Controlling Single-Pulse Magnetization Switching through Angular Momentum Reservoir Engineering

We report a systematic study of single-pulse all-optical switching in Co/Gd bilayers, revealing that magnetization reversal dynamics can be tuned over 3 orders of magnitude. By varying the Gd thickness or inserting a Pt spacer layer between Co and Gd, we control the angular momentum transferred from the rare-earth sublattice to the transition-metal sublattice. Our results show that when Gd is abundant and strongly coupled to Co, angular momentum is efficiently transferred during Gd demagnetization, leading to ultrafast Co reversal. Reducing the Gd thickness or introducing Pt impedes this transfer, resulting in a slow domain growth mediated reversal as observed in CoDy and CoHo alloys. So, replacing Gd with Dy or Ho rare-earth elements slows down the switching due to a reduced angular momentum transfer toward Co upon demagnetization as demonstrated in CoGdDy alloy. Our findings establish angular momentum availability and transfer pathways as key parameters governing reversal dynamics.