arXiv subjects

Francisco Javier Romero

Publications and source records attributed to Francisco Javier Romero.

2 recordsLinked to original sources

arXiv2024

Ultraslow calorimetric studies of the martensitic transformation of NiFeGa alloys: detection and analysis of avalanche phenomena

We study the thermal properties of a bulk Ni55Fe19Ga26 Heusler alloy in a conduction calorimeter. At slow heating and cooling rates (1K/h), we compare as-cast and annealed samples. We report a smaller thermal hysteresis after the thermal treatment due to the stabilization of the 14M modulated structure in the martensite phase. In ultraslow experiments (40mK/h), we detect and analyze the calorimetric avalanches associated with the direct and reverse martensitic transformation from cubic to 14M phase. This reveals a distribution of events characterized by a power law with exponential cutoff $p(u) \propto u^{-\varepsilon}\exp(-u/ξ)$ where $\varepsilon\sim 2$ and damping energies $ξ=370$uJ (direct) and $ξ=27$uJ (reverse) that characterize the asymmetry of the transformation.

cond-mat.mtrl-sci↗

arXiv2024

Phase transition and polar cluster behavior above Curie temperature in ferroelectric BaTi$_{0.8}$Zr$_{0.2}$O$_3$

We study the phase transition behavior of the ferroelectric BaTi$_{0.8}$Zr$_{0.2}$O$_3$ in the paraelectric region. The temperature dependencies of thermal, polar, elastic and dielectric properties indicate the presence of local structures above the paraelectric-ferroelectric transition temperature Tc = 292 K. The non-zero remnant polarization is measured up to a characteristic temperature T* ~350 K, which coincides with the temperature where the dielectric constant deviates from Curie-Weiss law. Resonant Piezoelectric Spectroscopy shows that DC field-cooling above Tc using fields smaller than the coercive field leads to an elastic response and remnant piezoelectricity below T*, which likely corresponds to the coherence temperature associated with polar nanostructures in ferroelectrics. The observed remnant effect is attributed to the reorientation of polar nanostructures above Tc.

cond-mat.mtrl-sci↗