98 research outputs found
Uniaxial anisotropy and enhanced magnetostriction of CoFeO induced by reaction under uniaxial pressure with SPS
In this study, we have compared magnetic and magnetostrictive properties of
polycrystalline CoFeO pellets, produced by three different methods,
focusing on the use of Spark Plasma Sintering (SPS). This technique allows a
very short heat treatment stage while a uniaxial pressure is applied. SPS was
utilized to sinter cobalt ferrite but also to make the reaction and the
sintering (reactive sintering) of the same ceramic composition. Magnetic and
magnetostrictive measurements show that the reactive sintering with SPS induces
a uniaxial anisotropy, while it is not the case with a simple sintering
process. The induced anisotropy is then expected to be a consequence of the
reaction under uniaxial pressure. This anisotropy enhanced the magnetostrictive
properties of the sample, where a maximum longitudinal magnetostriction of
~ppm is obtained. This process can be a promising alternative to the
magnetic-annealing because of the short processing time required (22 minutes)
Enhancement of the magnetoelectric effect in multiferroic CoFeO/PZT bilayer by induced uniaxial magnetic anisotropy
In this study we have compared magnetic, magnetostrictive and piezomagnetic
properties of isotropic and anisotropic cobalt ferrite pellets. The isotropic
sample was prepared by the ceramic method while the sample exhibiting uniaxial
anisotropy was made by reactive sintering using Spark Plasma Sintering (SPS).
This technique permits to induce a magnetic anisotropy in cobalt ferrite in the
direction of the applied pressure during SPS process. Sample with uniaxial
anisotropy revealed a higher longitudinal magnetostriction and piezomagnetism
compared to the isotropic sample, but the transversal magnetostriction and
piezomagnetism were dramatically reduced. In the case of magnetoelectric
layered composite, the magnetoelectric coefficient is directly related to the
sum of the longitudinal and transversal piezomagnetic coefficients. These two
coefficients being opposite in sign, the use of material exhibiting high
longitudinal and low transversal piezomagnetic coefficient (or vice versa) in
ME devices is expected to improve the ME effect. Hence, ME bilayer devices were
made using isotropic and anisotropic cobalt ferrite stuck with a PZT layer. ME
measurements at low frequencies revealed that bilayer with anisotropic cobalt
ferrite exhibits a ME coefficient three times higher than a bilayer with
isotropic cobalt ferrite. We also investigated the behavior of such composites
when excited at resonant frequency
A Method to Measure Using With Multibody Decay
We describe a new method to measure the angle of the CKM
Unitarity Triangle using amplitude analysis of the multibody decay of the
neutral meson produced via colour-suppressed decays.
The method employs the interference between and to directly
extract the value of , and thus resolve the ambiguity between
and in the measurement of using
\bar{B}^0 \to J/ \psi \ks.
We present a feasibility study of this method using Monte Carlo simulation.Comment: 12 pages, 7 figures, 2 tables. See also talk presented by T.G. at
CKM2005 (http://ckm2005.ucsd.edu/) Submitted to Physics Letters
Magnetic properties of Nd6Fe13Cu single crystals
The understanding of coercivity mechanism in high performance Nd-Fe-B
permanent magnets relies on the analysis of the magnetic properties of all
phases present in the magnets. By adding Cu in such compounds, a new Nd6Fe13Cu
grain boundary phase is formed, however, the magnetic properties of this phase
and its role in the magnetic decoupling of the matrix Nd2Fe14B grains are still
insufficiently studied. In this work, we have grown Nd6Fe13Cu single crystals
by the reactive flux method and studied their magnetic properties in detail. It
is observed that below the N\'eel temperature (TN = 410 K), the Nd6Fe13Cu is
antiferromagnetic in zero magnetic field; whereas when a magnetic field is
applied along the a-axis, a spin-flop transition occurs at approx. 6 T,
indicating a strong competition between antiferromagnetic and ferromagnetic
interactions in two Nd layers below and above the Cu layers. Our atomistic spin
dynamics simulation confirms that an increase in temperature and/or magnetic
field can significantly change the antiferromagnetic coupling between the two
Nd layers below and above the Cu layers, which, in turn, is the reason for the
observed spin-flop transition. These results suggest that the role of
antiferromagnetic Nd6Fe13Cu grain boundary phase in the coercivity enhancement
of Nd-Fe-B-Cu magnets is more complex than previously thought, mainly due to
the competition between its antiferro- and ferro-magnetic exchange
interactions.Comment: 15 pages, 4 figure
Designing magnetocaloric materials for hydrogen liquefaction with light rare-earth Laves phases
Magnetocaloric hydrogen liquefaction could be a "game-changer" for liquid
hydrogen industry. Although heavy rare-earth-based magnetocaloric materials
show strong magnetocaloric effects in the temperature range required by
hydrogen liquefaction (77 ~ 20 K), the high resource criticality of the heavy
rare-earth elements is a major obstacle for upscaling this emerging
liquefaction technology. In contrast, the higher abundances of the light
rare-earth elements make their alloys highly appealing for magnetocaloric
hydrogen liquefaction. Via a mean-field approach, it is demonstrated that
tuning the Curie temperature () of an idealized light rare-earth-based
magnetocaloric material towards lower cryogenic temperatures leads to larger
maximum magnetic and adiabatic temperature changes ( and ). Especially in the vicinity of the condensation point of hydrogen (20
K), and of the optimized light rare-earth-based
material are predicted to show significantly large values. Following the
mean-field approach and taking the chemical and physical similarities of the
light rare-earth elements into consideration, a method of designing light
rare-earth intermetallic compounds for hydrogen liquefaction is proposed:
tunning of a rare-earth alloy to approach 20 K by mixing light rare-earth
elements with different de Gennes factors. By mixing Nd and Pr in Laves phase
, and Pr and Ce in Laves phase , a fully light
rare-earth intermetallic series with large magnetocaloric effects covering the
temperature range required by hydrogen liquefaction is developed, demonstrating
a competitive maximum effect compared to the heavy rare-earth compound
Nitrogenation and sintering of (Nd-Zr)Fe10Si2 tetragonal compounds for permanent magnets applications
International audienceNd(1-x)Zr(x)Fe10Si2 alloys have been prepared in the tetragonal ThMn12-type structure by arc-melting and melt-spinning and then nitrogenated to improve their magnetic properties. For x = 0.4 and 0.6 the Curie temperature and magnetic anisotropy fields increase from 280-300 ºC to about 390 ºC and from 2.8-3 T to 4.5-5 T respectively. The saturation magnetization remains almost unchanged. The nitrogenated powders were processed by spark plasma sintering (SPS) leading to compact pellets, which retain the full nitrogen content and magnetic properties up to 600 ºC, but segregated Fe-Si at elevated temperatures. Nitrogenation and SPS processing are, therefore, appropriate for sintering metastable materials such as (Nd,Zr)Fe10Si2 into compact material without loosing functional properties. This opens a way towards a new family of permanent magnets, lean of critical raw materials
The Completed SDSS-IV Extended Baryon Oscillation Spectroscopic Survey: Growth rate of structure measurement from cosmic voids
We present a void clustering analysis in configuration-space using the
completed Sloan Digital Sky Survey IV (SDSS-IV) extended Baryon Oscillation
Spectroscopic Survey (eBOSS) DR16 samples. These samples consist of Luminous
Red Galaxies (LRG) combined with the high redshift tail of the SDSS-III Baryon
Oscillation Spectroscopic Survey (BOSS) DR12 CMASS galaxies (called as
LRG+CMASS sample), Emission Line Galaxies (ELG) and quasars (QSO). We build
void catalogues from the three eBOSS DR16 samples using a ZOBOV-based
algorithm, providing 2,814 voids, 1,801 voids and 4,347 voids in the LRG+CMASS,
ELG and QSO samples, respectively, spanning the redshift range . We
measure the redshift space distortions (RSD) around voids using the anisotropic
void-galaxy cross-correlation function and we extract the distortion parameter
. We test the methodology on realistic simulations before applying it to
the data, and we investigate all our systematic errors on these mocks. We find
, and , for
the LRG+CMASS, ELG and QSO sample, respectively. The quoted errors include
systematic and statistical contributions. In order to convert our measurements
in terms of the growth rate , we use consensus values of linear bias
from the eBOSS DR16 companion papers~\citep{eBOSScosmo}, resulting in the
following constraints: ,
and . Our
measurements are consistent with other measurements from eBOSS DR16 using
conventional clustering techniques.Comment: 17 pages, 8 figure
Probing Charge-Symmetry-Violating Quark Distributions in Semi-Inclusive Leptoproduction of Hadrons
Recent experiments by the HERMES group at HERA are measuring semi-inclusive
electroproduction of pions from deuterium. We point out that by comparing the
production of and from an isoscalar target, it is possible, in
principle, to measure charge symmetry violation in the valence quark
distributions of the nucleons. It is also possible in the same experiments to
obtain an independent measurement of the quark fragmentation functions. We
review the information which can be deduced from such experiments and show the
``signature'' for charge symmetry violation in such experiments. Finally, we
predict the magnitude of the charge symmetry violation, from both the valence
quark distributions and the pion fragmentation function, which might be
expected in these experiments.Comment: 19 pages plus 5 figures, used eps
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