Simple theory for spin-lattice relaxation in metallic rare-earth ferromagnets

The spin-lattice-relaxation time ${\mathrm{\tau }}_{\mathrm{SL}}$ is a key quantity both for the dynamical response of ferromagnets excited by laser pulses and as the speed limit of magneto-optical recording. Extending the theory for the electron paramagnetic resonance of magnetic impurities to spin-lattice relaxation in ferromagnetic rare earths we calculate ${\mathrm{\tau }}_{\mathrm{SL}}$ for Gd and find a value of 48 ps in very good agreement with time-resolved spin-polarized photoemission experiments. We argue that the time scale for ${\mathrm{\tau }}_{\mathrm{SL}}$ in metals is essentially given by the spin-orbit induced magnetocrystalline anisotropy energy. © 1996 The American Physical Society.