Magnetic Field Suppression of the Martensitic Transformation in Mn-Based MnNi(Fe)Sn Metamagnetic Shape Memory Heusler Alloys

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

Martensitic transformation (MT), magnetic properties, and magnetocaloric effect (MCE) in Heusler-type Ni47Mn40Sn13−x Cd x (x= 0, 0.75, 1, 1.25 at. %) metamagnetic shape memory alloys (MetaMSMAs) are investigated, both experimentally and theoretically, as a function of doping with Cd. Ab-initio computations reveal that the ferromagnetic (FM) configuration is energetically more favorable in the cubic phase than the antiferromagnetic (AFM) state in undoped and doped alloys as well. Moreover, it is revealed that the alloys in the ground state exhibit a tetragonal structure confirming the existence of MT, in agreement with the experiments. It was indicated, both in theory and practice, that a reduction of the unit cell volume and an increase of the MT temperature as a function of the Cd doping. Indirect estimations of MCE in the vicinity of MT were carried out by using thermomagnetization curves measured under different magnetic fields up to 5 T. The results demonstrated that the doped alloys exhibit enhanced values of the inverse MCE comparable with those of Ni-Mn-based MetaMSMAs. Maximum magnetic entropy change in a field change of 2 T increases from 3.0 J.kg−1K−1 for the undoped alloy to 3.4 and 5.0 J.kg−1K−1 for the alloys doped with 0.75 and 1 at.% of Cd, respectively. The inverse and conventional MCE were explored by direct measurements of the adiabatic temperature change under the magnetic field change of 1.96 T. The Cd doping increased the maximum of inverse MCE by nearly 78% from 0.9 K to 1.6 K for the undoped and doped alloys, respectively. The results depicted that Cd doping can effectively tailor the structural, magnetic, and MCE properties of the Ni–Mn–Sn MetaMSMAs.

Magnetocaloric effect (MCE), magnetization, specific heat, and magnetostriction measurements were performed in both pulsed and steady high magnetic fields to investigate the magnetocaloric properties of Heusler alloys Ni50-xCoxMn31.5Ga18.5 (x = 9 and 9.7). From direct MCE measurements for Ni41Co9Mn31.5Ga18.5 up to 56 T, a steep temperature drop was observed for magnetic-field-induced martensitic transformation (MFIMT), designated as inverse MCE. Remarkably, this inverse MCE is apparent not only with MFIMT, but also in the magnetic-field-induced austenite phase. Specific heat measurements under steady high magnetic fields revealed that the magnetic field variation of the electronic entropy plays a dominant role in the unconventional magnetocaloric properties of these materials. First-principles based calculations performed for Ni41Co9Mn31.5Ga18.5 and Ni45Co5Mn36.7In13.3 revealed that the magnetic-field-induced austenite phase of Ni41Co9Mn31.5Ga18.5 is more unstable than that of Ni45Co5Mn36.7In13.3 and that it is sensitive to slight tetragonal distortion. We conclude that the inverse MCE in the magnetic-field-induced austenite phase is realized by marked change in the electronic entropy through tetragonal distortion induced by the externally applied magnetic field.

Tuning martensitic transformation and magnetocaloric effect with Bi substitution in MnCo1-xBixGe alloys

NiMn-based metamagnetic shape memory alloys

In order to investigate behavior of magnetic field-induced reverse martensitic transformation for Ni-Co-Mn-Sb, magnetization experiments up to a static magnetic field of 18 T and a pulsed magnetic field of 40 T were carried out. In the thermomagnetization curves for Ni41Co9Mn39Sb11 alloy, the equilibrium transformation temperature T0 was observed to decrease with increasing applied magnetic field, μ0H, at a rate of dT0/dμ0H = 4.6 K/T. The estimated value of entropy change evaluated from the Clausius-Clapeyron relation was about 14.1 J/(K·kg), which was in good agreement with the value obtained by differential scanning calorimetric measurements. For the isothermal magnetization curves, metamagnetic behavior associated with the magnetic field-induced martensitic transformation was observed. The equilibrium magnetic field, μ0H0 = (μ0HAf + μ0HMs)/2, of the martensitic transformation tended to be saturated at lower temperature; that is, transformation arrest phenomenon was confirmed for the Ni-Co-Mn-Sb system, analogous with the Ni(Co)-Mn-Z (Z = In, Sn, Ga, Al) alloys. Temperature dependence of the magnetic field hysteresis, μ0Hhys = μ0HAf − μ0HMs, was analyzed based on the model for the plastic deformation introduced by the dislocations. The behavior can be explained by the model and the difference of the sweeping rate of the applied magnetic field was well reflected by the experimental results.

Martensitic transformation and magnetic properties in high-Mn content Mn 50Ni 50− xIn x ferromagnetic shape memory alloys

Non-equilibrium martensitic transformation in metamagnetic shape memory alloys

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 at TCA = 309 K. This result strongly motivates further development of different MetaMSMA refrigerants shaped as ribbons.

Metamagnetic shape memory alloys have aroused considerable attraction as potential magnetic refrigerants due to the large inverse magnetocaloric effect associated to the magnetic-field-induction of a reverse martensitic transformation (martensite to austenite). In some of these alloys, the austenite phase can be retained on cooling under high magnetic fields, being the retained phase metastable after field removing. Here, we report a giant direct magnetocaloric effect linked to the anomalous forward martensitic transformation (austenite to martensite) that the retained austenite undergoes on heating. Under moderate fields of 10 kOe, an estimated adiabatic temperature change of 9 K has been obtained, which is (in absolute value) almost twice that obtained in the conventional transformation under higher applied fields. The observation of a different sign on the temperature change associated to the same austenite to martensite transformation depending on whether it occurs on heating (retained) or on cooling is attributed to the predominance of the magnetic or the vibrational entropy terms, respectively.

In situ optical microscopic observation of NiCoMnIn metamagnetic shape memory alloy under pulsed high magnetic field

In situ microscopic study was carried out on Ni44.3Co5.1Mn31.4Al19.2 metamagnetic shape memory alloy under a pulsed magnetic field up to 35.5T. Magnetic field-induced reverse martensitic transformation was directly observed. The critical magnetic fields were determined from the contrast change of the microstructure, where good agreement had been found by magnetization measurement in a previous study. Comparison of the micrographs during thermal and magnetic field cycles between Ni45Co5Mn36.7In13.3 and Ni44.3Co5.1Mn31.4Al19.2 was conducted. It is concluded that though the energy barrier related to thermal activation strongly effects the martensitic transformation of Ni45Co5Mn36.7In13.3, its effect is very small on Ni44.3Co5.1Mn31.4Al19.2. Qualitative discussion yielded a reasonable understanding of the results of microstructure observation. [doi:10.2320/matertrans.MBW201202]

Metamagnetic shape memory alloys (MMSMAs) are attractive functional materials owing to their unique properties such as magnetostrain, magnetoresistance, and the magnetocaloric effect caused by magnetic‐field‐induced transitions. However, the energy loss during the martensitic transformation, that is, the dissipation energy, Edis, is sometimes large for these alloys, which limits their applications. In this paper, a new Pd2MnGa Heusler‐type MMSMA with an extremely small Edis and hysteresis is reported. The microstructures, crystal structures, magnetic properties, martensitic transformations, and magnetic‐field‐induced strain of aged Pd2MnGa alloys are investigated. A martensitic transformation from L21 to 10M structures is seen at 127.4 K with a small thermal hysteresis of 1.3 K. The reverse martensitic transformation is induced by applying a magnetic field with a small Edis (= 0.3 J mol−1 only) and a small magnetic‐field hysteresis (= 7 kOe) at 120 K. The low values of Edis and the hysteresis may be attributed to good lattice compatibility in the martensitic transformation. A large magnetic‐field‐induced strain of 0.26% is recorded, indicating the proposed MMSMA's potential as an actuator. The Pd2MnGa alloy with low values of Edis and hysteresis may enable new possibilities for high‐efficiency MMSMAs.

Effect of partial metamagnetic and magnetic transition coupling on the magnetocaloric effect of Ni-Mn-Sn-Fe alloy

The fcc → fct martensitic transformation of a Mn-rich γ-MnFe alloy with a (011) martensite twin was investigated using transmission electron microscopy, dynamic mechanical analysis, electrical resistance measurements, differential scanning calorimetry and dilatometry. Experimental results show that the start temperature (M s) of the martensitic transformation and the temperature (T N) of the antiferromagnetic transition in Mn85.5Fe9.5Cu5 alloys are close and almost coincide. Direct and reverse martensitic transformations, normally a first-order transformation, exhibit features similar to those of a second-order continuous transition, which is attributed to coupling between the first-order martensitic transformation and the second-order antiferromagnetic transition.