Laser-wavelength dependence of ultrafast demagnetization in ferromagnetic metals

Laser-induced ultrafast demagnetization (UD) in ferromagnetic metals opens a new frontier at the intersection between laser technology and materials sciences. However, a complete understanding is still missing even in simple 3 d metals. Prior studies have often concentrated on the effect of laser fluence on UD, but whether and how the wavelength affects UD remain under explored. Here, we propose a new perspective that is based on laser wavelength. We show, via the example of fcc Ni, that without intraband transitions, wavelength has a significant impact on UD but the spin moment reduction is small. With the intraband transition, UD weakly depends on wavelength, but with a large spin reduction. The time-resolved electron and spin density of states reveals that electrons around the Fermi energy are largely responsible for strong demagnetization, which almost wipes out the imprint of the photon energy on demagnetization, explaining the experimental observation. A significant spin reduction is found when a large portion of the unoccupied minority states slightly above the Fermi level becomes occupied.