Critical Behavior within 20fs Drives the Out-of-Equilibrium Laser-induced Magnetic Phase Transition in Nickel

Ferromagnets undergo a ferromagnetic to paramagnetic phase transition when the material is slowly heated above the Curie temperature under equilibrium conditions. However, to date it is not known how fast such phase transitions can proceed under out-of-equilibrium excitation conditions, nor what the connection is between out-of-equilibrium and equilibrium phase transitions. By combining time- and angle-resolved photoemission with time-resolved transverse magneto-optical Kerr spectroscopy, we show that critical behavior governs the ultrafast magnetic phase transition in nickel. For laser fluences that transiently drive the electron temperature above the Curie temperature, we find that the heat capacity of the electron and spin system diverges. Very surprisingly, the spin system absorbs sufficient energy within 20 fs to subsequently proceed through the phase transition, which defines a new timescale in the process of ultrafast demagnetization in ferromagnets. Demagnetization and the collapse of the exchange splitting then occur on longer timescales of ~176 fs. Our results connect the out-of-equilibrium material behavior to the strongly coupled equilibrium behavior, and also establish that the transient electron temperature dominantly dictates the magnetic response.