnet thin films, which consist of Co/Pt multilayers and an Au capping layer on the top. The switching behaviors with various laser repetition rates, scanning speeds, and fluencies have been systematically studied, demonstrating pronounced AO-HDS in this new material system. Then, by fabricating the Au cladding layer into plasmonic nanostructures, I have experimentally demonstrated all-optical manipulation of magnetization with reduced laser threshold, thanks to the hot spots produced by the plasmonic nanostructures. A hybrid multiphysics model is constructed to simulate the plasmon-enhanced magnetization switching. Next, I will present preliminary results of the plasmonic nanostructures fabricated with nanosphere lithograph. Finally, I will discuss future directions of this emerging field.