Direct Measure of Giant Magnetocaloric Entropy Contributions in Ni-Mn-In

Off-stoichiometric alloys based on Ni 2 MnIn have drawn attention due to the coupled first order magnetic and structural transformations, and the large magnetocaloric entropy associated with the transformations. Here we describe calorimetric and magnetic studies of four compositions. The results provide a direct measure of entropy changes contributions including at the first-order phase transitions, and thereby a determination of the maximum field-induced entropy change corresponding to the giant magnetocaloric effect. We find a large excess entropy change, attributed to magneto-elastic coupling, but only in compositions with no ferromagnetic order in the high-temperature austenite phase. Furthermore, a molecular field model corresponding to antiferromagnetism of the low-temperature phases is in good agreement, and nearly independent of composition, despite significant differences in overall magnetic response of these materials

Calorimetric and magnetic study for Ni50_{50}Mn36_{36}In14_{14} and relative cooling power in paramagnetic inverse magnetocaloric systems

The non-stoichiometric Heusler alloy Ni$_{50}$Mn$_{36}$In$_{14}$ undergoes a martensitic phase transformation in the vicinity of 345 K, with the high temperature austenite phase exhibiting paramagnetic rather than ferromagnetic behavior, as shown in similar alloys with lower-temperature transformations. Suitably prepared samples are shown to exhibit a sharp transformation, a relatively small thermal hysteresis, and a large field-induced entropy change. We analyzed the magnetocaloric behavior both through magnetization and direct field-dependent calorimetry measurements. For measurements passing through the first-order transformation, an improved method for heat-pulse relaxation calorimetry was designed. The results provide a firm basis for the analytic evaluation of field-induced entropy changes in related materials. An analysis of the relative cooling power (RCP), based on the integrated field-induced entropy change and magnetizing behavior of the Mn spin system with ferromagnetic correlations, shows that a significant RCP may be obtained in these materials by tuning the magnetic and structural transformation temperatures through minor compositional changes or local order changes

High-field magneto-thermo-mechanical testing system for characterizing multiferroic bulk alloys

Multiferroic meta-magnetic shape memoryalloys are well known for exhibiting large magnetic field induced actuation strains, giant magnetocaloric effects, magneto-resistance, and structural and magnetic glassy behaviors. Thus, they are candidates for improving modern day sensing, actuation, magneto-resistance, and solid-state refrigeration processes. Until now, however, experimental apparatuses have typically been able to probe a limited ferroic parameter space in these materials, i.e., only concurrent thermal and mechanical responses, or magnetic and thermal responses. To overcome this barrier and better understand the coupling of multiple fields on materials behavior, a magneto-thermo-mechanical characterization device has been designed and implemented. This device is capable of compressing a specimen at load levels up to 5300 N collinearly with applied fields up to 9 T between temperatures of −100 °C and 120 °C. Uniaxial stress, strain, temperature, magnetic field, and the volumetric average magnetization have been simultaneously measured under mixed loading conditions on a NiCoMnIn meta-magnetic shape memoryalloy and a few selected results are presented here

Glassy phonon heralds a strain glass state in a shape memory alloy

Shape memory strain glasses are frustrated ferroelastic materials with glasslike slow relaxation an

Orientation dependence of the elastocaloric effect in Ni54Fe19Ga27 ferromagnetic shape memory alloy

The crystallographic anisotropy of elastocaloric effect (ECE) and relative cooling power (RCP) in Ni54Fe19Ga27 shape memory alloy single crystals are studied via compression tests. Single crystals are studied along the [001], [123], and [011] austenite directions and yield different ECE behaviors and maximum RCPs for various strain levels. A thermodynamic framework using the Helmholtz free energy is employed to analyze the total entropy change as a function of strain. Thermodynamic losses are computed from the mechanical hysteresis of superelasticity experiments to quantify the strain dependent RCP. It is found that the [001] orientation generates the highest maximal RCP of 738 J kg−1 when unloaded from 200 MPa. This is attributed mainly to the large superelastic temperature window of 45 K. However, loading the crystals to stresses higher than 200 MPa causes a multistep transformation in the [011] direction, thus reducing the alloy's overall RCP by 135 J kg−1. This is a consequence of the negative entropy change and large transformation hysteresis generated by the second‐stage transformation in the [011] direction. Interestingly, if only the first‐stage transformation in [011] is employed for the ECE, the [011] direction yields the highest RCP compared to [001] and [123] for any strain up to 3.5%

High-field magneto-thermo-mechanical testing system for characterizing multiferroic bulk alloys

Multiferroic meta-magnetic shape memory alloys are well known for exhibiting large magnetic field induced actuation strains, giant magnetocaloric effects, magneto-resistance, and structural and magnetic glassy behaviors. Thus, they are candidates for improving modern day sensing, actuation, magneto-resistance, and solid-state refrigeration processes. Until now, however, experimental apparatuses have typically been able to probe a limited ferroic parameter space in these materials, i.e., only concurrent thermal and mechanical responses, or magnetic and thermal responses. To overcome this barrier and better understand the coupling of multiple fields on materials behavior, a magneto-thermo-mechanical characterization device has been designed and implemented. This device is capable of compressing a specimen at load levels up to 5300 N collinearly with applied fields up to 9 T between temperatures of −100 °C and 120 °C. Uniaxial stress, strain, temperature, magnetic field, and the volumetric average magnetization have been simultaneously measured under mixed loading conditions on a NiCoMnIn meta-magnetic shape memory alloy and a few selected results are presented here

The effect of heat treatments on Ni43Mn42Co4Sn11 meta-magnetic shape memory alloys for magnetic refrigeration

The inverse magnetocaloric effect (MCE) in bulk polycrystalline and melt-spun ribbons of the Ni43Mn42Co4Sn11 meta-magnetic shape memory alloy (MSMA) is investigated. The influence of several material properties on the MCE and relative cooling power (RCP) are discussed and the property combinations for optimum MCE and RCP identified for a given thermodynamic framework. These include a small slope of magnetic field vs. martensitic transformation temperature phase diagram, a narrow transformation range, low transformation thermal hysteresis and a large change in magnetization on martensitic transformation, which results in low levels of applied magnetic fields desired for repeated MCE on field cycling. The thermo-magnetic responses of the samples were measured before and after heat treatments. The heat-treated ribbons produced the most favorable MCE by exhibiting the highest magnetization change and smallest elastic energy storage through the transformation. This was attributed to the specific microstructural features, including grain size to thickness ratio and degree of L2(1) ordering. In addition, issues in the literature in determining RCP for MSMAs are discussed, and a new method to find RCP is proposed and implemented. Completely reversible magnetic-field-induced martensitic transformation cycles were used to investigate hysteresis losses relative to actual refrigeration cycles, whereby the RCP was calculated using the defined thermodynamic framework and indirectly measured entropy changes. The annealed ribbons exhibited the high RCP level of 242 J kg(-1) under the applied field of 7 T compared with a theoretical maximum of 343 J kg(-1). Similar values of RCP in other MSMAs can be achievable if microstructural elastic energy storage and hysteresis loss are minimized during the transformation with the help of annealing treatments. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

On the microstructural origins of martensitic transformation arrest in a NiCoMnIn magnetic shape memory alloy

The martensitic transformation arrest phenomenon in Ni45Co5Mn36.6In13.4 meta-magnetic shape memory alloy (MMSMA) single crystals was investigated as a function of secondary annealing heat treatments, using thermo-magnetometry and transmission electron microscopy (TEM). Dark-field images of the austenite phase at room temperature revealed the long range L21 and B2 ordered microstructural landscape with different morphologies in the annealed single crystals. Their measured thermomagnetic responses demonstrated full transformation arrest after certain heat treatments and unique microstructural morphologies. Martensitic transformation hysteresis, range, and enthalpy were measured in the annealed partially- or non-arrested single crystals. With the data, herein, we found that the samples with long range L21 ordering exhibit martensitic transformation at higher temperatures than some of the crystals of the same composition showing predominantly B2 order. This finding opposes the previous reports on the effect of annealing on the martensitic transformation characteristics of MMSMAs. We provide evidence that long range order promoted with high temperature annealing is not the only microstructural feature capable of influencing the martensitic transition in NiCoMnIn MMSMAs. Data suggests that quenched in vacancies, in addition to long range order, also influence the transformation characteristics