479 research outputs found
Rare earth magnetism and ferroelectricity in RMnO3
Magnetic rare earths R have been proven to have a significant effect on the
multiferroic properties of the orthorhombic manganites RMnO3. A re-examination
of previous results from synchrotron based x-ray scattering experiments
suggests that symmetric exchange striction between neighboring R and Mn ions
may account for the enhancement of the ferroelectric polarization in DyMnO3 as
well as the magnetic-field induced ferroelectricity in GdMnO3. In general,
adding a second magnetic species to a multiferroic material may be a route to
enhance its ferroelectric properties.Comment: Contribution to ICM 2009; accepted for publication in Journal of
Physics: Conference Serie
Commensurate Dy magnetic ordering associated with incommensurate lattice distortion in orthorhombic DyMnO3
Synchrotron x-ray diffraction and resonant magnetic scattering experiments on
single crystal DyMnO3 have been carried out between 4 and 40 K. Below TN(Dy) =
5K, the Dy magnetic moments order in a commensurate structure with propagation
vector 0.5 b*. Simultaneous with the Dy magnetic ordering, an incommensurate
lattice modulation with propagation vector 0.905 b* evolves while the original
Mn induced modulation is suppressed and shifts from 0.78 b* to 0.81 b*. This
points to a strong interference of Mn and Dy induced structural distortions in
DyMnO3 besides a magnetic coupling between the Mn and Dy magnetic moments.Comment: submitted to Phys. Rev. B Rapid Communication
Enhanced ferroelectric polarization by induced Dy spin-order in multiferroic DyMnO3
Neutron powder diffraction and single crystal x-ray resonant magnetic
scattering measurements suggest that Dy plays an active role in enhancing the
ferroelectric polarization in multiferroic DyMnO3 above TNDy = 6.5 K. We
observe the evolution of an incommensurate ordering of Dy moments with the same
periodicity as the Mn spiral ordering. It closely tracks the evolution of the
ferroelectric polarization which reaches a maximum value of 0.2 muC/m^2. Below
TNDy, where Dy spins order commensurately, the polarization decreases to values
similar for those of TbMnO3
Transition from a phase-segregated state to single-phase incommensurate sodium ordering in Na_xCoO_2 with x \approx 0.53
Synchrotron X-ray diffraction investigations of two single crystals of
Na_xCoO_2 from different batches with composition x = 0.525-0.530 reveal
homogeneous incommensurate sodium ordering with propagation vector (0.53 0.53
0) at room-temperature. The incommensurate (qq0) superstructure exists between
220 K and 430 K. The value of q varies between q = 0.514 and 0.529, showing a
broad plateau at the latter value between 260 K and 360 K. On cooling, unusual
reversible phase segregation into two volume fractions is observed. Below 220
K, one volume fraction shows the well-known commensurate orthorhombic x = 0.50
superstructure, while a second volume fraction with x = 0.55 exhibits another
commensurate superstructure, presumably with a 6a x 6a x c hexagonal supercell.
We argue that the commensurate-to-incommensurate transition is an intrinsic
feature of samples with Na concentrations x = 0.5 + d with d ~ 0.03.Comment: Corrected/improved versio
Coupling of frustrated Ising spins to magnetic cycloid in multiferroic TbMnO3
We report on diffraction measurements on multiferroic TbMnO3 which
demonstrate that the Tb- and Mn-magnetic orders are coupled below the
ferroelectric transition TFE = 28 K. For T < TFE the magnetic propagation
vectors (tau) for Tb and Mn are locked so that tauTb = tauMn, while below TNTb
= 7 K we find that tauTb and tauMn lock-in to rational values of 3/7 b* and 2/7
b*, respectively, and obey the relation 3tauTb - tauMn = 1. We explain this
novel matching of wave vectors within the frustrated ANNNI model coupled to a
periodic external field produced by the Mn-spin order. The tauTb = tauMn
behavior is recovered when Tb magnetization is small, while the tauTb = 3/7
regime is stabilized at low temperatures by a peculiar arrangement of domain
walls in the ordered state of Ising-like Tb spins.Comment: 5 pages, 3 figure
Albanian grapevine cultivars: preliminary results of molecular, phenolic and ampelometric profiles and relatedness
'Shesh i Zi', 'Shesh i Bardhë', 'Kallmet' and 'Vlosh' are commercialised grapevine wine varieties which best represent the high variability of the Albanian autochthonous germplasm because of their different origin within the country and their great wine potential. We characterised these cultivars at molecular, phenolic and ampelometric level as well as assessing their relatedness. The leaves were scanned and images were analyzed using SuperAmpelo, which allows to measure leaves, clusters, berries, and seeds, and to record the main descriptive ampelographic characters, including carpometric indices. The content of total anthocyanins, total flavonoids, non-anthocyanic flavonoids, proanthocyanidins and total polyphenols were determined in the extracts of the grape skins and seeds of 10 berries from each of 10 representative clusters as well as in samples of one year old wines. For the molecular characterization, 10 SSR loci were used. The similarity of cultivars was studied by applying the Ward's method for hierarchical clustering along with other Albanian genotypes. This work has completed and made more accurate the existing ampelographic descriptions found in literature with the aim to valorize these autochthonous cultivars, which could represent a valuable instrument for improving the local economies
Ga substitution as an effective variation of Mn-Tb coupling in multiferroic TbMnO3
Ga for Mn substitution in multiferroic TbMnO has been performed in
order to study the influence of Mn-magnetic ordering on the Tb-magnetic
sublattice. Complete characterization of TbMnGaO ( = 0,
0.04, 0.1) samples, including magnetization, impedance spectroscopy, and x-ray
resonant scattering and neutron diffraction on powder and single crystals has
been carried out. We found that keeping the same crystal structure for all
compositions, Ga for Mn substitution leads to the linear decrease of and , reflecting the reduction of the exchange
interactions strength and the change of the Mn-O-Mn bond
angles. At the same time, a strong suppression of both the induced and the
separate Tb-magnetic ordering has been observed. This behavior unambiguously
prove that the exchange fields have a strong influence on the
Tb-magnetic ordering in the full temperature range below
and actually stabilize the Tb-magnetic ground state.Comment: 9 pages, 8 figure
Characterization of the optical properties of the buried contact of the JWST MIRI Si:As infrared blocked impurity band detectors
The Mid-Infrared Instrument MIRI on-board the James Webb Space Telescope uses
three Si:As impurity band conduction detector arrays. MIRI medium resolution
spectroscopic measurements (R3500-1500) in the 5~ to 28~
wavelength range show a 10-30\% modulation of the spectral baseline; coherent
reflections of infrared light within the Si:As detector arrays result in
fringing. We quantify the shape and impact of fringes on spectra of optical
sources observed with MIRI during ground testing and develop an optical model
to simulate the observed modulation. We use our optical model in conjunction
with the MIRI spectroscopic data to show that the properties of the buried
contact inside the MIRI Si:As detector have a significant effect on the
fringing behavior.Comment: 11 pages, 7 figures, SPIE Astronomical Telescopes + Instrumentation
2020, submitted to SPI
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