H. Flores-Zúñiga

Elastocaloric and magnetocaloric effects in Ni-Mn-Sn(Cu) shape-memory alloy

We have studied magnetocaloric and elastocaloric properties of a Ni-Mn-Sn(Cu) metamagnetic shape-memory alloy undergoing a magneto-structural transition (martensitic type) close to room temperature. Changes of entropy have been induced by isothermally applying both mechanical (uniaxial stress) and magnetic fields. These entropy changes have been, respectively, estimated from dilatometric measurements giving the length of the sample as a function of temperature at selected applied forces and magnetic fields and from magnetization measurements as a function of temperature at selected applied magnetic fields. Our results indicate that the elastocaloric effect is conventional and occurs in two steps which reflect the interplay between the martensitic and the incipient magnetic transitions. By contrast, the magnetocaloric effect is inverse and occurs in a single step that encompasses the effect arising from both transitions.

Fe and Co selective substitution in Ni2MnGa: Effect of magnetism on relative phase stability

We studied the effect of selective substitution in Fe and Co 3d elements on the structural and magnetic phase transitions in the prototypical magnetic shape memory alloy Ni–Mn–Ga. We determined the phase diagram for each elemental doping substitution by means of calorimetry and ac susceptibility measurements. An effort was made to substitute each constituent element by the same amounts of doping and to study the role of parameters other than electronic concentration in controlling phase stability. Specifically, selective doping with atoms of similar atomic radii but different magnetic properties allowed us to investigate the role of magnetic interactions on the relative phase stability of the ternary compound. We determined the entropy change associated with the martensitic transition for each quaternary alloy to obtain further information on the effect of magnetism on the relative stability of the involved phases exhibited by these compounds.

Phase diagram of Fe-doped Ni-Mn-Ga ferromagnetic shape-memory alloys

ThyssenKrupp Electrical Steel GmbH, D-45881 Gelsenkirchen, Germany(Dated: April 8, 2008)We have studied the effect of Fe addition on the structural and magnetic transitions in themagnetic shape memory alloy Ni-Mn-Ga by substituting systematically each atomic species byFe. Calorimetric and AC susceptibility measurements have been carried out in order to study themagnetic and structural transformation properties. We find that the addition of Fe modifies thestructural and magnetic transformation temperatures. Magnetic transition temperatures are dis-placed to higher values when Fe is substituted into Ni-Mn-Ga, while martensitic and premartensitictransformation temperatures shift to lower values. Moreover, it has been found that the electronper atom concentration essentially governs the phase stability in the quaternary system. However,the observed scaling of transition temperatures with e/a differs from that reported in the relatedternary system Ni-Mn-Ga.

Stress- and magnetic field-induced entropy changes in Fe-doped Ni–Mn–Ga shape-memory alloys

Isothermal stress- and magnetic field-induced entropy changes in a Fe-doped Ni–Mn–Ga alloy have been measured in the limits of low applied stress and magnetic field. We have obtained that in this limit while elastocaloric is conventional, giving rise to an increase of entropy when a stress is applied, magnetocaloric effect is inverse, which means that entropy decreases by application of an applied magnetic field. This inverse effect is a consequence of the magnetostructural coupling driven by the martensitic transition.

Structural and magnetic characterization of the intermartensitic phase transition in NiMnSn Heusler alloy ribbons

Phase transitions and structural and magnetic properties of rapidly solidified Ni50Mn38Sn12 alloy ribbons have been studied. Ribbon samples crystallize as a single-phase, ten-layered modulated (10M) monoclinic martensite with a columnar-grain microstructure and a magnetic transition temperature of 308 K. By decreasing the temperature, martensite undergoes an intermartensitic phase transition around 195 K. Above room temperature, the high temperature martensite transforms into austenite. Below 100 K, magnetization hysteresis loops shift along the negative H-axis direction, confirming the occurrence of an exchange bias effect. On heating, the thermal dependence of the coercive field HC shows a continuous increase, reaching a maximum value of 1017 Oe around 50 K. Above this temperature, HC declines to zero around 195 K. But above this temperature, it increases again up to 20 Oe falling to zero close to 308 K. The coercivity values measured in both temperature intervals suggest a significant difference in the...

Refrigerant capacity of austenite in as-quenched and annealed Ni51.1Mn31.2In17.7 melt spun ribbons

The thermal dependence of the magnetic entropy change (ΔSM(T)) and refrigerant capacity (RC) of austenite in as-quenched ribbons of chemical composition Ni51.1Mn31.2In17.7 produced by melt spinning at a high cooling rate of 48 ms−1 is reported. The effect of annealing at 1073 K on the structure and the magnetic properties was studied. The as-quenched sample is a single-phase austenite that presents a B2 ordered structure. The annealing on the melt spun samples produced a L21-type ordered structure. Austenite is characterized by a broad ΔSM(T) curve that, for a field change of 5.0 T, exhibits a full-width at half-maximum δTFMHW of 107 K, a peak value of the magnetic entropy change ΔSMpeak of −3.1 Jkg−1 K−1, and RC = 345 Jkg−1. Although annealed samples show larger ΔSMpeak values the narrower ΔSM(T) curves leads to a reduction in RC. Thus, the as-quenched sample shows a higher efficiency for a refrigerant cycle.

Adiabatic magnetocaloric effect in Ni50Mn35In15 ribbons

Heusler-type Ni-Mn-based metamagnetic shape memory alloys (MetaMSMAs) are promising candidates for magnetic refrigeration. To increase heat exchange rate and efficiency of cooling, the material should have a high surface/volume ratio. In this work, the typical Ni50Mn35In15 MetaMSMA was selected to fabricate thin ribbons by melt-spinning. The characteristic transformations of the ribbons were determined by calorimetry, X-ray diffraction, scanning electron microscopy and thermomagnetization measurements. The inverse and conventional magnetocaloric effects (MCEs) associated with the martensitic transformation (MT) and the ferromagnetic transition of the austenite (TCA), respectively, were measured directly by the adiabatic method (ΔTad) and indirectly by estimating the magnetic entropy change from magnetization measurements. It is found that the ribbons exhibit large values of ΔTad = −1.1 K at μ0ΔH = 1.9 T, in the vicinity of the MT temperature of 300 K for inverse MCE, and ΔTad = 2.3 K for conventional MCE ...

Expanding the magnetocaloric operation range in Ni–Mn–In Heusler alloys by Cu-doping

We study the magnetocaloric properties of a set of Cu-doped Ni–Mn–In metamagnetic shape memory alloys with the aim of designing composites with large operation range constituted of components of selected Cu-fractions. The studied Cu-doped alloys are characterized by a low hysteresis (~5 K) and by a strong sensitivity of their transition temperatures to an applied magnetic field which ensures excellent reproducibility of the magnetocaloric entropy change ( J K−1 kg−1) upon cyclic application of 6 T. Within the studied composition region, fine-tuning of the Cu-fraction enables shifting the transition temperature by desired amounts. By means of calorimetric measurements under applied magnetic field we show that by assembling samples of tailored compositions it is possible to build a composite showing reversible large magnetocaloric effect (~7.5 J kg−1 K−1) over a broad temperature range of ~30 K.

Recrystallization of a Ti-45Ni-5Cu cold-worked shape memory alloy characterized by thermoelectric power and electrical properties

The recrystallization of a cold-worked shape memory alloy (SMA) without R-phase transition, i.e. the Ti-45.0Ni-5.0Cu (% at.), has been studied by thermoelectric power (TEP) technique and electrical resistivity change (Dρ) on several heat treated samples after cold working. This study was also supported by differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and hardness Vickers (HV) measurements. Transformation temperatures show an increase with heat treatment temperature, while hysteresis decreases. Hardness values decrease with aging temperature for the same heat treatment time. No important changes were observed in electrical resistivity with heat treatment temperature, except at 300oC. TEP results show an important increase with heat treatment temperature, and a linear relationship was found between hardness and thermoelectric power. With this correlation between TEP and microhardness it is possible to predict its mechanical response by knowing its TEP, so this correlation could be important for applications.

Large and reversible elastocaloric effect near room temperature in a Ga-doped Ni–Mn–In metamagnetic shape-memory alloy

We report a giant elastocaloric effect near room temperature in a polycrystalline Ga-doped Ni–Mn–In ferromagnetic shape-memory alloy. The elastocaloric effect has been quantified by measuring both isothermal stress-induced entropy changes and adiabatic stress-induced temperature changes. A reproducible maximum entropy change, ΔSrev≃ 25 J⋅K−1⋅kg−1, upon cycling across the martensitic transition was obtained by application of a compressive stress of 100MPa. The corresponding maximum amount of cooling, ΔTadi≃−4.9K, was measured when this stress was rapidly removed. These values are comparable with those reported for giant magnetocaloric materials, which are induced by application and release of a high magnetic field. Therefore, the studied material is a good candidate to be used in solid-state refrigeration devices based on the elastocaloric effect.