Separation of ultrafast spin currents and spin-flip scattering in Co/Cu(001) driven by femtosecond laser excitation employing the complex magneto-optical Kerr effect

Ultrafast magnetization dynamics in metallic heterostructures consists of a combination of local demagnetization in the ferromagnetic constituent and spin-dependent transport contributions within and in between the constituents. Separation of these local and nonlocal contributions is essential to obtain microscopic understanding and for potential applications of the underlying microscopic processes. By comparing the ultrafast changes of the polarization rotation and ellipticity in the magneto-optical Kerr effect we observe a time-dependent magnetization profile $M(z,t)$ in Co/Cu(001) films by exploiting the effective depth sensitivity of the method. By analyzing the spatiotemporal correlation of these profiles we find that on time scales before hot electron thermalization ($<100$ fs) the transient magnetization of Co films is governed by spin-dependent transport effects, while after hot electron thermalization ($>200$ fs) local spin-flip processes dominate.