N. Pontius

Distinguishing the ultrafast dynamics of spin and orbital moments in solids

For an isolated quantum particle, such as an electron, the orbital (L) and spin (S) magnetic moments can change provided that the total angular momentum of the particle is conserved. In condensed matter, an efficient transfer between L and S can occur owing to the spin–orbit interaction, which originates in the relativistic motion of electrons. Disentangling the absolute contributions of the orbital and spin angular momenta is challenging, however, as any transfer between the two occurs on femtosecond timescales. Here we investigate such phenomena by using ultrashort optical laser pulses to change the magnetization of a ferromagnetic film and then probe its dynamics with circularly polarized femtosecond X-ray pulses. Our measurements enable us to disentangle the spin and orbital components of the magnetic moment, revealing different dynamics for L and S. We highlight the important role played by the spin–orbit interaction in the ultrafast laser-induced demagnetization of ferromagnetic films, and show also that the magneto-crystalline anisotropy energy is an important quantity to consider in such processes. Our study provides insights into the dynamics in magnetic systems as well as perspectives for the ultrafast control of information in magnetic recording media.

Ultrafast spin transport as key to femtosecond demagnetization

Irradiating a ferromagnet with a femtosecond laser pulse is known to induce an ultrafast demagnetization within a few hundred femtoseconds. Here we demonstrate that direct laser irradiation is in fact not essential for ultrafast demagnetization, and that electron cascades caused by hot electron currents accomplish it very efficiently. We optically excite a Au/Ni layered structure in which the 30 nm Au capping layer absorbs the incident laser pump pulse and subsequently use the X-ray magnetic circular dichroism technique to probe the femtosecond demagnetization of the adjacent 15 nm Ni layer. A demagnetization effect corresponding to the scenario in which the laser directly excites the Ni film is observed, but with a slight temporal delay. We explain this unexpected observation by means of the demagnetizing effect of a superdiffusive current of non-equilibrium, non-spin-polarized electrons generated in the Au layer.

Hot-Electron-Driven Enhancement of Spin-Lattice Coupling in Gd and Tb 4 f Ferromagnets Observed by Femtosecond X-Ray Magnetic Circular Dichroism

Femtosecond x-ray magnetic circular dichroism was used to study the time-dependent magnetic moment of 4f electrons in the ferromagnets Gd and Tb, which are known for their different spin-lattice coupling. We observe a two-step demagnetization with an ultrafast demagnetization time of 750 fs identical for both systems and slower times which differ sizeably with 40 ps for Gd and 8 ps for Tb. We conclude that spin-lattice coupling in the electronically excited state is enhanced up to 50 times compared to equilibrium.

Stimulated X-ray emission for materials science

Resonant inelastic X-ray scattering and X-ray emission spectroscopy can be used to probe the energy and dispersion of the elementary low-energy excitations that govern functionality in matter: vibronic, charge, spin and orbital excitations. A key drawback of resonant inelastic X-ray scattering has been the need for high photon densities to compensate for fluorescence yields of less than a per cent for soft X-rays. Sample damage from the dominant non-radiative decays thus limits the materials to which such techniques can be applied and the spectral resolution that can be obtained. A means of improving the yield is therefore highly desirable. Here we demonstrate stimulated X-ray emission for crystalline silicon at photon densities that are easily achievable with free-electron lasers. The stimulated radiative decay of core excited species at the expense of non-radiative processes reduces sample damage and permits narrow-bandwidth detection in the directed beam of stimulated radiation. We deduce how stimulated X-ray emission can be enhanced by several orders of magnitude to provide, with high yield and reduced sample damage, a superior probe for low-energy excitations and their dispersion in matter. This is the first step to bringing nonlinear X-ray physics in the condensed phase from theory to application.

Speed limit of the insulator–metal transition in magnetite

As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator-metal, or Verwey, transition has long remained inaccessible. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase. Here we investigate the Verwey transition with pump-probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator-metal transition. We find this to be a two-step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5±0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics.

Metal-to-ligand and ligand-to-metal charge transfer in thin films of Prussian blue analogues investigated by X-ray absorption spectroscopy

A series of thin films of Prussian blue analogues is investigated by X-ray absorption spectroscopy (XAS) at the Fe, Co and Mn L(2,3)-edges. The ligand field multiplet theory enables us to examine accurately the electronic structure of these materials. Experimental XAS spectra of CoFe Prussian blue analogues are successfully reproduced using a ground state configuration including metal-to-ligand (MLCT) and ligand-to-metal charge transfer (LMCT) at the Co and Fe L(2,3)-edges. In particular, a huge improvement is achieved for satellite peaks at the Co(iii) L(2,3)-edges compared to previous calculations in the literature based on LMCT effects only. On the other hand, XAS spectra of MnFe analogues synthesized for the first time, can be reproduced conveniently by taking into account either MLCT or LMCT depending on the conditions of the sample preparation. For each thin film, the proportion of the different oxidation states of Co, Fe and Mn is evaluated. Unexpectedly, this analysis reveals the presence of a significant amount of a reduced phase, which turns out to be strongly dependent on the sample synthesis and storage conditions.

Ultrafast angular momentum transfer in multisublattice ferrimagnets

Femtosecond laser pulses can be used to induce ultrafast changes of the magnetization in magnetic materials. However, one of the unsolved questions is that of conservation of the total angular momentum during the ultrafast demagnetization. Here we report the ultrafast transfer of angular momentum during the first hundred femtoseconds in ferrimagnetic Co0.8Gd0.2 and Co0.74Tb0.26 films. Using time-resolved X-ray magnetic circular dichroism allowed for time-resolved determination of spin and orbital momenta for each element. We report an ultrafast quenching of the magnetocrystalline anisotropy and show that at early times the demagnetization in ferrimagnetic alloys is driven by the local transfer of angular momenta between the two exchange-coupled sublattices while the total angular momentum stays constant. In Co0.74Tb0.26 we have observed a transfer of the total angular momentum to an external bath, which is delayed by ~150 fs.

Femtosecond x-ray absorption spectroscopy of spin and orbital angular momentum in photoexcited Ni films during ultrafast demagnetization

BESSY II, Helmholtz-Zentrum Berlin fu¨r Materialien und Energie GmbH,Albert-Einstein-Strasse 15, 12489 Berlin, Germany(Dated: March 19, 2010)We follow for the first time the evolution of the spin and orbital angular momentum of a thinNi film during ultrafast demagnetization, by means of x-ray magnetic circular dichroism. Bothcomponents decrease with a 130 ± 40 fs time constant upon excitation with a femtosecond laserpulse. Additional x-ray absorption measurements reveal an increase in the spin-orbit interaction by6±2 % during this process. This is the experimental observation of a transient change in spin-orbitinteraction during ultrafast demagnetization.

A novel monochromator for experiments with ultrashort X‐ray pulses

Aiming at advancing storage-ring-based ultrafast X-ray science, over the past few years many upgrades have been undertaken to continue improving beamline performance and photon flux at the Femtoslicing facility at BESSY II. In this article the particular design upgrade of one of the key optical components, the zone-plate monochromator (ZPM) beamline, is reported. The beamline is devoted to optical pump/soft X-ray probe applications with 100 fs (FWHM) X-ray pulses in the soft X-ray range at variable polarization. A novel approach consisting of an array of nine off-axis reflection zone plates is used for a gapless coverage of the spectral range between 410 and 1333 eV at a designed resolution of E/ΔE = 500 and a pulse elongation of only 30 fs. With the upgrade of the ZPM the following was achieved: a smaller focus, an improved spectral resolution and bandwidth as well as excellent long-term stability. The beamline will enable a new class of ultrafast applications with variable optical excitation wavelength and variable polarization.

Time-resolved resonant soft x-ray diffraction with free-electron lasers: Femtosecond dynamics across the Verwey transition in magnetite

Resonant soft x-ray diffraction (RSXD) with femtosecond (fs) time resolution is a powerful tool for disentangling the interplay between different degrees of freedom in strongly correlated electron materials. It allows addressing the coupling of particular degrees of freedom upon an external selective perturbation, e.g., by an optical or infrared laser pulse. Here, we report a time-resolved RSXD experiment from the prototypical correlated electron material magnetite using soft x-ray pulses from the free-electron laser FLASH in Hamburg. We observe ultrafast melting of the charge-orbital order leading to the formation of a transient phase, which has not been observed in equilibrium.