Multiple Ultrafast Pathways of Photo-Induced Magnetic Phase Transition in FeRh Thin Films.

Strong interplay among electron, spin, and lattice degrees of freedom in condensed matter provides a wide range of emergent phenomena, and also serves as a firm basis for the useful cross-coupled electronic, magnetic, and structural functionalities. Here, we present metallic and magnetic FeRh as a representative system where such coupling effects highlight a unique phase evolution in the non-equilibrium state, which is inaccessible in the equilibrium state. Employing terahertz spectroscopy, we examined and compared the changes in free electron dynamics across thermally-driven and photo-induced antiferromagnetic-ferromagnetic transitions in FeRh thin films. In particular, we identified a transient paramagnetic state emerging within a few picoseconds after photoexcitation, preceding both lattice expansion and long-range ferromagnetic ordering. Based on these results and thermodynamic simulations, we could demonstrate that the photo-induced magnetic phase transition proceeds through distinct and multiple pathways, governed by decoupled evolutions of electron, spin, and lattice systems. These findings provide a foundation for understanding and controlling ultrafast phase transitions by exploiting coupled degrees of freedom.