Correlating EUV TMOKE and ARPES measurements to understand the temporal and spatial length scales underlying ultrafast demagnetization in nickel*

Time-resolved magneto-optical Kerr effect spectroscopies spanning the M-shell absorption edges of magnetic materials provide powerful probes of the spin dynamics. However, to date, the disparate dynamics that occur at different depths in a laser-heated material were not disentangled. Here we show that this spatial averaging masks the true behavior of the material, where parts of the material at the surface can go through the ferromagnetic-paramagnetic phase transition, while deeper layers will not. We quantitatively compare depth sensitive time-resolved transverse magneto-optical Kerr effect measurements with time- and angle-resolved photoemission spectroscopy surface measurements on nickel. We show that the first critical behavior of the material occurs when the electron temperature as measured directly through ARPES and transient reflectivity reaches the Curie temperature, followed by a second critical behavior when the lattice temperature reaches the Curie temperature. These observations coupled with knowledge of the probing depth of each method, completely explain the results obtained from both experiments.