Ever since its discovery ultrafast demagnetization has remained one of the most intriguing research areas in magnetism. Here, we demonstrate that in [Co (tCo )/ Pd (0.9 nm)] 8 multilayers, the characteristic decay time in femtosecond time- scale varies non-monotonically with tCo in the range 0.07 nm B tCo B 0.75 nm. Further investigation reveals higher spin fluctuation at higher ratio of electron to Curie temperature to be responsible for this. Microscopic three-temperature modelling unravels a similar trend in the spin–lattice interaction strength, which strongly supports our experimental observation. The knowledge of the fem- tosecond magnetization decay mechanism in ultrathin ferromagnetic films is unique and important for the advancement of fundamental magnetism besides their potential applications in ultrahigh speed spintronic devices.