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 non-local 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 (MOKE) 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 spatio-temporal 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.

After excitation by femtosecond laser pulses, Gd and Tb exhibit ultrafast demagnetization in two steps, with the time constant of the second step linked to the coupling strength of the 4f magnetic moments to the lattice. In time-resolved magneto-optical Kerr effect measurements of Gd 1−x Tb x alloys, we observe a decrease in this time constant from 33 to 9 ps with Tb content x increasing from 0 to 0.7. We explain this behavior by the stronger spin-lattice coupling of Tb compared to Gd, which increases the effective spin-lattice coupling in Gd 1−x Tb x with x. In contrast, the faster time constant of the first demagnetization step exhibits no dependence on x. Additional time-and element-resolved x-ray magnetic circular dichroism measurements show a two-step demagnetization of Gd and Tb in Gd 0.6 Tb 0.4 with an equivalent time scale of the second step but a different magnitude of demagnetization which persists for 15 ps. We explain this by an increased coupling of the Gd 4f magnetic moments to the lattice compared to pure Gd, via interatomic exchange coupling to the neighboring Tb 4f moments mediated by 5d electrons, which has limited efficiency and allows an estimation of a characteristic time scale of the interatomic exchange coupling. We assign the first demagnetization step to the dynamics of the laser-excited 5d electrons, while the second demagnetization step is dominated by the strength of spin-lattice coupling of the 4f electrons.

We analyze laser-induced ultrafast, spatially inhomogeneous magnetization dynamics of epitaxial Co/Cu(001) films in a 0.4-10 nm thickness range with time-resolved magnetization-induced second harmonic generation, which probes femtosecond spin dynamics at the vacuum/Co and Co/Cu interfaces. The interference of these two contributions makes the overall signal particularly sensitive to differences in the transient magnetization redistribution between the two interfaces, i.e. ultrafast magnetization profiles in the ferromagnetic film. We find in films of up to 3 nm thickness a stronger demagnetization at the surface, because the film thickness is smaller than the effective mean free path of the spin current mediating the demagnetization, i.e. the difference between the mean free paths of the majority and minority carriers. For film thicknesses larger than 3 nm, the magnetization profile reverses, since majority spins can escape into the conducting substrate only from the interface-near region.

The polarization of the two beam (driver-probe) high-order harmonic generation from solids is measured. The experiments, together with computer simulations, allow us to distinguish two different coupling mechanisms of the driver and the probe, resulting in different harmonic efficiencies and spectral slopes. We find that in the nonrelativistic regime the coupling is mostly due to the nonlinear plasma density modulation.