Ultrafast magnetization dynamics of Gd(0001): Bulk vs. surface

Ultrafast laser-induced demagnetization of Gd(0001) has been investigated by magneto-induced optical second harmonic generation and the magneto-optical Kerr effect which facilitate a comparison of surface and bulk dynamics. We observe pronounced differences in the transient changes of the surface and bulk sensitive magneto-optical signals which we attribute to transfer of optically excited, spin-polarized carriers between surface and bulk states of the Gd(0001) film. A fluence dependent analysis of the bulk magnetization dynamics results in a weak variation of the demagnetization time constant, which starts at about 700 fs and increases by 10% within a fluence variation up to 1 mJ/cm2. We compare these results with fluence dependent changes in the transient energy density calculated by the two temperature model. The determined characteristic times of excess energy transfer from the electron system to the lattice, which is mediated by e-ph scattering, range from 0.2 - 0.6 ps. Such a more pronounced fluence dependent change in the characteristic time compared to the observed rather weakly varying demagnetization times suggests a more advanced description of the optically excited state than by the two-temperature model.