379 research outputs found
Comment on "Magnetic field effects on neutron diffraction in the antiferromagnetic phase of UPt3"
Moreno and Sauls [Phys. Rev. B 63, 024419 (2000)] have recently tried to
reanalyze earlier neutron scattering studies of the antiferromagnetic order in
UPt3 with a magnetic field applied in the basal plane. In their calculation of
the magnetic Bragg peak intensities, they perform an average over different
magnetic structures belonging to distinct symmetry representations. This is
incorrect. In addition, they have mistaken the magnetic field direction in one
of the experiments, hence invalidating their conclusions concerning the
experimental results.Comment: Revised 5 June 2001: Added group theory analysis and modified
discussion of S and K domain
Magnetic frustration in the spinel compounds Ge Co_2 O_4 and Ge Ni_2 O_4
In both spinel compounds GeCoO and GeNiO which order
antiferromagnetically (at and , ) with different Curie Weiss temperatures (=80.5 K and -15 K),
the usual magnetic frustration criterion is not fulfilled.
Using neutron powder diffraction and magnetization measurements up to 55 T,
both compounds are found with a close magnetic ground state at low temperature
and a similar magnetic behavior (but with a different energy scale), even
though spin anisotropy and first neighbor exchange interactions are quite
different. This magnetic behavior can be understood when considering the main
four magnetic exchange interactions. Frustration mechanisms are then
enlightened.Comment: submitted to Phys.Rev.B (2006
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
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
ABS Inflows to the United States and the Global Financial Crisis
The “global saving glut” (GSG) hypothesis argues that the surge in capital inflows from emerging market economies to the United States led to significant declines in long-term interest rates in the United States and other industrial economies. In turn, these lower interest rates, when combined with both innovations and deficiencies of the U.S. credit market, are believed to have contributed to the U.S. housing bubble and to the buildup in financial vulnerabilities that led to the financial crisis. Because the GSG countries for the most part restricted their U.S. purchases to Treasuries and Agency debt, their provision of savings to ultimately risky subprime mortgage borrowers was necessarily indirect, pushing down yields on safe assets and increasing the appetite for alternative investments on the part of other investors. We present a more complete picture of how capital flows contributed to the crisis, drawing attention to the sizable inflows from European investors into U.S. private-label asset-backed securities (ABS), including mortgage-backed securities and other structured investment products. By adding to domestic demand for private-label ABS, substantial foreign acquisitions of these securities contributed to the decline in their spreads over Treasury yields. Through a combination of empirical estimation and model simulation, we verify that both GSG inflows into Treasuries and Agencies, as well as European acquisitions of ABS, played a role in contributing to downward pressures on U.S. interest rates.
Symmetry of Magnetically Ordered Quasicrystals
The notion of magnetic symmetry is reexamined in light of the recent
observation of long range magnetic order in icosahedral quasicrystals [Charrier
et al., Phys. Rev. Lett. 78, 4637 (1997)]. The relation between the symmetry of
a magnetically-ordered (periodic or quasiperiodic) crystal, given in terms of a
``spin space group,'' and its neutron diffraction diagram is established. In
doing so, an outline of a symmetry classification scheme for magnetically
ordered quasiperiodic crystals is provided. Predictions are given for the
expected diffraction patterns of magnetically ordered icosahedral crystals,
provided their symmetry is well described by icosahedral spin space groups.Comment: 5 pages. Accepted for publication in Phys. Rev. Letter
Spin fluctuations in the stacked-triangular antiferromagnet YMnO3
The spectrum of spin fluctuations in the stacked-triangular antiferromagnet
YMnO3 was studied above the Neel temperature using both unpolarized and
polarized inelastic neutron scattering. We find an in-plane and an out-of-plane
excitation. The in-plane mode has two components just above TN, a
resolution-limited central peak and a Debye-like contribution. The
quasi-elastic fluctuations have a line-width that increases with q like Dq^z
and the dynamical exponent z=2.3. The out-of-plane fluctuations have a gap at
the magnetic zone center and do not show any appreciable q-dependence at small
wave-vectors.Comment: JETP LETTERS, in pres
Crystal structure and high-field magnetism of La2CuO4
Neutron diffraction was used to determine the crystal structure and magnetic
ordering pattern of a La2CuO4 single crystal, with and without applied magnetic
field. A previously unreported, subtle monoclinic distortion of the crystal
structure away from the orthorhombic space group Bmab was detected. The
distortion is also present in lightly Sr-doped crystals. A refinement of the
crystal structure shows that the deviation from orthorhombic symmetry is
predominantly determined by displacements of the apical oxygen atoms. An
in-plane magnetic field is observed to drive a continuous reorientation of the
copper spins from the orthorhombic b-axis to the c-axis, directly confirming
predictions based on prior magnetoresistance and Raman scattering experiments.
A spin-flop transition induced by a c-axis oriented field previously reported
for non-stoichiometric La2CuO4 is also observed, but the transition field (11.5
T) is significantly larger than that in the previous work
Neutron diffraction study of spin and charge ordering in SrFeO(3-delta)
We report a comprehensive neutron diffraction study of the crystal structure
and magnetic order in a series of single-crystal and powder samples of
SrFeO in the vacancy range . The data
provide detailed insights into the interplay between the oxygen vacancy order
and the magnetic structure of this system. In particular, a crystallographic
analysis of data on Sr8Fe8O23 revealed a structural transition between the
high-temperature tetragonal and a low-temperature monoclinic phase with a
critical temperature T = 75 K, which originates from charge ordering on the Fe
sublattice and is associated with a metal-insulator transition. Our experiments
also revealed a total of seven different magnetic structures of
SrFeO in this range of , only two of which (namely an
incommensurate helix state in SrFeO3 and a commensurate, collinear
antiferromagnetic state in Sr4Fe4O11) had been identified previously. We
present a detailed refinement of some of the magnetic ordering patterns and
discuss the relationship between the magneto-transport properties of
SrFeO samples and their phase composition and magnetic
microstructure.Comment: 37 page
Magnetic Order and Dynamics in an Orbitally Degenerate Ferromagnetic Insulator
Neutron scattering was used to determine the spin structure and the magnon
spectrum of the Mott--Hubbard insulator YTiO. The magnetic structure is
complex, comprising substantial G-type and A-type antiferromagnetic components
in addition to the predominant ferromagnetic component. The magnon spectrum, on
the other hand, is gapless and nearly isotropic. We show that these findings
are inconsistent with the orbitally ordered states thus far proposed for
YTiO and discuss general implications for a theoretical description of
exchange interactions in orbitally degenerate systems.Comment: to appear in Phys. Rev. Let
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