121 research outputs found
Performance of the elliptically polarizing undulator on SPEAR
This is a description of the first tests of the Elliptically Polarizing Undulator (EPU) on the SPEAR storage ring at SSRL. The EPU is the first device of its type; it is capable of producing plane polarized light in the vertical and horizontal, and right and left circularly polarized light in the 500-1000 eV range. Tests of the EPU were done to characterize its effect on the electron beam in SPEAR. Even at minimum gap, motion of the EPU magnets to vary the polarization of the output radiation caused negligible changes in the tune or the steering of the electron beam, even with no compensation of the steering trim coils. We also measured the polarization of x-rays generated by the EPU using a newly developed multilayer polarimeter built to be efficient in the EPU`s energy range. The EPU produces nearly 100% plane and circularly polarized x-rays. Using left and right circularly polarized radiation, we also performed tests of magnetic circular dichroism on magnetic multilayers
Prediction of huge X-ray Faraday rotation at the Gd N_4,5 threshold
X-ray absorption spectra in a wide energy range around the 4d-4f excitation
threshold of Gd were recorded by total electron yield from in-plane magnetized
Gd metal films. Matching the experimental spectra to tabulated absorption data
reveals unprecedented short light absorption lengths down to 3 nm. The
associated real parts of the refractive index for circularly polarized light
propagating parallel or antiparallel to the Gd magnetization, determined
through the Kramers-Kronig transformation, correspond to a magneto-optical
Faraday rotation of 0.7 degrees per atomic layer. This finding shall allow the
study of magnetic structure and magnetization dynamics of lanthanide elements
in nanosize systems and dilute alloys.Comment: 4 pages, 2 figures, final version resubmitted to Phys. Rev. B, Brief
Reports. Minor change
Disorder-induced magnetic memory: Experiments and theories
Beautiful theories of magnetic hysteresis based on random microscopic
disorder have been developed over the past ten years. Our goal was to directly
compare these theories with precise experiments. We first developed and then
applied coherent x-ray speckle metrology to a series of thin multilayer
perpendicular magnetic materials. To directly observe the effects of disorder,
we deliberately introduced increasing degrees of disorder into our films. We
used coherent x-rays to generate highly speckled magnetic scattering patterns.
The apparently random arrangement of the speckles is due to the exact
configuration of the magnetic domains in the sample. In effect, each speckle
pattern acts as a unique fingerprint for the magnetic domain configuration.
Small changes in the domain structure change the speckles, and comparison of
the different speckle patterns provides a quantitative determination of how
much the domain structure has changed. How is the magnetic domain configuration
at one point on the major hysteresis loop related to the configurations at the
same point on the loop during subsequent cycles? The microscopic return-point
memory(RPM) is partial and imperfect in the disordered samples, and completely
absent when the disorder was not present. We found the complementary-point
memory(CPM) is also partial and imperfect in the disordered samples and
completely absent when the disorder was not present. We found that the RPM is
always a little larger than the CPM. We also studied the correlations between
the domains within a single ascending or descending loop. We developed new
theoretical models that do fit our experiments.Comment: 26 pages, 25 figures, Accepted by Physical Review B 01/25/0
Disorder-induced microscopic magnetic memory
Using coherent x-ray speckle metrology, we have measured the influence of
disorder on major loop return point memory (RPM) and complementary point memory
(CPM) for a series of perpendicular anisotropy Co/Pt multilayer films. In the
low disorder limit, the domain structures show no memory with field cycling--no
RPM and no CPM. With increasing disorder, we observe the onset and the
saturation of both the RPM and the CPM. These results provide the first direct
ensemble-sensitive experimental study of the effects of varying disorder on
microscopic magnetic memory and are compared against the predictions of
existing theories.Comment: 4 pages, 4 figures. Accepted for publication in Physical Review
Letters in Nov. 200
Suppressed Magnetization at the Surfaces and Interfaces of Ferromagnetic Metallic Manganites
What happens to ferromagnetism at the surfaces and interfaces of manganites?
With the competition between charge, spin, and orbital degrees of freedom, it
is not surprising that the surface behavior may be profoundly different than
that of the bulk. Using a powerful combination of two surface probes, tunneling
and polarized x-ray interactions, this paper reviews our work on the nature of
the electronic and magnetic states at manganite surfaces and interfaces. The
general observation is that ferromagnetism is not the lowest energy state at
the surface or interface, which results in a suppression or even loss of
ferromagnetic order at the surface. Two cases will be discussed ranging from
the surface of the quasi-2D bilayer manganite
(LaSrMnO) to the 3D Perovskite
(LaSrMnO)/SrTiO interface. For the bilayer manganite,
that is, ferromagnetic and conducting in the bulk, these probes present clear
evidence for an intrinsic insulating non-ferromagnetic surface layer atop
adjacent subsurface layers that display the full bulk magnetization. This
abrupt intrinsic magnetic interface is attributed to the weak inter-bilayer
coupling native to these quasi-two-dimensional materials. This is in marked
contrast to the non-layered manganite system
(LaSrMnO/SrTiO), whose magnetization near the interface
is less than half the bulk value at low temperatures and decreases with
increasing temperature at a faster rate than the bulk.Comment: 15 pages, 13 figure
On the Location and Composition of the Dust in the MCG-6-30-15 Warm Absorber
Hubble Space Telescope images of MCG-6-30-15 show a dust lane crossing the
galaxy just below the nucleus. In this paper, we argue that this dust lane is
responsible for the observed reddening of the nuclear emission and the Fe I
edge hinted at in the Chandra spectrum of MCG-6-30-15. We further suggest that
the gas within the dust lane can comprise much of the low ionization component
(i.e., the one contributing the O VII edge) of the observed warm absorber.
Moreover, placing the warm absorbing material at such distances (hundreds of
pc) can account for the small outflow velocities of the low ionization
absorption lines as well as the constancy of the O VIII edge. Photoionization
models of a dusty interstellar gas cloud (with a column appropriate for the
reddening toward MCG-6-30-15) using a toy Seyfert 1 spectral energy
distribution show that it is possible to obtain a significant O VII edge
(\tau~0.2) if the material is ~150 pc from the ionizing source. For
MCG-6-30-15, such a distance is consistent with the observed dust lane. The
current data on MCG-6-30-15 is unable to constrain the dust composition within
the warm absorber. Astronomical silicate is a viable candidate, but there are
indications of a very low O abundance in the dust, which is inconsistent with a
silicate origin. If true, this may indicate that there were repeated cycles of
grain destruction and growth from shocks in the interstellar medium of
MCG-6-30-15. Pure iron grains are an unlikely dust constituent due to the limit
on their abundance in the Galaxy, yet they cannot be ruled out. The high column
densities inferred from the highly ionized zone of the warm absorber implies
that this gas is dust-free.Comment: 7 pages, 3 Figures, A&A accepte
ELECTROCHEMICALLY ADSORBED Pb ON Ag (111) STUDIED WITH GRAZING-INCIDENCE X-RAY SCATTERING
Probing Adsorption Interactions in Metal-Organic Frameworks using X-ray Spectroscopy
We explore the local electronic signatures of molecular adsorption at coordinatively unsaturated binding sites in the metal-organic framework Mg-MOF-74 using X-ray spectroscopy and first-principles calculations. In situ measurements at the Mg K-edge reveal distinct pre-edge absorption features associated with the unique, open coordination of the Mg sites which are suppressed upon adsorption of CO2 and N,N′-dimethylformamide. Density functional theory shows that these spectral changes arise from modifications of local symmetry around the Mg sites upon gas uptake and are strongly dependent on the metal-adsorbate binding strength. The expanded MOF Mg2(dobpdc) displays the same behavior upon adsorption of CO 2 and N,N′-dimethylethylenediamine. Similar sensitivity to local symmetry is expected for any open metal site, making X-ray spectroscopy an ideal tool for examining adsorption in such MOFs. Qualitative agreement between ambient-temperature experimental and 0 K theoretical spectra is good, with minor discrepancies thought to result from framework vibrational motion. © 2013 American Chemical Society
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