The laser-induced femtosecond demagnetization in ferromagnetic metals is investigated theoretically. Different from the conventional nanosecond one, this ultrafast demagnetization is a cooperative effect of the external laser field and the internal spin-orbit coupling. The spin-orbit coupling smears out the original identities of triplets and singlets while the laser field uses it as an avenue to influence demagnetization. Importantly, this demagnetization can be manipulated controllably, an essential point to future applications, such as ultrafast control of magneto-optical gating. Finally, the polarization filter effect on the ultrafast time scale is discussed based upon the present theoretical results.

• Received 18 August 1999

DOI:https://doi.org/10.1103/PhysRevLett.85.3025