arXiv subjects

João P. Araújo

Publications and source records attributed to João P. Araújo.

4 recordsLinked to original sources

arXiv2026

Self-thermometry measurements of the adiabatic temperature change in first-order phase transition magnetocaloric materials

Accurately measuring the magnetocaloric effect is necessary to foster the development of magnetic refrigeration devices. However, current methods are inconvenient, requiring different instruments to measure each individual property or a custom-made setup. By measuring the time-varying magnetization in a commercially available VersaLab\textsuperscript{\textregistered} PPMS\textsuperscript{\textregistered} from Quantum Design, we have determined the adiabatic temperature change ($Δ$T$_{\textrm{ad}}$) of the first-order phase transition material Gd$_5$Si$_2$Ge$_2$, for a magnetic field change of 0 to 1 T, under high vacuum ($<$ 0.1 mTorr). For each temperature and magnetic field, the equilibrium magnetization is used as the magnetization-to-temperature conversion curve, allowing us to extend the validity of a previously proposed technique to the first-order phase transition material Gd$_5$Si$_2$Ge$_2$, which exhibits significant hysteresis. Our method thus enables full characterization (magnetic entropy change, adiabatic temperature change, and heat capacity) of any magnetocaloric material, whether it has a first-order or a second-order phase transition, using a single instrument. Comparing to a directly measured $Δ$T$_{\textrm{ad}}$, our method resulted in a peak $Δ$T$_{\textrm{ad}}$ value of 4.47 K, within 1\% of the directly measured value for a sample of the same composition.

cond-mat.mtrl-sci↗

arXiv2025

Ge-based Clinopyroxene series: first principles and experimental local probe study

The structural and electronic properties of the CaMnGe$_2$O$_6$ and SrMnGe$_2$O$_6$ clinopyroxene systems have been investigated by means of perturbed angular correlation (PAC) measurements, performed at ISOLDE, combined with $ab-initio$ electronic structure calculations within the density functional theory (DFT) framework. The partial density of states (PDOS) of the CaMnGe$_2$O$_6$ and SrMnGe$_2$O$_6$ stable compounds has been determined, and it has been observed that the requirement of including an on-site Hubbard-$U$ potential was necessary in order to describe the highly correlated Mn $3d$-states. By considering $U_{eff}$=4 eV, we obtained a band gap width of 1.82 eV and 1.70 eV, for the CaMnGe$_2$O$_6$ and SrMnGe$_2$O$_6$, respectively. Combining electric field gradient (EFG) first principles calculations, using a supercell scheme, with experimental PAC results, we were able to infer that the Cd probe can replace either the $A$ (Ca, Sr) or the Mn sites in the crystalline structures. We also showed that Cd substitution is expected to lead to a reduction in the width of the band gap in these systems, evidencing opportunities for potential band-gap engineering.

cond-mat.mtrl-sci↗

arXiv2019

Freestanding and flexible composites of magnetocaloric Gd$_5$(Si,Ge)$_4$ microparticles embedded in thermoplastic poly(methyl methacrylate) matrix

The implementation of processed magnetic materials onto thermoplastics can be an approach for practical application of brittle intermetallic materials with the advantage of enlarging the range of applications. In the present work, we evaluate the effect of blending magnetocaloric Gd$_5$Si$_{2.4}$Ge$_{1.6}$ micrometric particles with 3.4 $μ$m in different weight fractions onto a flexible, transparent and non-magnetic poly(methyl methacrylate) (PMMA). A close to homogeneous grain distribution along the polymer surface were achiever by using a simple solvent casting method for evaluation of their magnetocaloric properties. From XRD analysis, it was found a relative unit cell volume reduction of $\sim$2.5$\times$10$^3$ ppm for the composite with 70 wt.\% of powder as a result of interfacial interactions between the components. Although PMMA does not influence the magnetic nature of microparticles main phase, a reduction on the amount of secondary monoclinic phase occurs for all produced composite samples. As a consequence, a weakening on the effect of secondary phases on the micropowder magnetocaloric response is observed as a result of hydrostatic pressure from the difference between thermal expansions of matrix and filler.

cond-mat.mtrl-sci↗

arXiv2017

On the nature of the (de)coupling of the magnetostructural transition in Er$_5$Si$_4$

In this report, a successful thermodynamical model was employed to understand the structural transition in Er$_5$Si$_4$, able to explain the decoupling of the magnetic and structural transition. This was achieved by the DFT calculations which were used to determine the energy differences at 0 K, using a LSDA+U approximation. It was found that the M structure as the stable phase at low temperatures as verified experimentally with a $ΔF_0 = -$0.262 eV. Finally, it was achieved a variation of Seebeck coefficient ($\sim$ 6 $μ$V) at the structural transition which allow to conclude that the electronic entropy variation is negligible in the transition.

cond-mat.mtrl-sci↗