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

The spin-lattice-relaxation time tau(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 tau(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 tau(SL) in metals is essentially given by the spin-orbit induced magnetocrystalline anisotropy energy.