Femtosecond signatures of optically induced magnons beforeultrafast demagnetization

The dynamics of magnetization span a wide range of timescales. At the slow end, nanosecond (and beyond) spin dynamics can be driven with microwaves or external magnetic fields, and are central to technologies such as magnetic sensors. At the opposite extreme, femtosecond laser pulses can trigger ultrafast demagnetization, a sub-picosecond dissipation of spin, opening the door to applications in ultrafast magnetization writing, spin transport, and intense broadband THz sources.
Understanding what happens to magnetization on these ultrafast timescales is, however, highly challenging and has been an active field of study for almost three decades. The standard description relies on “three-temperature” models, where electrons, spins, and the lattice exchange energy with each other. Yet such models neglect the simultaneous transfer of angular momentum, which is essential to understand the loss of spin. Moreover, a central question has remained unresolved: what types of spin excitations actually occur during the first instants of demagnetization? Both single-electron spin flips and collective transverse spin excitations (magnons) have been proposed, but experimental access has been limited. Traditional optical probes measure only the macroscopic magnetization, while techniques that are sensitive to microscopic excitations generally lack the temporal resolution to capture sub-100-fs processes.