87 research outputs found
Spin-driven electric polarization from strong 4d-4f cross-coupling in 6H-perovskite Ba3HoRu2O9
The occurrence of spin-driven polarization is rarely observed in 4d or 5d
transition-metal oxides, compared to their 3d-transition-metal oxide
counterparts, despite theoretical predictions of the good possibility of
multiferroicity in higher d-orbital systems. Here we have studied a prototype
4d-4f compound, Ba3HoRu2O9, through time-of-flight neutron diffraction, ac
susceptibility, and complex dielectric spectroscopy. We demonstrate that the
non-colinear structure involving two different magnetic ions, Ru(4d) and
Ho(4f), breaks the spatial inversion symmetry via inverse Dzyaloshinskii Moriya
(D-M) interaction through strong 4d-4f magnetic correlation, which results in
non-zero polarization. The stronger spin-orbit coupling of 4d-orbital might
play a major role in creating D-M interaction of non-linear spins. We have
systematically investigated the magnetoelectric domain dynamics in this system,
which exhibits intriguing behavior with shorter correlation lengths. Further,
we have investigated the effect of external pressure on the magnetic
transition. The result reveals an enhancement of ordering temperature by the
application of external pressure (1.6K per GPa). We speculate that external
pressure might favor stabilizing the magnetoelectric phase. Our study shows a
route to designing magnetoelectric multiferroic compounds containing larger
d-orbital through strong 4d-4f cross-coupling
Ordering process and ferroelectricity in a spinel derived from FeV2O4
The spinel FeV2O4 is known to exhibit peculiar physical properties, which is
generally ascribed to the unusual presence of two cations showing a pronounced
interplay between spin, orbital and lattice degrees of freedom (Fe2+ and V3+ on
the tetrahedral and octahedral sites, respectively). The present work reports
on an experimental re-investigation of this material based on a broad
combination of techniques, including x-ray diffraction, energy dispersive and
M\"ossbauer spectroscopies, as well as magnetization, heat capacity, dielectric
and polarization measurements. Special attention was firstly paid to establish
the exact cationic composition of the investigated samples, which was found to
be Fe1.18V1.82O4. All the physical properties were found to point out a complex
ordering process with a structural transition at TS = 138 K, followed by two
successive magnetostructural transitions at TN1 = 111 K and TN2 = 56 K. This
latter transition marking the appearance of electric polarization,
magnetization data were analysed in details to discuss the nature of the
magnetic state at T< TN2. An overall interpretation of the sequence of
transitions was proposed, taking into account two spin couplings, as well as
the Jahn-Teller effects and the mechanism of spin-orbit stabilization. Finally,
the origin of ferroelectricity in Fe1.18V1.82O4 is discussed on the basis of
recent models.Comment: 26 pages, 9 figures,59 references.Accepted by Physical Review
Competition between Ferrimagnetism and Magnetic Frustration in Zinc Substituted YBaFe4O7
The substitution of zinc for iron in YBaFe4O7 has allowed the oxide series
YBaFe4-xZnxO7, with 0.40 < x < 1.50, belonging to the "114" structural family
to be synthesized. These oxides crystallize in the hexagonal symmetry (P63mc),
as opposed to the cubic symmetry (F-43m) of YBaFe4O7. Importantly, the d.c.
magnetization shows that the zinc substitution induces ferrimagnetism, in
contrast to the spin glass behaviour of YBaFe4O7. Moreover, a.c. susceptibility
measurements demonstrate that concomitantly these oxides exhibit a spin glass
or a cluster glass behaviour, which increases at the expense of ferrimagnetism,
as the zinc content is increased. This competition between ferrimagnetism and
magnetic frustration is interpreted in terms of lifting of the geometric
frustration, inducing the magnetic ordering, and of cationic disordering, which
favours the glassy state.Comment: 24 pages, 10 figure
Experimental study of a cavitating centrifugal pump during fast starts-up
The start-up of rocket engine turbopumps is generally performed only in a few seconds. It implies that these pumps reach their nominal operating conditions after only a few rotations. During these first rotations of the blades, the flow evolution in the pump is governed by transient phenomena, based mainly on the flow rate and rotation speed evolution. These phenomena progressively become negligible when the steady behavior is reached. The pump transient behavior induces significant pressure fluctuations which may result in partial flow vaporization, i.e. cavitation. An existing experimental test rig has been updated in the LML laboratory (Lille, France) for the start-ups of a centrifugal pump. The study focuses on cavitation induced during the pump start-up. Instantaneous measurement of torque, flow rate, inlet and outlet unsteady pressures, and pump rotation velocity enable to characterize the pump behavior during rapid starting periods. Three different types of fast start-up behaviors have been identified. According to the final operating point, the start-up is characterized either by a single drop of the delivery static pressure, by several low-frequency drops, or by a water hammer phenomenon that can be observed both a the inlet and outlet of the pump. A physical analysis is proposed to explain these three different types of transient flow behavior
Magnetic and Electrical Properties of Ordered 112-type Perovskite LnBaCoMnO5+\delta (Ln = Nd, Eu)
Investigation of the oxygen-deficient 112-type ordered oxides of the type
LnBaCoMnO5+\delta (Ln = Nd, Eu) evidences certain unusual magnetic behavior at
low temperatures, compared to the LnBaCo2O5+\delta cobaltites. One observes
that the substitution of manganese for cobalt suppresses the ferromagnetic
state and induces strong antiferromagnetic interactions. Importantly,
NdBaCoMnO5.9 depicts a clear paramagnetic to antiferromagnetic type transition
around 220 K, whereas for EuBaCoMnO5.7 one observes an unusual magnetic
behavior below 177 K which consists of ferromagnetic regions embedded in an
antiferromagnetic matrix. The existence of two sorts of crystallographic sites
for Co/Mn and their mixed valence states favor the ferromagnetic interaction
whereas antiferromagnetism originates from the Co3+-O-Co3+ and Mn4+-O-Mn4+
interactions. Unlike the parent compounds, the present Mn-substituted phases do
not exhibit prominent magnetoresistance effects in the temperature range
75-400K.Comment: 23 pages including figure
FEDSM2005-77038 ROTOR-STATOR INTERACTIONS IN A VANED DIFFUSER RADIAL FLOW PUMP
ABSTRACT The paper presents an analysis of unsteady effects associated with rotor stator interactions in a vaned diffuser radial flow pump. The experimental data have been obtained by 2D Particle Image Velocimetry at mid height between hub and shroud, in the impeller outlet zone and within a vaned diffuser passage. Unsteadiness is discussed first in the impeller outlet zone and then along a passage of the vaned diffuser
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