Femtosecond versus nanosecond magnetization dynamics in ultrathin ferromagnetic bilayers

We investigate the magnetization dynamics of an ultrathin Co (1.5 nm)/Py (1.5 nm) bilayer system from femtosecond (fs) to nanosecond (ns) timescales. Magnetization dynamics in the fs timescales is characterized as a highly nonequilibrium regime due to an ultrafast reduction of magnetization by laser excitation. On the other hand, the dynamics in the ns timescales is characterized as a close-to-equilibrium regime involving the excitation of coherent magnons. We demonstrate that the interfacial interaction between the Co and Py layers in these two nonequilibrium regimes across the timescales is dynamic and simultaneously influences the magnetization loss in the fs timescales and the magnon dynamics in the ns timescales. On ultrafast (fs) timescales, comparison between time-resolved magneto-optical Kerr effect (TR-MOKE) measurements and temperature-based <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>μ</a:mi><a:mi>T</a:mi></a:mrow></a:math> model simulations reveals that the bilayer exhibits demagnetization dynamics intermediate between those of its individual layers. When driven far from equilibrium by ultrashort laser pulse excitation, the magnetization dynamics of the individual Co and Py layers appear to remain decoupled and evolve independently in the initial stages of the ultrafast response. On the other hand, in the ns regime, the two individual layers of the bilayer precess together at the same frequency in a coupled manner as one effective single layer. These results improve our understanding of magnetization dynamics across timescales in ultrathin exchanged-coupled ferromagnetic bilayers and provide valuable insights for the design of high-frequency and energy-efficient spintronic device concepts.