Phase effects in magnetic second-harmonic generation on ultrathin Co and Ni films on Cu(001) - art. no. 144417

Phase relations in second-harmonic generation (SHG) have been studied syste matically for Ni and Ca films on Cu(001) as a function of thickness in the range 1-12 ML. Below two monolayers we observe, with s-polarized fundamenta l light and P-polarized second-harmonic light, phase shifts close to 180 de grees of both films against the Cu substrate. Such large phase shifts in th is polarization combination appear to be characteristic of the transition f rom s- to d-band metals. In the ferromagnetic thickness range the relative phase between odd and even tensor elements becomes important. For Ni films it is independent of film thickness for both p --> P and s --> P polarizati ons, and leads to the ratio chi (odd)(xyy)/chi (even)(zyy) = 0.047 +/- 0.01 3, in agreement with theoretical predictions. In the case of Co films the r elative phase differs dramatically between these two polarization combinati ons. It is nearly independent of film thickness for p --> P polarized SHG. However, for s --> P polarization it varies from 135 degrees at 3 ML to 60 degrees at 10 ML, causing a sign change of the magnetic contrast at 90 degr ees near 6 ML. The ratio of odd to even second-harmonic field amplitudes wa s found to vary from 22% at 3 ML to 12.5% above 6 ML. From these results we conclude that measurements of the relative phase are necessary for derivin g reliable information from SHG about the magnetization state of a sample.