171 research outputs found
A cluster model with random anisotropy for hysteresis jumps in CeNiCu alloys
Some Cerium compounds exhibit hysteresis cycles with sharp macroscopic jumps
in the magnetization at very low temperatures. This effect is attributed to the
formation of clusters in which the anisotropy competes with the applied
magnetic field. Here, we present a simple model where a lattice of
ferromagnetically coupled spins is separated in clusters of random sizes and
with random anisotropy. Within this model, we obtain hysteresis cycles
presenting jumps that behave in a similar way that the experimental ones, and
that disappear when increasing the temperature. The results are in good
agreement with the hysteresis cycles measured at very low temperatures in
CeNiCu and the comparison with these experimental results allows
to discriminate the relative importance of the mechanisms driving the thermal
evolution of the cycles.Comment: Accepted in PR
Influence of primary particle density in the morphology of agglomerates
Agglomeration processes occur in many different realms of science such as
colloid and aerosol formation or formation of bacterial colonies. We study the
influence of primary particle density in agglomerate structure using
diffusion-controlled Monte Carlo simulations with realistic space scales
through different regimes (DLA and DLCA). The equivalence of Monte Carlo time
steps to real time scales is given by Hirsch's hydrodynamical theory of
Brownian motion. Agglomerate behavior at different time stages of the
simulations suggests that three indices (fractal exponent, coordination number
and eccentricity index) characterize agglomerate geometry. Using these indices,
we have found that the initial density of primary particles greatly influences
the final structure of the agglomerate as observed in recent experimental
works.Comment: 11 pages, 13 figures, PRE, to appea
Size effects in the magnetic behaviour of TbAl_2 milled alloys
The study of the magnetic properties depending upon mechanical milling of the
ferromagnetic polycrystalline TbAl_2 material is reported. The Rietveld
analysis of the X-ray diffraction data reveals a decrease of the grain size
down to 14 nm and -0.15 % of variation of the lattice parameter, after 300
hours of milling time. Irreversibility in the zero field cooled - field cooled
(ZFC-FC) DC-susceptibility and clear peaks in the AC susceptibility between 5
and 300 K show that the long-range ferromagnetic structure is inhibited in
favour of a disordered spin arrangement below 45 K. This glassy behaviour is
also deduced from the variation of the irreversibility transition with the
field (H^{2/3}) and frequency. The magnetization process of the bulk TbAl_2 is
governed by domain wall thermal activation processes. By contrast, in the
milled samples, cluster-glass properties arise as a result of cooperative
interactions due to the substitutional disorder. The interactions are also
influenced by the nanograin structure of the milled alloys, showing a variation
of coercivity with the grain size, below the crossover between the multi- and
single-domain behaviours.Comment: 23 pages, 11 figures, to appear in J. Phys.: Condens. Ma
Cryogenic study of the magnetic and thermal stability of retained austenite in nanostructured bainite
First-order nature of the ferromagnetism in CeIn2 investigated using muon spin rotation and by systematic substitution of La for Ce
he nature of the first-order ferromagnetic transition in binary CeIn2 alloy is investigated by muon spin rotation (ÎŒSR) measurements and chemical substitution of Ce by La in the La1âxCexIn2 (0.9 x 1.0) series of alloys. Below 22 K, the analysis of ÎŒSR spectra shows two spin precession frequencies associated with the local field at the muon site created by the surrounding ferromagnetic ordered magnetic moments. These frequencies abruptly disappear above TC, indicating the first-order character of this transition, as previously reported. For temperatures between 22 and 24 K, the shape of the ÎŒSR spectra indicates the existence of an additional magnetic phase with features of an incommensurate magnetic structure. The presence of this magnetic phase is supported by dc(ac)-magnetic susceptibility and specific-heat results obtained on chemical diluted samples,which also show a magnetic contribution above the ferromagnetic transition. The combined analysis of these results clarifies the first-order character of the ferromagnetic transition in CeIn2, based on the existence of an intermediate magnetic phase between the paramagnetic and ferromagnetic state
On the 'centre of gravity' method for measuring the composition of magnetite/maghemite mixtures, or the stoichiometry of magnetite-maghemite solid solutions, via Fe-57 Mossbauer spectroscopy
We evaluate the application of 57Fe Mössbauer spectroscopy to the determination of the
composition of magnetite (Fe3O4)/maghemite (Îł-Fe2O3) mixtures and the stoichiometry
of magnetite-maghemite solid solutions. In particular, we consider a recently proposed
model-independent method which does not rely on a priori assumptions regarding the
nature of the sample, other than that it is free of other Fe-containing phases. In it a single
parameter, ÎŽRTâthe âcentre of gravityâ, or area weighted mean isomer shift at room
temperature, T = 295 ± 5 Kâis extracted by curve-fitting a sampleâs Mössbauer spectrum,
and is correlated to the sampleâs composition or stoichiometry. We present data on highpurity
magnetite and maghemite powders, and mixtures thereof, as well as comparison
literature data from nanoparticulate mixtures and solid solutions, to show that a linear
correlation exists between ÎŽRT and the numerical proportion of Fe atoms in the magnetite
environment: α = Femagnetite/Fetotal = â ( ) ÎŽ ÎŽ RT o /m, where ÎŽo = 0.3206 ± 0.0022mm sâ1
and m = 0.2135 ± 0.0076mm sâ1
. We also present equations to relate α to the weight
percentage w of magnetite in mixed phases, and the magnetite stoichiometry x = Fe2+/Fe3+
in solid solutions. The analytical method is generally applicable, but is most accurate when
the absorption profiles are sharp; in some samples this may require spectra to be recorded
at reduced temperatures. We consider such cases and provide equations to relate Ύ ( ) T to the
corresponding α value
Structural control of the non-ionic surfactant alcohol ethoxylates (AEOs) on transport in natural soils
Surfactants, after use, enter the environment through diffuse and point sources such as irrigation with treated and non-treated waste water and urban and industrial wastewater discharges. For the group of non-ionic synthetic surfactant alcohol ethoxylates (AEOs), most of the available information is restricted to the levels and fate in aquatic systems, whereas current knowledge of their behavior in soils is very limited. Here we characterize the behavior of different homologs (C12-C18) and ethoxymers (E03, E06, and E08) of the AEOs through batch experiments and under unsaturated flow conditions during infiltration experiments. Experiments used two different agricultural soils from a region irrigated with reclaimed water (Guadalete River basin, SW Spain). In parallel, water flow and chemical transport were modelled using the HYDRUS-1D software package, calibrated using the infiltration experimental data. Estimates of water flow and reactive transport of all surfactants were in good agreement between infiltration experiments and simulations. The sorption process followed a Freundlich isotherm for most of the target compounds. A systematic comparison between sorption data obtained from batch and infiltration experiments revealed that the sorption coefficient (K-d) was generally lower in infiltration experiments, performed under environmental flow conditions, than in batch experiments in the absence of flow, whereas the exponent (beta) did not show significant differences. For the low clay and organic carbon content of the soils used, no clear dependence of K-d on them was observed. Our work thus highlights the need to use reactive transport parameterization inferred under realistic conditions to assess the risk associated with alcohol ethoxylates in subsurface environments. (C) 2020 The Authors. Published by Elsevier Ltd
Structural defects in Hg1âxCdxI2 layers grown on CdTe substrates by vapor phase epitaxy
Hg1âxCdxI2 20â25-ÎŒm-thick layers with a uniform composition in the range of x = 0.1â0.2 were grown on CdTe substrates by vapor phase epitaxy (VPE). The growth was carried out using an α-HgI2 polycrystalline source at 200â°C and in the time range of 30â100 h. The layers were studied by scanning electron microscopy (SEM) and high resolution synchrotron x-ray topography (SXRT). The SEM and SXRT images of Hg1âxCdxI2 VPE layers allow one to identify the defects affecting the layer structure. The two main types of structural defects in the layers are subgrain boundaries and densely spaced striations similar to those referred generally to as vapor grown HgI2 bulk crystals. The effect of the growth time on these defects has been analyzed and on the basis of this it has been possible to grow Hg1âxCdxI2 layers with low defect [email protected]
Substitutional effects of in by Cu in CeIn2
We have investigated the evolution of the magnetic properties on the Ce(In1'xCux)2 (0 < x †0.3) series of alloys. The orthorhombic structure of the CeIn2 alloy (Imma) changes into the hexagonal AlB2-type (P6/mmm) for x = 0.05 and, then, into the hexagonal CaIn2-type (P63/mmm) for higher Cu concentrations, up to x = 0.3. The dc (ac) magnetic susceptibility shows an abrupt decrease of the magnetic transition temperature from 22 K to 5.4 K (x = 0.05). The results indicate the influence of the crystallographic type of structure and disorder effects on the magnetic behavior along the series. © Owned by the authors, published by EDP Sciences, 2014
- âŠ