Influence of Thermal and Magnetic History on Direct DTad Measurements of Ni49+xMn36-xIn15 Heusler Alloys

In the present work, using Heusler Ni49+xMn36-xIn15 (with x = 0 and 0.5) alloys, it is shown that the choice of the appropriate measurement protocol (erasing the prior state of the sample in between experiments) in DTad first shot characterization is crucial for obtaining reliable results. Unlike indirect measurements, for which incorrect protocols produce overestimates of the characteristics of the material, erroneous direct measurements underestimate DTad in the region close to its first order phase transition. The error in DTad is found to be dependent on the temperature step used, being up to ~40% underestimation, including a slight shift in its peak temperature.AEI/FEDER-UE (project MAT-2016-77265-R)US Army Research Laboratory W911NF-19-2-021

AC conduction in sol-gel-derived glasses in the SiO<SUB>2</SUB>-As<SUB>2</SUB>O<SUB>3</SUB> system

AC-resistivity measurements on sol-gel-derived glasses in the system xAs<SUB>2</SUB>O<SUB>3</SUB>·(1-x)SiO<SUB>2</SUB> with 0.05 &lt; x &lt; 0.16 have been carried out over the frequency range 2-100 kHz in the temperature range 80-400 K. The ac resistivity for all the glasses shows a sharp minimum at around 317 K. This is ascribed to a change in chemical equilibrium of the ratio [As<SUP>5+</SUP>]/[As<SUP>3+</SUP>] as a function of temperature. In the temperature range 190-300 K the ac resistivity is shown to arise due to a correlated-barrier-hopping mechanism involving bipolarons. A small fraction of total arsenic sites is found to participate in the hopping conduction. The ac resistivity at temperatures below 190 K shows a frequency exponent s, the variation of which can be qualitatively explained on the basis of the overlapping-large-polaron model

AC conductivity of Sb<SUB>2</SUB>O<SUB>3</SUB>-P<SUB>2</SUB>O<SUB>5</SUB> glasses

AC resistivity has been measured for glasses in the system (Sb<SUB>2</SUB>O<SUB>3</SUB>)<SUB>x</SUB>(P<SUB>2</SUB>O<SUB>5</SUB>)<SUB>(1-x)</SUB> with 0.03 &lt; x &lt; 0.2 over a frequency range 2-100 kHz for temperatures varying from 100 to 350 K. The resistivity shows a frequency dependence given by ρ(ω)∝ω<SUP>−s</SUP> with s having values in the range 0.8-0.95. The conductivity in these glasses arises due to the presence of Sb<SUP>3+</SUP> and Sb<SUP>5+</SUP> ions. A correlated-barrier-hopping model of bipolarons predicts hopping-length values which are inconsistent with the overall concentration of antimony ions in the glasses concerned. The overlapping-large-polaron model, on the other hand, gives the correct variation of tunneling distance with glass composition

High dielectric permittivity in sol-gel derived SiO<SUB>2</SUB>-As<SUB>2</SUB>O<SUB>3</SUB> glasses

Glasses in the system x As2O3(1-x) SiO2 with 0.05&lt;x&lt;0.16 have been prepared by sol-gel route. They exhibit anomalously large dielectric permittivity in the temperature range 275 to 325 K. The permittivity values are found to be in the range 400-100000 depending on the glass composition and the frequency of measurement. This effect is believed to arise due to a fairly large value of ratio [As5+]/[As3+] in these glasses and its drastic change as a function of temperature

Magnetic properties of Sb<SUB>2</SUB>O<SUB>3</SUB>-doped Ba-M hexagonal ferrites

Ba-M hexagonal ferrites with Sb<SUB>2</SUB>O<SUB>3</SUB> doping have been prepared in the composition series BaO(Fe<SUB>2</SUB>O<SUB>3</SUB>)6-x(Sb<SUB>2</SUB>O<SUB>3</SUB>)x with 0 &#8804; x &#8804; 0.4. Cold pressed mixture of the starting oxides has been given heat treatment in the temperature range 1100 to 1300&#176; C for periods extending to 12 h. Sb<SUB>2</SUB>O<SUB>3</SUB> doping reduces the values of saturation magnetization and Curie temperature. A cationic distribution scheme with Sb<SUP>3+</SUP> ions occupying the octahedral sites in the S block of the Ba-M hexagonal crystal structure predicts magnetization changes similar to those observed experimentally

Iron nanoparticles in copper matrix prepared by sol-gel route

Iron particles having diameters around 8 nm and loosely packed with nanosized copper particles have been prepared by a sol-gel route. The samples exhibit coercivities in the range 200 to 500 Oe that are typical of single-domain iron grains. The Mössbauer spectrum is consistent with the presence of α-Fe particles in the system. However, a finite value of the isomer shift is obtained that is ascribed to possible electron transfer between the iron atoms and the surrounding copper matrix

Analysis of the magnetic field dependence of the isothermal entropy change of inverse magnetocaloric materials

In this work, the magnetic field dependence of the inverse magnetocaloric (MC) effect is analyzed using a mean field approach for describing antiferromagnetic to ferromagnetic magnetoelastic transitions. The model is able to describe both second- and first-order transition through the introduction of a magnetovolume energy term. The power law dependence for the field dependence of the isothermal magnetic entropy change (ΔSiso∝ΔHn), has an exponent n with an overshoot above 2 for first-order transitions, while it is not present for the second-order case. This is in excellent agreement with previous phenomenological observations, supporting the validity of recently proposed criterion to distinguish between first- and second-order thermomagnetic transitions. A main difference with respect to direct MC effect is that negative values of the exponent n are obtained at temperatures close to the transition. This is ascribed to the reduction of the inverse MC response due to the influence of the unavoidable ferromagnetic to paramagnetic transition at higher temperatures. The obtained features are qualitatively compared to those of GdBaCo2O6 (antiferromagnetic to ferromagnetic magnetoelastic transition), showing a good agreement between both experiments and the model. The obtained information is extrapolated to understand the behavior of the exponent n for a Ni49Mn36In15 sample (magnetostructural transition).Agencia Estatal de Investigación PID2019-105720RB-I0Universidad de Sevilla US-126017Consejería de Economía, Conocimiento, Empresas y Universidad P18-RT-746US Army Research Laboratory W911NF-19-2-021

Influence of Thermal and Magnetic History on Direct DTad Measurements of Ni49+xMn36xIn15 Heusler Alloys

In the present work, using Heusler Ni49+xMn36-xIn15 (with x = 0 and 0.5) alloys, it is shown that the choice of the appropriate measurement protocol (erasing the prior state of the sample in between experiments) in DTad first shot characterization is crucial for obtaining reliable results. Unlike indirect measurements, for which incorrect protocols produce overestimates of the characteristics of the material, erroneous direct measurements underestimate DTad in the region close to its first order phase transition. The error in DTad is found to be dependent on the temperature step used, being up to ~40% underestimation, including a slight shift in its peak temperature.Peer reviewe