237 research outputs found
Magnetic domain fluctuations in an antiferromagnetic film observed with coherent resonant soft x-ray scattering
We report the direct observation of slow fluctuations of helical
antiferromagnetic domains in an ultra-thin holmium film using coherent resonant
magnetic x-ray scattering. We observe a gradual increase of the fluctuations in
the speckle pattern with increasing temperature, while at the same time a
static contribution to the speckle pattern remains. This finding indicates that
domain-wall fluctuations occur over a large range of time scales. We ascribe
this non-ergodic behavior to the strong dependence of the fluctuation rate on
the local thickness of the film.Comment: to appear in Phys. Rev. Let
The cleavage surface of the BaFe_(2-x)Co_(x)As_(2) and Fe_(y)Se_(1-x)Te_(x) superconductors: from diversity to simplicity
We elucidate the termination surface of cleaved single crystals of the
BaFe_(2-x)Co_(x)As_(2) and Fe_(y)Se_(1-x)Te_(x) families of the high
temperature iron based superconductors. By combining scanning tunneling
microscopic data with low energy electron diffraction we prove that the
termination layer of the Ba122 systems is a remnant of the Ba layer, which
exhibits a complex diversity of ordered and disordered structures. The observed
surface topographies and their accompanying superstructure reflections in
electron diffraction depend on the cleavage temperature. In stark contrast,
Fe_(y)Se_(1-x)Te_(x) possesses only a single termination structure - that of
the tetragonally ordered Se_(1-x)Te_(x) layer.Comment: 4 pages, 4 figure
A lattice of microtraps for ultracold atoms based on patterned magnetic films
We have realized a two dimensional permanent magnetic lattice of
Ioffe-Pritchard microtraps for ultracold atoms. The lattice is formed by a
single 300 nm magnetized layer of FePt, patterned using optical lithography.
Our magnetic lattice consists of more than 15000 tightly confining microtraps
with a density of 1250 traps/mm. Simple analytical approximations for the
magnetic fields produced by the lattice are used to derive relevant trap
parameters. We load ultracold atoms into at least 30 lattice sites at a
distance of approximately 10 m from the film surface. The present result
is an important first step towards quantum information processing with neutral
atoms in magnetic lattice potentials.Comment: 7 pages, 7 figure
Fabrication of magnetic atom chips based on FePt
We describe the design and fabrication of novel all-magnetic atom chips for
use in ultracold atom trapping. The considerations leading to the choice of
nanocrystalline exchange coupled FePt as best material are discussed. Using
stray field calculations, we designed patterns that function as magnetic atom
traps. These patterns were realized by spark erosion of FePt foil and e-beam
lithography of FePt film. A mirror magneto-optical trap (MMOT) was obtained
using the stray field of the foil chip.Comment: 5 pages, 5 figure
A high resolution, hard x-ray photoemission investigation of La_(2-2x)Sr_(1+2x)Mn_2O_7 (0.30<x<0.50): on microscopic phase separation and the surface electronic structure of a bilayered CMR manganite
Photoemission data taken with hard x-ray radiation on cleaved single crystals
of the bilayered, colossal magnetoresistant manganite La_(2-2x)Sr_(1+2x)Mn_2O_7
(LSMO) with 0.30<x<0.50 are presented. Making use of the increased
bulk-sensitivity upon hard x-ray excitation it is shown that the core level
footprint of the electronic structure of the LSMO cleavage surface is identical
to that of the bulk. Furthermore, by comparing the core level shift of the
different elements as a function of doping level x, it is shown that
microscopic phase separation is unlikely to occur for this particular manganite
well above the Curie temperature.Comment: 7 pages, 5 figure
Nanoscale superconducting gap variations, strong coupling signatures and lack of phase separation in optimally doped BaFe1.86Co0.14As2
We present tunneling data from optimally-doped, superconducting
BaFe1.86Co0.14As2 and its parent compound, BaFe2As2. In the superconductor,
clear coherence-like peaks are seen across the whole field of view, and their
analysis reveals nanoscale variations in the superconducting gap value, Delta.
The average magnitude of 2Delta is ~7.4 kBTC, which exceeds the BCS weak
coupling value for either s- or d-wave superconductivity. The characteristic
length scales of the deviations from the average gap value, and of an
anti-correlation discovered between the gap magnitude and the zero bias
conductance, match well with the average separation between the Co dopant ions
in the superconducting FeAs planes. The tunneling spectra themselves possess a
peak-dip-hump lineshape, suggestive of a coupling of the superconducting
electronic system to a well-defined bosonic mode of energy 4.7 kBTC, such as
the spin resonance observed recently in inelastic neutron scattering.Comment: 4 figures, corrected typos, reduced size of image
Magnetization reversal, asymmetry, and role of uncompensated spins in perpendicular exchange coupled systems
This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.Soft x-ray resonant magnetic scattering has been used to investigate the element-selective microscopic magnetization reversal behavior of room temperature perpendicular exchange coupled ferromagnetic∕antiferromagnetic (F∕AF) systems and to study the role of the interfacial coupling strength on it. Different nucleation processes and domain size distributions along the decreasing and increasing branches of the reversal have been found. The size of the magnetic domains during reversal depends on both the F anisotropy and F∕AF coupling strength, decreasing when one of them increases. Evidence of the exchange bias(coercivity enhancement) being induced by pinned (unpinned) uncompensated AF interfacial spins is also shown
Magnetic x‐ray dichroism of rare‐earth materials
We discuss recent developments in the magnetic x‐ray dichroism of rare‐earth materials. The application of this technique to the study of magnetic materials is discussed. Also, other work on magneto‐optical effects in the x‐ray range is reviewed
Surface adatom conductance filtering in scanning tunneling spectroscopy of Co-doped BaFe2As2 iron pnictide superconductors
We establish in a combination of ab initio theory and experiments that the
tunneling process in scanning tunneling microscopy/spectroscopy on the A-122
iron pnictide superconductors - in this case BaFeCoAs - involve
a strong adatom filtering of the differential conductance from the near-EF Fe3d
states, which in turn originates from the top-most sub-surface Fe layer of the
crystal. The calculations show that the dominance of surface Ba-related
tunneling pathways leaves fingerprints found in the experimental differential
conductance data, including large particle-hole asymmetry and an
energy-dependent contrast inversion.Comment: 4.5 pages, 4 figures. To appear in Physical Review Letter
Rare earth contributions to the X-ray magnetic circular dichroism at the Co K edge in rare earth-cobalt compounds investigated by multiple-scattering calculations
The X-ray magnetic circular dichroism (XMCD) has been measured at the Co K
edge in Co-hcp and R-Co compounds (R=La, Tb, Dy). The structure of the
experimental XMCD spectra in the near-edge region has been observed to be
highly sensitive to the magnetic environment of the absorbing site.
Calculations of the XMCD have been carried out at the Co K edge in Co metal,
LaCo and TbCo within the multiple-scattering framework including the
spin-orbit coupling. In the three systems, the XMCD spectra in the near-edge
region are well reproduced. The possibility to separate and quantitatively
estimate the local effects from those due to the neighboring atoms in the XMCD
cross section makes possible a more physical understanding of the spectra. The
present results emphasize the major role played by the states of the Tb
ions in the XMCD spectrum at the Co K edge in the TbCo compound.Comment: 34 pages, revtex, 10 eps figures included with epsf, after referee
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