859 research outputs found
Magnons in ultrathin Fe/Co multi-layers on W(110) and Ir(111)
Probing the fundamental magnetic interactions in layered magnetic structures is
essential to the understanding of the properties of these systems. Such physical
quantities can be experimentally measured by probing the magnon dispersion
relation over the whole Brillouin zone. On this regard, high wavevector magnons
in ultrathin Fe and Co multilayers grown on W(110) and Ir(111) are studied using
spin polarized high resolution electron energy loss spectroscopy.
In layers of Fe and Co films deposited on W(110) the magnons are probed
along the [001] direction in the wavevector range of 0.25 inverse angstrom to 1.3 inverse angstrom.
Experimental results reveal two magnon modes which exhibit a clear dispersion as expected. The
results are described within the Heisenberg model. In the case of 1ML Co/1ML
Fe/W(110) structure, the interface Fe intralayer exchange coupling constants are
found to be enhanced when compared to 1ML Fe/W(110). The interlayer exchange
coupling was found to be weak (4.5 meV) relative to the 2ML Fe deposited on the
same substrate (7.6 meV). The antisymmteric Dzyaloshinskii-Moriya interaction
(DMI) was observed to be enhanced when compared to the 2ML Fe/W(110) system.
Fe films with the thickness of 2ML and 3ML showed very soft magnons when they
were grown on Ir(111). This indicates rather small exchange coupling constants in
these systems. For the case of 2ML Co/1ML Fe/Ir(111) two magnon modes were
observed. Comparing the experimental results with those of ab initio calculations
it is observed that the exchange interaction in the Fe layer at the interface is very
weak, with a large antiferromagnetic contribution.
The results provide a deeper understanding of the magnetic interactions in layered
structures and may help to advance the understanding of the role of symmetric and
antisymmetric exchange interaction in low dimensional magnetism
Biological wastewater treatment in aeration tanks
Development of mathematical model for prediction of output parameters of aeration
tank with account of dissolved oxygen, oxygen , sludge, substrate transfer and biological treatment. The mathematical model may be used in predicting the effectiveness of aeration tank under different regimes of work
Magnetic susceptibility of the frustrated spinels ZnCr2O4, MgCr2O4 and CdCr2O4
We analyzed the magnetic susceptibilities of several Cr spinels using two
recent models for the geometrically frustrated pyrochlore lattice, the Quantum
Tetrahedral Mean Field model and a Generalized Constant Coupling model. Both
models can describe the experimental data for ACr2 O4 (with A = Zn, Mg, and Cd)
satisfactorily, with the former yielding a somewhat better agreement with
experiment for A = Zn, Mg. The obtained exchange constants for nearest and
next-nearest neighbors are discussed.Comment: 4 pages, 1 figure, 1 table, conferenc
Evolution of optical properties of chromium spinels CdCrO, HgCrS, and ZnCrSe under high pressure
We report pressure-dependent reflection and transmission measurements on
ZnCrSe, HgCrS, and CdCrO single crystals at room
temperature over a broad spectral range 200-24000 cm. The pressure
dependence of the phonon modes and the high-frequency electronic excitations
indicates that all three compounds undergo a pressure-induced structural phase
transition with the critical pressure 15 GPa, 12 GPa, and 10 GPa for
CdCrO, HgCrS, and ZnCrSe, respectively. The
eigenfrequencies of the electronic transitions are very close to the expected
values for chromium crystal-field transitions. In the case of the chalcogenides
pressure induces a red shift of the electronic excitation which indicates a
strong hybridization of the Cr d-bands with the chalcogenide bands.Comment: Accepted for publication in Phys. Rev.
Low-temperature structural transition in FeCr_2S_4
Transmission electron microscopy studies of [110] and [111] oriented
FeCr_2S_4 single crystals at different temperatures reveal a structural
transition at low temperatures indicating a cubic-to-triclinic symmetry
reduction within crystallographic domains. The overall crystal symmetry was
found to be reduced from Fd3m to F-43m. The triclinic distortions were
suggested to result from the combined actions of tetragonal distortions due to
the Jahn-Teller active Fe^2+ ions and trigonal distortions due to a
displacement of the Cr^3+ ions in the direction.Comment: 4 pages, 8 figure
Multiferroicity and colossal magneto-capacitance in Cr-thiospinels
The sulfur based Cr-spinels RCr2S4 with R = Cd and Hg exhibit the coexistence
of ferromagnetic and ferroelectric properties together with a pronounced
magnetocapacitive coupling. While in CdCr2S4 purely ferromagnetic order is
established, in HgCr2S4 a bond-frustrated magnetic ground state is realized,
which, however, easily can be driven towards a ferromagnetic configuration in
weak magnetic fields. This paper shall review our recent investigation for both
compounds. Besides the characterization of the magnetic properties, the complex
dielectric permittivity was studied by means of broadband dielectric
spectroscopy as well as measurements of polarization hysteresis and
pyro-currents. The observed colossal magneto-capacitive effect at the magnetic
transition seems to be driven by an enormous variation of the relaxation
dynamics.Comment: 10 pages, 11 figure
Tuning orbital-selective correlation effects in superconducting RbFeSeS
We report on terahertz time-domain spectroscopy on superconducting and
metallic iron chalcogenides RbFeSeS. The
superconducting transition is reduced from 32 K () to 22 K
(), and finally suppressed () by isoelectronic substitution of Se
with S. Dielectric constant and optical conductivity exhibit a
metal-to-insulator transition associated with an orbital-selective Mott phase.
This orbital-selective Mott transition appears at higher temperature
with increasing sulfur content, identifying sulfur substitution as an efficient
parameter to tune orbital-dependent correlation effects in iron-chalcogenide
superconductors. The reduced correlations of the charge carriers can
account for the suppression of the superconductivity and the pseudogap-like
feature between and that was observed for .Comment: 6 pages, 4 figure
Berezinskii-Kosterlitz-Thouless Type Scenario in Molecular Spin Liquid CrO
The spin relaxation in chromium spinel oxides CrO ( Mg,
Zn, Cd) is investigated in the paramagnetic regime by electron spin resonance
(ESR). The temperature dependence of the ESR linewidth indicates an
unconventional spin-relaxation behavior, similar to spin-spin relaxation in the
two-dimensional (2D) chromium-oxide triangular lattice antiferromagnets. The
data can be described in terms of a generalized Berezinskii-Kosterlitz-Thouless
(BKT) type scenario for 2D systems with additional internal symmetries. Based
on the characteristic exponents obtained from the evaluation of the ESR
linewidth, short-range order with a hidden internal symmetry is suggested.Comment: 7 pages, 4 figure
Multiferroic behavior in CdCr2X4 (X = S, Se)
The recently discovered multiferroic material CdCr2S4 shows a coexistence of
ferromagnetism and relaxor ferroelectricity together with a colossal
magnetocapacitive effect. The complex dielectric permittivity of this compound
and of the structurally related CdCr2Se4 was studied by means of broadband
dielectric spectroscopy using different electrode materials. The observed
magnetocapacitive coupling at the magnetic transition is driven by enormous
changes of the relaxation dynamics induced by the development of magnetic
order
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