Unifying Ultrafast Magnetization Dynamics

Abstract

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.