232 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
Intrinsic and extrinsic x-ray absorption effects in soft x-ray diffraction from the superstructure in magnetite
We studied the (001/2) diffraction peak in the low-temperature phase of
magnetite (Fe3O4) using resonant soft x-ray diffraction (RSXD) at the Fe-L2,3
and O-K resonance. We studied both molecular-beam-epitaxy (MBE) grown thin
films and in-situ cleaved single crystals. From the comparison we have been
able to determine quantitatively the contribution of intrinsic absorption
effects, thereby arriving at a consistent result for the (001/2) diffraction
peak spectrum. Our data also allow for the identification of extrinsic effects,
e.g. for a detailed modeling of the spectra in case a "dead" surface layer is
present that is only absorbing photons but does not contribute to the
scattering signal.Comment: to appear in Phys. Rev.
Electronic structure, magnetic and dielectric properties of the edge-sharing copper-oxide chain compound NaCuO
We report an experimental study of \nco, a Mott insulator containing chains
of edge-sharing CuO plaquettes, by polarized x-ray absorption spectroscopy
(XAS), resonant magnetic x-ray scattering (RMXS), magnetic susceptibility, and
pyroelectric current measurements. The XAS data show that the valence holes
reside exclusively on the Cu sites within the copper-oxide spin chains
and populate a -orbital polarized within the CuO plaquettes. The RMXS
measurements confirm the presence of incommensurate magnetic order below a
N\'eel temperature of K, which was previously inferred from
neutron powder diffraction and nuclear magnetic resonance data. In conjunction
with the magnetic susceptibility and XAS data, they also demonstrate a new
"orbital" selection rule for RMXS that is of general relevance for magnetic
structure determinations by this technique. Dielectric property measurements
reveal the absence of significant ferroelectric polarization below , which
is in striking contrast to corresponding observations on the isostructural
compound \lco. The results are discussed in the context of current theories of
multiferroicity.Comment: 7 pages, 7 figure
Direct observation of t2g orbital ordering in magnetite
Using soft-x-ray diffraction at the site-specific resonances in the Fe L23
edge, we find clear evidence for orbital and charge ordering in magnetite below
the Verwey transition. The spectra show directly that the (001/2) diffraction
peak (in cubic notation) is caused by t2g orbital ordering at octahedral Fe2+
sites and the (001) by a spatial modulation of the t2g occupation.Comment: to appear in Phys. Rev. Let
Momentum-dependent charge correlations in YBaCuO superconductors probed by resonant x-ray scattering: Evidence for three competing phases
We have used resonant x-ray scattering to determine the momentum dependent
charge correlations in YBaCuO samples with highly ordered
chain arrays of oxygen acceptors (ortho-II structure). The results reveal
nearly critical, biaxial charge density wave (CDW) correlations at in-plane
wave vectors (0.315, 0) and (0, 0.325). The corresponding scattering intensity
exhibits a strong uniaxial anisotropy. The CDW amplitude and correlation length
are enhanced as superconductivity is weakened by an external magnetic field.
Analogous experiments were carried out on a YBaCuO crystal with
a dilute concentration of spinless (Zn) impurities, which had earlier been
shown to nucleate incommensurate magnetic order. Compared to pristine crystals
with the same doping level, the CDW amplitude and correlation length were found
to be strongly reduced. These results indicate a three-phase competition
between spin-modulated, charge-modulated, and superconducting states in
underdoped YBaCuO.Comment: 6 pages, 3 figures revised version, to appear in Phys. Rev. Let
Spectroscopy of stripe order in La1.8Sr0.2NiO4 using resonant soft x-ray diffraction
Strong resonant enhancements of the charge-order and spin-order
superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when
x-ray energies in the vicinity of the Ni L2,3 absorption edges are used. The
pronounced photon-energy and polarization dependences of these diffraction
intensities allow for a critical determination of the local symmetry of the
ordered spin and charge carriers. We found that not only the antiferromagnetic
order but also the charge-order superstructure resides within the NiO2 layers;
the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with
the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the
cuprates.Comment: 4 pages, 3 figure
Long range antiferromagnetic order of formally nonmagnetic Eu3 Van Vleck ions observed in multiferroic Eu1 xYxMnO3
We report on resonant magnetic x ray scattering and absorption spectroscopy studies of exchange coupled antiferromagnetic ordering of Eu3 magnetic moments in multiferroic Eu1 amp; 8722;xYxMnO3 in the absence of an external magnetic field. The observed resonant spectrum is characteristic of a magnetically ordered 7F1 state that mirrors the Mn magnetic ordering, due to exchange coupling between the Eu 4f and Mn 3d spins. Here, we observe long range magnetic order generated by exchange coupling of magnetic moments of formally nonmagnetic Van Vleck ions, which is a step further towards the realization of exotic phases induced by exchange coupling in systems entirely composed of non magnetic ion
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