J. Güdde, U. Conrad, V. Jähnke, and E. Matthias

Second harmonic generation in reflection (SHG) is a versatile tool to optically investigate electronic, structural and magnetic properties of surfaces and interfaces of centrosymmetric materials. For magnetic materials a large intensity contrast can be observed when the magnetization direction is reversed. This intensity contrast is the result of an interference of two contributions of the nonlinear susceptibility c ⁽²⁾ which respond either symmetric (even) or antisymmetric (odd) to a reversal of the magnetization direction. Since this interference depends not only on the magnitude but also on the relative phase of these two contributions, it is important to measure both quantities for a reliable investigation of the dependence of magnetization on external parameters. In this contribution we will demonstrate the importance of the relative phase for Co and Ni films.

In our first example, phase angles for up to 12 ML Co/Cu(001) films were measured by frequency-domain SHG interferometry with ultrashort laser pulses under UHV conditions at 80°C substrate temperature. We found dramatic differences in the thickness dependence of the magnetic contrast for the two different polarization combinations p-P and s-P. Beyond 2 ML when the films is ferromagnetic, the magnetic contrast for p-P polarized SHG decreases but the sign does not change. To the contrary, for s-P polarization the magnetic contrast first reaches a maximum around 3 ML but then decreases and changes sign around 6 ML. Phase measurements unambiguously show that the sign change is caused by a phase angle of 90°, changing from 133° near 2 ML to about 60° around 11 ML. We interpret this phase change by a superposition of two effects. One is that SHG from the Cu substrate and the Co film is out of phase by 180°, the other is that with increasing film thickness SHG at the film surface dominates the interface contribution.

In our second example, the goal was to prove that the time-dependence of the magnetic SHG contrast is actually a reliable measure of the magnetization dynamics. Here, we examined the time-dependence of the relative phase in an ultrafast pump-probe experiment on 60 nm polycrystalline Ni films on fused silica at room temperature in air. In these experiments we used time-domain SHG interferometry and measured the relative phase for different pump-probe delays. No dependence of the phase on delay time was found which allows to infer from time-resolved SHG data the magnetization dynamics of the film.