Manipulating femtosecond magnetization in ferromagnets and molecular magnets through laser chirp

Laser chirp has found many applications in pulse shaping, high harmonic generation, and dynamical control of the vibrational excitation. Here, we show that the laser chirp can significantly affect the amount of the magnetic moment change in the laser-induced femtosecond magnetism. Under the same laser condition, an increase in the chirp from 0 to 0.04 rad/fs(2) results in the magnetic moment change from 0.4% to 5% in ferromagnetic nickel. We find that the laser pulse duration has a direct effect on the effectiveness of the chirp. For a shorter pulse of duration 12 fs, the spin change with the chirp is small. We attribute this to a shorter interaction time and consequently a smaller instantaneous photon energy. We also investigate the effect of chirp on laser-induced local-spin-switching on molecular magnets consisting of two magnetic centers (Fe, Co, and Ni) bridged by Na atoms. We find that the effect of chirping differs depending on the original switching behavior of the atom. On Fe the chirp renders the process slightly less effective, on Ni it leads to a more dramatic loss of fidelity due to quantum interference, and for Co the effect lies in-between. Our study opens a new way to manipulate spin on femtosecond time scales. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3673404]