Direct Measurement of the Static and Transient Magneto-Optical Permittivity of Cobalt Across the Entire M-edge in a Reflection Geometry by Use of Polarization Scanning*

The microscopic state of a magnetic material is characterized by its resonant magneto-optical response through the off-diagonal dielectric tensor component $\epsilon_{xy}$. However, the measurement of the full complex $\epsilon_{xy}$ in the extreme ultraviolet region covering the M absorption edges of 3d ferromagnets is associated with multiple experimental challenges. We demonstrate a new technique to extract $\epsilon_{xy}$ simply by scanning the polarization angle of linearly polarized high harmonics to measure the magneto-optical asymmetry in reflection geometry. Because this technique is more practical to implement than previous approaches, we can directly measure the time evolution of $\epsilon_{xy}$ (t) during laser-induced demagnetization across the entire M2,3 absorption edge of cobalt with femtosecond resolution. For polycrystalline Co films on an insulating substrate, the changes in $\epsilon_{xy}$ are uniform throughout the spectrum. This result suggests that, for strong demagnetization, the ultrafast demagnetization response is primarily driven by magnon generation.