We investigate the magnetic properties of Mn adsorbates on Fe͑100͒ in the regime up to a few monolayers.Magnetic circular dichroism in absorption shows long-range ferromagnetic order for the Mn adsorbate, with antiferromagnetic alignment with respect to the Fe substrate. Element-specific magnetic domain imaging and hysteresis measurements show that the macroscopic magnetic behavior of the Mn adlayer is fully determined by the Fe substrate. For coverages below 0.5 ML the Mn absorption spectra show rich structures that are typical for localized d states. From this the Mn ground state is identified as a mixture of atomiclike d 5 and d 6 states, with a local spin moment of 4.5 B . However, the circular dichroism is 2.4 times smaller than expected for this ground state, suggesting disorder within the Mn adsorbate with an ordered moment of 1.9 B at 120 K. The magnetic signal vanishes near 1 ML coverage, consistent with the theoretically predicted c(2ϫ2) antiferromagnetic ground state of the monolayer.
We report a dependence of the photoabsorption cross section as measured by total electron yield at the Fe and Co 3p thresholds on the sign of the magnetization for p-polarized light at oblique incidence.Peak-to-peak asymmetries up to 4% are observed. The asymmetry increases towards grazing incidence, where the total sample current is smallest.The transverse magneto-optic Kerr effect measured simultaneously shows a peak-to-peak asymmetry of up to 23% at the Co 3p threshold. The dichroism is used to image magnetic domains on an Fe (100) surface in a photoelectron emission microscope. PACS numbers: 78.20.Ls, 75.60.Ch, 78.70.Dm Spectroscopy of the core levels of the ferromagnetic transition elements has shown in recent years to be surprisingly rich in features caused by the Coulomb and exchange interactions between the core hole and the spinpolarized valence electrons. X-ray absorption experiments (XAS) with linearly polarized light [1] show a change of the spectra when the polarization vector is either parallel or perpendicular to the magnetization axis, according to the selection rules Am = 0 or Am =~1 , respectively. Magnetic circular dichroism (MCD) in x-ray absorption [2,3]can be understood as a spin-dependent excitation of core electrons into empty states above the Fermi level, whichbecause of the ferromagnetism are spin polarized [4].The spin dependence in the excitation arises from coupling between light helicity and the orbital momentum of the excited electron, which is in turn coupled to the spin of the photoexcited electron by spin-orbit interaction. Sum rules, which allow one to extract quantitatively spin and orbital moments, make MCD in absorption a very attractive technique [5]. By measuring the photoabsorption cross section via the total photoelectron yield of the sample, it is even possible to obtain spatially resolved and chemically specific magnetic information [6].In photoemission, even though the final state of the photoelectron far above the Fermi level has only negligible exchange and spin-orbit interactions, related effects can also be observed. It is well known that core level photoelectrons from ferromagnets in general carry a spin polarization due to exchange [7]. Apart from magnetic circular and "conventional" (as described above for XAS) linear dichroisrn, a new type of magnetic linear dichroism was recently reported for photoemission appearing on magnetization reversal [8]. This effect requires that the vectors of magnetization, electric radiation field, and electron emission form a chiral system with the angle between light polarization and electron emission different from 90, and occurs only in angle-resolved experiments. In photoabsorption, the angular acceptance of the excited electrons is not restricted, so that it is, in principle, an angle-integrating experiment. However, Kao and co-workers [9] reported in 1990 for photon energies near the Fe 2p excitation thresholds a change of the refIectivity for obliquely incident p-polarized light when the magnetization is switched between th...
Multilayer Laue lenses are diffractive optics for hard X-rays. To achieve high numerical aperture and resolution, diffracting structures of nanometer periods are required in such lenses, and a thickness (in the direction of propagation) of several micrometers is needed for high diffracting efficiency. Such structures must be oriented to satisfy Bragg’s law, which can only be achieved consistently over the entire lens if the layers vary in their tilt relative to the incident beam. The correct tilt, for a particular wavelength, can be achieved with a very simple technique of using a straight-edge mask to give the necessary gradient of the layers. An analysis of the properties of lenses cut from such a shaded profile is presented and it is shown how to design, prepare, and characterize matched pairs of lenses that operate at a particular wavelength and focal length. It is also shown how to manufacture lenses with ideal curved layers for optimal efficiency.
Improvements in x-ray optics critically depend on the measurement of their optical performance. The knowledge of wavefront aberrations, for example, can be used to improve the fabrication of optical elements or to design phase correctors to compensate for these errors. At present, the characterization of such optics is made using intense x-ray sources, such as synchrotrons. However, the limited access to these facilities can substantially slow down the development process. Improvements in the brightness of lab-based x-ray micro-sources in combination with the development of new metrology methods, particularly ptychographic x-ray speckle tracking, enable characterization of x-ray optics in the lab with a precision and sensitivity not possible before. Here, we present a laboratory setup that utilizes a commercially available x-ray source and can be used to characterize different types of x-ray optics. The setup is used in our laboratory on a routine basis to characterize multilayer Laue lenses of high numerical aperture and other optical elements. This typically includes measurements of the wavefront distortions, optimum operating photon energy, and focal length of the lens. To check the sensitivity and accuracy of this laboratory setup, we compared the results to those obtained at the synchrotron and saw no significant difference. To illustrate the feedback of measurements on performance, we demonstrated the correction of the phase errors of a particular multilayer Laue lens using a 3D printed compound refractive phase plate.
We demonstrate that X-ray fluorescence emission, which cannot maintain a stationary interference pattern, can be used to obtain images of structures by recording photon-photon correlations in the manner of the stellar intensity interferometry of Hanbury Brown and Twiss. This is achieved utilising femtosecond-duration pulses of a hard X-ray free-electron laser to generate the emission in exposures comparable to the coherence time of the fluorescence. Iterative phasing of the photon correlation map generated a model-free real-space image of the structure of the emitters. Since fluorescence can dominate coherent scattering, this may enable imaging uncrystallised macromolecules.
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