arXiv subjects

A. A. Coelho

Publications and source records attributed to A. A. Coelho.

12 recordsLinked to original sources

arXiv2018

Experimental setup for measuring the barocaloric effect in polymers: Application to natural rubber

Barocaloric materials have shown to be promising alternatives to the conventional vapor-compression refrigeration technologies. Nevertheless, barocaloric effect (σb-CE) has not been extensively examined for many classes of materials up to now. Aiming at fulfilling this gap, this paper describes the development of a high-pressure experimental setup for measuring the σb-CE in polymers. The design allows simultaneous measurements of temperature, pressure and strain during the barocaloric cycle. The system proved to be fully-functional through basic experiments using natural rubber. Samples exhibited large temperature variations associated to the σb-CE. Strain-temperature curves were also obtained, which could allow indirect measurements of the isothermal entropy change.

cond-mat.soft↗

arXiv2018

Unusual effects of manual grinding and subsequent annealing process observed in Gd5.09Ge2.03Si1.88 compound

The Gd5.09Ge2.03Si1.88 compound, as well as other magnetocaloric materials, certainly will not be used in their un-manufactured as-cast condition in future magnetic refrigeration applications or other devices. In this work, we have studied the Gd5.09Ge2.03Si1.88 compound processed in different ways, mainly, the as-cast powder, the annealed powder and the pressed and sintered powder. The annealed powder (1370 K / 20 hours) does not present the monoclinic phase and the first-order-magneto-structural transition observed in the as-cast powder. The pressed and sintered powder also do not present the first-order transition. Furthermore, the compacting pressure shifts the second-order-magnetic transition to lower temperatures. The behavior of cell parameters as a function of the compacting pressure indicates that TC is directly affected by parameter c change.

cond-mat.mtrl-sci↗

arXiv2016

X-ray powder diffraction of high-absorption materials at the XRD1 beamline off the best conditions: Application to (Gd,Nd)5Si4 compounds

Representative compounds of the new family of magnetic materials Gd5-xNdxSi4 were analyzed by X-ray diffraction at the XRD1 beamline at LNLS. To reduce X-ray absorption, thin layers of the powder samples were mounted outside the capillaries and measured in Debye-Scherrer geometry as usual. The X-ray diffraction analyses and the magnetometry results indicate that the behavior of the magnetic transition temperature as a function of Nd content may be directly related to the average of the four smallest interatomic distances between different rare earth sites of the majority phase of each compound. The quality and consistency of the results show that the XRD1 beamline is able to perform satisfactory X-ray diffraction experiments on high-absorption materials even off the best conditions.

physics.ins-det↗

arXiv2015

Multifunctional Heusler alloy: experimental evidences of enhanced magnetocaloric properties at room temperature and half-metallicity

Heusler alloys are widely studied due to their interesting structural and magnetic properties, like magnetic memory shape ability, coupled magneto-structural phase transitions and half-metallicity; ruled, for many cases, by the valence electrons number ($N_v$). The present work focuses on the magnetocaloric potentials of half-metals, exploring the effect of $N_v$ on the magnetic entropy change, preserving half-metallicity. The test bench is the Si-rich side of the half-metallic series Fe$_2$MnSi$_{1-x}$Ga$_x$. From the obtained experimental results it was possible to obtain $|ΔS|_{max}=ΔH^{0.8}(α+βN_v)$, i.e., the maximum magnetic entropy change depends in a linear fashion on $N_v$, weighted by a power law on the magnetic field change $ΔH$ ($α$ and $β$ are constants experimentally determined). In addition, it was also possible to predict a new multifunctional Heusler alloy, with enhanced magnetocaloric effect, Curie temperature close to 300 K and half-metallicity.

cond-mat.mtrl-sci↗

arXiv2015

A new spin gapless semiconductor: quaternary Heusler CoFeCrGa alloy

Despite a plethora of materials suggested for spintronic applications, a new class of materials has emerged, namely spin gapless semiconductors (SGS), that offers potentially more advantageous properties than existing ones. These magnetic semiconductors exhibit a finite band gap for one spin channel and a closed gap for the other. Here, supported by the first-principles, electronic-structure calculations, we report the first experimental evidence of SGS behavior in equiatomic quaternary CoFeCrGa, having a cubic Heusler (L21) structure but exhibiting chemical disorder (DO3 structure). CoFeCrGa is found to transform from SGS to half-metallic phase under pressure, which is attributed to unique electronic-structure features. The saturation magnetization (MS) obtained at 8 K agrees with the Slater-Pauling rule and the Curie temperature (TC) is found to exceed 400 K. Carrier concentration (up to 250 K) and electrical conductivity are observed to be nearly temperature independent, prerequisites for SGS. The anomalous Hall coefficient is estimated to be 185 S/cm at 5 K. Considering the SGS properties and high TC, this material appears to be promising for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2015

High spin polarization and large spin splitting in equiatomic quaternary CoFeCrAl Heusler alloy

In this paper, we investigate CoFeCrAl alloy by means of various experimental techniques and ab-initio calculations to look for half-metallic nature. The alloy is found to exist in the cubic Heusler structure, with presence of B2 ordering. Saturation magnetization (MS) value of about 2 Bohr magneton/f.u. is observed at 8 K under ambient pressure, which is in good agreement with the Slater-Pauling rule. MS values are found to be independent of pressure, which is a prerequisite for half-metals. The ab-initio electronic structure calculations predict half-metallic nature for the alloy with a spin slitting energy of 0.31 eV. Importantly, this system shows a high current spin polarization value of 0.67 [with error of 0.02], as deduced from the point contact Andreev reflection (PCAR) measurements. Linear dependence of electrical resistivity with temperature indicates the possibility of reasonably high spin polarization at elevated temperatures (~150 K) as well. All these suggest that CoFeCrAl is a promising material for the spintronic devices.

cond-mat.mtrl-sci↗

arXiv2010

Pressure induced magnetic and magnetocaloric properties in NiCoMnSb Heusler alloy

The effect of pressure on the magnetic and the magnetocaloric properties around the martensitic transformation temperature in NiCoMnSb Heusler alloy has been studied. The martensitic transition temperature has significantly shifted to higher temperatures with pressure, whereas the trend is opposite with the application of applied magnetic field. The maximum magnetic entropy change around the martensitic transition temperature for Ni45Co5Mn38Sb12 is 41.4 J/kg K at the ambient pressure, whereas it is 33 J/kg K at 8.5 kbar. We find that by adjusting the Co concentration and applying suitable pressure, NiCoMnSb system can be tuned to achieve giant magnetocaloric effect spread over a large temperature span around the room temperature, thereby making it a potential magnetic refrigerant material for applications.

cond-mat.str-el↗

arXiv2010

Stabilization of antiferromagnetism in CeFe2 alloys: Effects of chemical and hydrostatic pressure

Effects of Al, Mn and Sb dopings in CeFe2 and effect of applied pressure have been investigated. Al doping gives rise to the FM-AFM transition and a reduction in the magnetic moment and TC values, clearly indicating the growth of the AFM component. Mn and Sb dopings only cause a reduction in TC value. It is found that in general external pressure enhances the antiferromagnetism in both the pure and the doped alloys. Enhancement of the Ce 4f- Fe 3d hybridization as a result of dopings and with the external pressure may be the reason for the stabilization of antiferromagnetism in these alloys.

cond-mat.str-el↗

arXiv2006

Correlation between magnetism and magnetocaloric effect in RCo2-based Laves phase compounds

By virtue of the itinerant electron metamagnetism (IEM), the RCo2 compounds with R=Er, Ho and Dy are found to show first order magnetic transition at their ordering temperatures. The inherent instability of Co sublattice magnetism is responsible for the occurrence of IEM, which leads to interesting magnetic and related properties. The systematic studies of the variations in the magnetic and magnetocaloric properties of the RCo2-based compounds show that the magnetovolume effect plays a decisive role in determining the nature of magnetic transitions and hence the magnetocaloric effect (MCE) in these compound. It is found that the spin fluctuations arising due to the magnetovolume effect reduce the strength of IEM in these compounds, which subsequently lead to a reduction in the MCE. Most of the substitutions at the Co site are found to result in a positive magnetovolume effect, leading to an initial increase in the ordering temperature. Application of pressure, on the other hand, causes a reduction in the ordering temperature due to the negative magnetovolume effect. A comparative study of the magnetic and magnetocaloric properties of RCo2 compounds under various substitutions and applied pressure is presented. Analysis of the magnetization data using the Landau theory has shown that there is a strong correlation between the Landau coefficient (B) and the MCE. The variations seen in the order of magnetic transition and the MCE values seem to support the recent model proposed by Khmelevskyi and Mohn for the occurrence of IEM in RCo2 compounds.

cond-mat.mtrl-sci↗

arXiv2006

Measurement of pressure effects on the magnetic and the magnetocaloric properties of the intermetallic compounds R(Co1-xSix)2 [R=Dy and Er]

The effect of external pressure on the magnetic properties and magnetocaloric effect of R(Co1-xSix)2 [R= Er, Dy and x=0, 0.025 and 0.05] compounds has been studied. The ordering temperatures of both the parent as well as the Si substituted compounds are found to decrease with pressure. In all the compounds, the critical field for metamagnetic transition increases with pressure. It is seen that the magnetocaloric effect in the parent compounds is almost insensitive to pressure, while there is considerable enhancement in the case of Si substituted compounds. Spin fluctuations arising due to the magnetovolume effect play a crucial role in determining the pressure dependence of magnetocaloric effect in these compounds. Analysis of the magnetization data using the Landau theory has shown that the magnitude of the Landau coefficient (C3) decreases with Si concentration whereas it is found to increase with pressure. The isothermal magnetic entropy change is found to behave in the same manner as C3, both with Si concentration (at ambient pressure) as well as with the applied pressure.

cond-mat.mtrl-sci↗

arXiv2001

Angular dependence of the bulk nucleation field Hc2 of aligned MgB2 crystallites

Studies on the new MgB2 superconductor, with a critical temperature Tc ~ 39 K, have evidenced its potential for applications although intense magnetic relaxation effects limit the critical current density, Jc, at high magnetic fields. This means that effective pinning centers must be added into the material microstructure, in order to halt dissipative flux movements. Concerning the basic microscopic mechanism to explain the superconductivity in MgB2, several experimental and theoretical works have pointed to the relevance of a phonon-mediated interaction, in the framework of the BCS theory. Questions have been raised about the relevant phonon modes, and the gap and Fermi surface anisotropies, in an effort to interpret spectroscopic and thermal data that give values between 2.4 and 4.5 for the gap energy ratio. Preliminary results on the anisotropy of Hc2 have shown a ratio, between the in-plane and perpendicular directions, around 1.7 for aligned MgB2 crystallites and 1.8 for epitaxial thin films. Here we show a study on the angular dependence of Hc2 pointing to a Fermi velocity anisotropy around 2.5. This anisotropy certainly implies the use of texturization techniques to optimize Jc in MgB2 wires and other polycrystalline components.

cond-mat.supr-con↗

arXiv2001

Anisotropic superconducting properties of aligned MgB2 crystallites

Samples of aligned MgB2 crystallites have been prepared, allowing for the first time the direct identification of an upper critical field anisotropy Hc2^{ab}/Hc2^{c}= xi_{ab}/xi_{c} ~ 1.73; with xi_{o,ab} ~ 70 A, xi_{o,c} ~ 40 A, and a mass anisotropy ratio m_{ab}/m_{c} ~ 0.3. A ferromagnetic background signal was identified, possibly related to the raw materials purity.

cond-mat.supr-con↗