Laser-wavelength dependence of ultrafast demagnetization in ferromagnetic metals

Laser-induced ultrafast demagnetization in ferromagnetic metals opens a new frontier at the intersection between laser technologyand material sciences. However, a complete understanding is still missing even in simple 3d metals. Prior studies have often concentrated on the effect of laser fluence on ultrafast demagnetization, but whether and how the wavelength affects ultrafast demagnetization 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 ultrafast demagnetization but the spin moment reduction is small. With the intraband transition, 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.