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

Abstract

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 Author to whom correspondence should be addressed; electronic mail: a_scholl@lbl.gov. 2Ultrafast 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 [1][2][3][4][5]. Here, the orbital momentum is crucial as it links the electron spin, which carries most of the magnetic moment, to the lattice via the spinorbit interaction. In this letter, we investigate the orbital momentum dynamics during an ultrafast demagnetization in the model system Fe/Gd using X-ray magnetic circular dichroism (XMCD) [6].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 [7] would, for example, appear as a temporary accumulation of orbital and concomitant reduction of spin momentum. In contrast, a bottleneck caused by the spin-orbit interaction would be visible as a reduced orbital to spin momentum ratio. The second component of the multilayer, Gd, is best described as a Heisenberg ferromagnet with localized 4f electrons. Gd does not exhibit an orbital momentum in the 4f shell, which is half full. A large exchange energy of about 11 eV separates the majority and minority 4f states, inhibiting low-energy spin-flip excitations. Magnetic long range order in Gd is established via 4f-5d exchange with the Gd 5d valence states and their exchange interaction with Gd 5d orbitals of nearest neighbors [8]. Therefore, the Gd 4f demagnetization occurs indirectly via 4f-5d exchange and subsequent 5d electronphonon scattering while the Fe demagnetization occurs directly via 3d electron-phonon scattering. The Gd orbital momentum should t...

Nonequilibrium Magnetization Dynamics of Gadolinium Studied by Magnetic Linear Dichroism in Time-Resolved4fCore-Level Photoemission

“…Recently, Bartelt et al reported on an element-specific magnetization dynamics in Fe=Gd multilayers using x-ray magnetic circular dichroism (MCD). Interestingly, an almost simultaneous demagnetization is observed probing the Fe valence and Gd 4f shell, which could occur, e.g., due to hybridization of both constituents or the intense excitation by F abs 20 mJ=cm 2 [19].…”

Subpicosecond Magnetization Reversal across Ferrimagnetic Compensation Points

“…An interesting ferrimagnetic system for this study is the family of amorphous rare-earth (RE)-transition-metal (TM) alloys, such as GdFeCo, for which the compensation points can be tuned in a wide temperature range by simply varying its composition [20]. However, due to the localized character of the spins responsible for magnetism in the RE ferrimagnetic sublattice, it is not clear whether its magnetization can be changed by laser pulses on a fs time scale [21].…”

Role of the inter-sublattice exchange coupling in short-laser-pulse-induced demagnetization dynamics of GdCo and GdCoFe alloys

“…The interaction of a subpicosecond optical laser pulse with spins in a magnetic system has become a rapidly growing area of science [1][2][3][4][5][6][7][8][9][10][11] since the pioneering work by Beaurepaire et al Using a 60 fs optical laser pulse, they demonstrated an ultrafast (within about 1 ps) demagnetization in a Ni thin film. 1 After this observation, research on laser-induced demagnetization has been conducted at large in ferromagnetic transition metals (TMs) (e.g., Co, Ni), rare-earth (RE) metals (e.g., Gd, Tb), and their alloys (see Ref.…”