arXiv subjects

M. C. Gallardo

Publications and source records attributed to M. C. Gallardo.

5 recordsLinked to original sources

arXiv2025

Phase dependence of the Thermal Memory Effect in Polycrystalline Ribbon and Bulk Ni55Fe19Ga26 Heusler Alloys

The thermal memory effect, TME, has been studied in Ni55Fe19Ga26 shape memory alloys, fabricated as ribbons via melt-spinning and as pellets via arc-melting, to evaluate its dependence on the martensitic structure and the macrostructure of the samples. When the reverse martensitic transformation is interrupted, a kinetic delay in the subsequent complete transformation is only evident in the ribbon samples, where the 14M modulated structure is the dominant phase. In contrast, degradation of the modulated structure or the presence of the gamma-phase significantly reduces the observed TME. In such cases, the magnitude of the TME approaches the detection limits of commercial calorimeters, and only high-resolution calorimeter at very low heating rate (40 mK h-1) can show the effect. Following the kinetic arrest and subsequent cooling, the reverse martensitic transformation was completed at several heating rates to confirm the athermal nature of the phenomenon.

cond-mat.mtrl-sci↗

arXiv2021

Thermo-magnetic characterization of phase transitions in a Ni-Mn-In metamagnetic shape memory alloy

The partially overlapped ferroelastic/martensitic and para-ferromagnetic phase transitions of a Ni$_{50.53}$Mn${33.65}$In$_{15.82}$ metamagnetic shape memory alloy have been studied from calorimetric, magnetic and acoustic emission measurement. We have taken advantage of the existence of thermal hysteresis of the first order ferroelastic/martensitic phase transition ($\sim2.5$K) to discriminate the latent heat contribution $Δ$Ht = 7.21(15) kJ/kg and the specific heat contribution $Δ$Hc = 216(1) J/kg to the total excess enthalpy of the phase transition. The specific heat was found to follow a step-like behavior at this phase transition. The intermittent dynamics of the ferroelastic/martensitic transition has been characterized as a series of avalanches detected both from acoustic emission and calorimetric measurements. The energy distribution of these avalanche events was found to follow a power law with a characteristic energy exponent $ε\sim2$ which is in agreement with the expected value for a system undergoing a symmetry change from cubic to monoclinic. Finally, the critical behavior of the para-ferromagnetic austenite phase transition that takes place at $\sim 311$K has been studied from the behavior of the specific heat. A critical exponent $α\sim0.09$ has been obtained, which has been shown to be in agreement with previous values reported for Ni-Mn-Ga alloys but different from the critical divergence reported for pure Ni.

cond-mat.mtrl-sci↗

arXiv2021

Experimental method to determine specific heat and transition enthalpy at a first order phase transition: fundamentals and application to a NiMnIn Heusler alloy

A new method that characterizes thermal properties during a first-order phase transition is described. The technique consists in exciting the sample by a series of constant frequency thermal pulses which one in every N pulses (N is a small number like four) being exceedingly large in amplitude. This pulse induces phase transformation which is inhibited during the following smaller pulses due to thermal hysteresis. That way the specific heat for a given mixture of phases can be determined. The results obtained are independent of experimental parameters like the rate and the amplitude of the pulses, unlike what happens in other calorimetric techniques. The method also provides the enthalpy excess by analyzing the energy balance between the dissipated heat and the heat flowing during each pulse of measurement. The protocol is tested to analyze the phase transitions of a Heusler alloy Ni 50.54 Mn 33.65 In 15.82 . The paramagnetic-ferromagnetic transition for the austenite phase is continuous and the specific heat shows a lambda anomaly. The martensitic phase transition shows a first-order character and the specific heat follows a step-like behaviour in contrast with previously reported large-peak anomalies.

cond-mat.mtrl-sci↗

arXiv2018

First Accurate Normalization of the $β$-delayed $α$ Decay of $^{16}$N and Implications for the $^{12}$C$(α,γ)^{16}$O Astrophysical Reaction Rate

The $^{12}\text{C}(α,γ){}^{16}\text{O}$ reaction plays a central role in astrophysics, but its cross section at energies relevant for astrophysical applications is only poorly constrained by laboratory data. The reduced $α$ width, $γ_{11}$, of the bound $1^-$ level in $^{16}$O is particularly important to determine the cross section. The magnitude of $γ_{11}$ is determined via sub-Coulomb $α$-transfer reactions or the $β$-delayed $α$ decay of $^{16}$N, but the latter approach is presently hampered by the lack of sufficiently precise data on the $β$-decay branching ratios. Here we report improved branching ratios for the bound $1^-$ level [$b_{β,11} = (5.02\pm 0.10)\times 10^{-2}$] and for $β$-delayed $α$ emission [$b_{βα} = (1.59\pm 0.06)\times 10^{-5}$]. Our value for $b_{βα}$ is 33% larger than previously held, leading to a substantial increase in $γ_{11}$. Our revised value for $γ_{11}$ is in good agreement with the value obtained in $α$-transfer studies and the weighted average of the two gives a robust and precise determination of $γ_{11}$, which provides significantly improved constraints on the $^{12}$C$(α,γ)$ cross section in the energy range relevant to hydrostatic He burning.

nucl-ex↗

arXiv2003

The order parameter-entropy relation in some universal classes: experimental evidence

The asymptotic behaviour near phase transitions can be suitably characterized by the scaling of $Δs/Q^2$ with $ε=1-T/T_c$, where $Δs$ is the excess entropy and $Q$ is the order parameter. As $Δs$ is obtained by integration of the experimental excess specific heat of the transition $Δc$, it displays little experimental noise so that the curve $\log(Δs/Q^2)$ versus $\logε$ is better constrained than, say, $\logΔc$ versus $\logε$. The behaviour of $Δs/Q^2$ for different universality classes is presented and compared. In all cases, it clearly deviates from being a constant. The determination of this function can then be an effective method to distinguish asymptotic critical behaviour. For comparison, experimental data for three very different systems, Rb2CoF4, Rb2ZnCl4 and SrTiO3, are analysed under this approach. In SrTiO3, the function $Δs/Q^2$ does not deviate within experimental resolution from a straight line so that, although Q can be fitted with a non mean-field exponent, the data can be explained by a classical Landau mean-field behaviour. In contrast, the behaviour of $Δs/Q^2$ for the antiferromagnetic transition in Rb2CoF4 and the normal-incommensurate phase transition in Rb2ZCl4 is fully consistent with the asymptotic critical behaviour of the universality class corresponding to each case. This analysis supports, therefore, the claim that incommensurate phase transitions in general, and the A$_2$BX$_4$ compounds in particular, in contrast with most structural phase transitions, have critical regions large enough to be observable.

cond-mat↗