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Ultrafast control of the magnetization in ps timescales by fs laser pulsesoffers an attractive avenue for applications such as fast magnetic devices forlogic and memory. However, ultrafast helicity-independent all-optical switching(HI-AOS) of the magnetization has thus far only been observed in Gd-based,ferrimagnetic amorphous (\textit{a}-) rare earth-transition metal(\textit{a}-RE-TM) systems, and a comprehensive understanding of the reversalmechanism remains elusive. Here, we report HI-AOS in ferrimagnetic\textit{a}-Gd$_{22-x}$Tb$_x$Co$_{78}$ thin films, from x = 0 to x = 18, andelucidate the role of Gd in HI-AOS in \textit{a}-RE-TM alloys and multilayers.Increasing Tb content results in increasing perpendicular magnetic anisotropyand coercivity, without modifying magnetization density, and slowerremagnetization rates and higher critical fluences for switching but stillshows picosecond HI-AOS. Simulations of the atomistic spin dynamics based onthe two-temperature model reproduce these results qualitatively and predictthat the lower damping on the RE sublattice arising from the small spin-orbitcoupling of Gd (with $L = 0$) is instrumental for the faster dynamics and lowercritical fluences of the Gd-rich alloys. Annealing\textit{a}-Gd$_{10}$Tb$_{12}$Co$_{78}$ leads to slower dynamics which we argueis due to an increase in damping. These simulations strongly indicate thatacounting for element-specific damping is crucial in understanding HI-AOSphenomena. The results suggest that engineering the element specific damping ofmaterials can open up new classes of materials that exhibit low-energy,ultrafast HI-AOS.

Role of Element-Specific Damping on the Ultrafast, Helicity-Independent All-Optical Switching Dynamics in Amorphous (Gd,Tb)Co Thin Films