Abstract
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.
Highlights
The exchange stiffness coefficient, Aex, represents the strength of direct exchange interactions among neighboring spins
We report, the dynamics of Aex on a femtosecond timescale in Co/Pt multilayers for a range of FP, demonstrating that Aex varies rapidly, affecting spin dynamics and its variation can be controlled by the pump fluence
timeresolved magneto-optical Kerr effect (TR-MOKE) signals were measured for 1.7 ≤ FP ≤ 28.5 mJ cm−2 for time delays of up to 30 ps
Summary
The exchange stiffness coefficient, Aex, represents the strength of direct exchange interactions among neighboring spins. Thermal agitation reduces the degree of the ordering of neighboring spins, effectively lowering the value of Aex. the temperature-dependence of Aex is relatively well recognized in static cases[17], very little is known regarding how Aex varies on an in the ultrafast timescale. We report, the dynamics of Aex on a femtosecond timescale in Co/Pt multilayers for a range of FP, demonstrating that Aex varies rapidly, affecting spin dynamics and its variation can be controlled by the pump fluence. While the electron-spin interaction strength is kept constant all the time in the conventional three-temperature model (3TM) in the study of ultrafast magnetism, the dynamic change of Aex is considered, adopting the generalized three-temperature model (G-3TM) developed by A.
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