Ultrafast dynamics of exchange stiffness in Co/Pt multilayer

The exchange stiffness coefficient, Aex, represents the strength of direct exchange interactions among neighboring spins. Aex is linked to most of the magnetic properties such as skyrmion formation, magnetic vortex, magnetic domain wall width, and exchange length. Hence, the quantification of Aex is essential to understanding fundamental magnetic properties, but little is known for the dynamics of Aex on a sub-picosecond timescale. We report the ultrafast dynamcis of Aex in an ordered magnetic state in Co/Pt ferromagnetic multilayer. Time-resolved magneto-optical Kerr effect and reflectivity measurements were analyzed for various pump fluences. We reveal that the significant dynamical reduction of Aex is responsible for the dramatic increase of remagnetization time for high fluences. The analysis shows that Aex dynamically varies, strongly affecting overall ultrafast demagnetization/remagnetization process. The investigation demonstrates the possibility of Aex engineering in femtosecond timescale and thereby provides a way to design ultrafast spintronic devices. The exchange stiffness coefficient describes the interaction between neighbouring spins of a ferromagnetic system and analysing these interactions at ultrafast timescales can reveal information on their demagnetisation and remagnetisation mechanisms. Here, the authors investigate the ultrafast dynamics of the exchange stiffness coefficient of a ferromagnetic multilayer system and model the data using a generalised three temperature model.