Theoretical foundation of the time-resolved magneto-optical Kerr effect for femtosecond magnetism*

Laser-induced femtosecond magnetism or femtomagnetism opens a new frontier for a faster magnetic storage device, but to probe such a rapid magnetization change challenges the existing experimental and theoretical wisdom. We establish a new paradigm through a first-principles investigation in ferromagnetic nickel [1]. We show that the time-resolved optical and magnetic responses energetically follow their respective optical and magneto-optical susceptibilities; as a result, the one-to-one correspondence between them sensitively depends on the incident photon energy. For a shorter laser pulse, a delay of 10 fs in the magnetic signal with respect to the optical one is revealed through a phase-sensitive polarization-magnetization diagram; for a longer pulse, such a delay diminishes and the correlation can be established unambiguously [2].\\[4pt] [1] Zhang {\it et al}, Nature Physics {\bf 5}, 499 (2009);\\[0pt] [2] Zhang, Phys. Rev. Lett {\bf 101}, 187203 (2008).