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

The spin-lattice relaxation time τ SL is a key quantity both for the dynam-ical response of ferromagnets excited by laser pulses and as the speed limit of magneto-optical recording. Extending the theory for the electron param-agnetic resonance of magnetic impurities to spin-lattice relaxation in ferro-magnetic rare earths we calculate τ 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 τ SL in metals is essentially given by the spin-orbit induced magnetocrystalline anisotropy energy.