Unifying Ultrafast Magnetization Dynamics

We present a microscopic model that successfully explains the ultrafast equilibration of magnetic order in ferromagnetic metals at a time scale ${ensuremath{tau}}_{M}$ of only a few hundred femtoseconds after pulsed laser excitation. It is found that ${ensuremath{tau}}_{M}$ can be directly related to the so-called Gilbert damping factor $ensuremath{alpha}$ that describes damping of GHz precessional motion of the magnetization vector. Independent of the spin-scattering mechanism, an appealingly simple equation relating the two key parameters via the Curie temperature ${T}_{C}$ is derived, ${ensuremath{tau}}_{M}ensuremath{approx}{c}_{0}ensuremath{hbar}/{k}_{B}{T}_{C}ensuremath{alpha}$, with $ensuremath{hbar}$ and ${k}_{B}$ the Planck and Boltzmann constants, respectively, and the prefactor ${c}_{0}ensuremath{sim}frac{1}{4}$. We argue that phonon-mediated spin-flip scattering may contribute significantly to the sub-ps response.