Element-specific spin and orbital momentum dynamics of Fe/Gd multilayers

The role of orbital magnetism in the laser-induced demagnetization of Fe/Gd multilayers was investigated using time-resolved X-ray magnetic circular dichroism at 2-ps time resolution given by an xray streak camera. An ultrafast transfer of angular momentum from the spin via the orbital momentum to the lattice was observed which was characterized by rapidly thermalizing spin and orbital momenta. Strong interlayer exchange coupling between Fe and Gd led to a simultaneous demagnetization of both layers. 1 Author to whom correspondence should be addressed; electronic mail: afbartelt@lbl.gov. 2 Ultrafast magnetic storage and processing is founded on our ability to control magnetism on picosecond and femtosecond time scales. Magnetic phase transitions conserve the total angular momentum and usually involve the crystal lattice as a quasi-infinite reservoir of angular momentum. A prototypical ultrafast magnetic phenomenon is the demagnetization after excitation by an intense laser pulse The Fe/Gd multilayer consists of two metals of very different electronic structure. Fe has exchange-split 3d spin bands which intersect the Fermi surface, allowing both low-energy spin-flip (Stoner) and spin wave excitations (magnons). The spin momentum dominates the total angular momentum while the orbital momentum is quenched by the strong ligand field and only partially restored by the spin-orbit interaction. The coupling of the orbital momentum to the anisotropic ligand field enables the flow of angular momentum from the spin system to the lattice during the demagnetization. A direct photon-driven exchange of spin and orbital momentum as proposed by Hübner 3 Early experiments on Gd suggested a slow laser-induced demagnetization in tens of picoseconds Our experiments were performed on a stack of 20 alternating 0.5-nm Fe and Gd layers grown on top of a 200-nm Al heat sink, protected by a thin Al cap layer, and supported by a 100 nm silicon nitride membrane. At and above room temperature the easy magnetization direction was out-of-plane. The thin layers were antiferromagnetically coupled with a common Curie temperature of about 230°C. In order to separate the transient dynamics of the Fe 3d and Gd 4f spin and orbital momenta in the Fe/Gd multilayer, we extended time-resolved XMCD [12, 13] into a laser pump -x-ray probe technique. XMCD has the unique ability to separate and quantify spin and orbital momenta with element specificity Also, XMCD avoids laser pump-induced state-filling effects since the spin-dependent band occupation is determined by recording the absorption cross-section for circularly polarized x-rays, exciting electrons from a spin-orbit-split core level into the valence states. Integration over the absorption resonances accounts for all unoccupied states. The demagnetization dynamics was initiated by heating the sample above the Curie temperature with 60-fs (full width at half maximum (FWHM)) long 800-nm laser pulses at an intensity of 20 mJ/cm 2 and 5 kHz, and probed with 60-ps (FWHM) x-ray pulses from the elliptically polarizing undulator beamline 4.0 at the Advanced Light Source Two representative streaked x-ray pulses are shown in Starting from the integrated transient Fe L 3,2 and Gd M 4,5 dichroism, the transient spin momentum m s (t) = <S z > and orbital momentum m l (t) = <L z > were determined by using sum rules orbital momentum m l (t) before and after t=0 is zero-within experimental errors-in agreement with our expectation that the Gd 4f demagnetization occurs indirectly via exchange with Gd 5d states. The Fe 3d orbital momentum m l (t) decays simultaneously with the Fe spin momentum m s (t). This can be seen more clearly when 5 m l (t) and m s (t) are normalized to their values before t=0, The ultrafast dynamics in Fe/Gd is a true demagnetization as angular momentum is transferred from spin and orbital momentum to the lattice, which acts as a sink. The demagnetization is not primarily the result of a rearrangement of angular momentum between spin and orbit, which would be visible as a change in the orbital to spin momentum ratio. Note that a partial demagnetization in a non-equilibrium situation may be possible without coupling to the lattice because of the different g-factors of electron spin and orbital momenta. It is clear that the Fe spin-orbit interaction does not constitute a bottleneck in the demagnetization of Fe/Gd because the Fe spin and orbital momenta are in or close to equilibrium. The slow dynamics in Fe/Gd, compared to N