We present the development and characterization of a dedicated resonant soft x-ray scattering facility. Capable of operation over a wide energy range, the beamline and endstation are primarily used for scattering from soft matter systems around the carbon K-edge (∼285 eV). We describe the specialized design of the instrument and characteristics of the beamline. Operational characteristics of immediate interest to users such as polarization control, degree of higher harmonic spectral contamination, and detector noise are delineated. Of special interest is the development of a higher harmonic rejection system that improves the spectral purity of the x-ray beam. Special software and a user-friendly interface have been implemented to allow real-time data processing and preliminary data analysis simultaneous with data acquisition.
Ceramic matrix composites are the emerging material of choice for structures that will see temperatures above ~1,500 °C in hostile environments, as for example in next-generation gas turbines and hypersonic-flight applications. The safe operation of applications depends on how small cracks forming inside the material are restrained by its microstructure. As with natural tissue such as bone and seashells, the tailored microstructural complexity of ceramic matrix composites imparts them with mechanical toughness, which is essential to avoiding failure. Yet gathering three-dimensional observations of damage evolution in extreme environments has been a challenge. Using synchrotron X-ray computed microtomography, we have fully resolved sequences of microcrack damage as cracks grow under load at temperatures up to 1,750 °C. Our observations are key ingredients for the high-fidelity simulations used to compute failure risks under extreme operating conditions.
With a membrane based mechanism to allow for pressure change of a sample in a radial diffraction diamond anvil cell (rDAC) and simultaneous infra-red laser heating, it is now possible to investigate texture changes during deformation and phase transformations over a wide range of temperature-pressure conditions. The device is used to study bcc (α), fcc (γ) and hcp (ε) iron. In bcc iron, room temperature compression generates a texture characterized by (100) and (111) poles parallel to the compression direction. During the deformation induced phase transformation to hcp iron, a subset of orientations are favored to transform to the hcp structure first and generate a texture of (01-10) at high angles to the compression direction. Upon further deformation, the remaining grains transform, resulting in a texture that obeys the Burgers relationship of (110) bcc // (0001) hcp . This is in contrast to high temperature results that indicate that texture is developed through dominant pyramidal 〈a+c〉 {2-1-12}〈2-1-13〉 and basal (0001)〈2-1-10〉 slip based on polycrystal plasticity modeling. We also observe that the high temperature fcc phase develops a 110 texture typical for fcc metals deformed in compression.
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...
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