The magnetocaloric effect (MCE) in paramagnetic materials has been widely used for attaining very low temperatures by applying a magnetic field isothermally and removing it adiabatically. The effect can also be exploited for room-temperature refrigeration by using giant MCE materials. Here we report on an inverse situation in Ni-Mn-Sn alloys, whereby applying a magnetic field adiabatically, rather than removing it, causes the sample to cool. This has been known to occur in some intermetallic compounds, for which a moderate entropy increase can be induced when a field is applied, thus giving rise to an inverse magnetocaloric effect. However, the entropy change found for some ferromagnetic Ni-Mn-Sn alloys is just as large as that reported for giant MCE materials, but with opposite sign. The giant inverse MCE has its origin in a martensitic phase transformation that modifies the magnetic exchange interactions through the change in the lattice parameters.

Structural and magnetic transformations in the Heusler-based system Ni 0.50 Mn 0.50−x Sn x are studied by x-ray diffraction, optical microscopy, differential scanning calorimetry, and magnetization. The structural transformations are of austenitic-martensitic character. The austenite state has an L2 1 structure, whereas the structures of the martensite can be 10M, 14M, or L1 0 depending on the Sn composition. For samples that undergo martensitic transformations below and around room temperature, it is observed that the magnetic exchange in both parent and product phases is ferromagnetic, but the ferromagnetic exchange, characteristic of each phase, is found to be of different strength. This gives rise to different Curie temperatures for the austenitic and martensitic states.

The magnetic and structural transformations in the Heusler-based system Ni 0.50 Mn 0.50−x In x are studied in the composition range 0.05ഛ x ഛ 0.25. While the cubic phase is preserved in the range 0.165ഛ x ഛ 0.25, we find the presence of martensitic transformations in alloys with x ഛ 0.16. In a critical composition range 0.15ഛ x ഛ 0.16, the magnetic coupling in both austenitic and martensitic states is ferromagnetic. Magnetic field-induced structural transitions are also found in the x = 0.16 alloy, whereby the structural transition temperature shifts by 42 K in a field of 50 kOe.

Applying a magnetic field to a ferromagnetic Ni 50 Mn 34 In 16 alloy in the martensitic state induces a structural phase transition to the austenitic state. This is accompanied by a strain which recovers on removing the magnetic field, giving the system a magnetically superelastic character. A further property of this alloy is that it also shows the inverse magnetocaloric effect. The magnetic superelasticity and the inverse magnetocaloric effect in Ni-Mn-In and their association with the first-order structural transition are studied by magnetization, strain, and neutron-diffraction studies under magnetic field.

We report on measurements of the adiabatic temperature change in the inverse magnetocaloric Ni 50 Mn 34 In 16 alloy. It is shown that this alloy heats up with the application of a magnetic field around the Curie point due to the conventional magnetocaloric effect. In contrast, the inverse magnetocaloric effect associated with the martensitic transition results in the unusual decrease of temperature by adiabatic magnetization. We also provide magnetization and specific heat data which enable to compare the measured temperature changes to the values indirectly computed from thermodynamic relationships. Good agreement is obtained for the conventional effect at the second-order paramagnetic-ferromagnetic phase transition. However, at the first-order structural transition the measured values at high fields are lower than the computed ones. Irreversible thermodynamics arguments are given to show that such a discrepancy is due to the irreversibility of the first-order martensitic transition.

We report magnetization and differential thermal analysis measurements as a function of pressure accross the martensitic transition in magnetically superelastic Ni-Mn-In alloys. It is found that the properties of the martensitic transformation are significantly affected by the application of pressure. All transition temperatures shift to higher values with increasing pressure. The largest rate of temperature shift with pressure has been found for Ni50Mn34In16 as a consequence of its small entropy change at the transition. Such a strong pressure dependence of the transition temperature opens up the possibility of inducing the martensitic transition by applying relatively low hydrostatic pressures. [4], which derive from the coupling between the martensitic transition and the magnetic order. In this family of alloys, magnetic moments are mainly confined to the Mn atoms. The exchange interaction between magnetic moments is long range and oscillatory, and is mediated by the conduction electrons. As a consequence, the magnetic properties of these alloys are sensitive to the distance between neighboring Mn atoms, and, indeed, different magnetic behavior has been reported for alloys with different X element. At the martensitic transition, the change in the lattice cell modifies the distance between Mn-atoms which can lead to antiferromagnetic interactions. Antiferromagnetic interactions are expected to be present in off-stoichiometric Ni-Mn-X compounds with X as Ga [5], Sn [6], In [7] and Sb [8].In the present paper, we investigate the effect of hydrostatic pressure on Ni-Mn-In magnetic shape memory alloys. The application of pressure modifies the distance between Mn atoms thereby affecting the magnetic exchange. The relative stability between the high temperature cubic phase and the low temperature martensitic phase is also be affected by pressure.Two samples were prepared by arc melting pure metals under argon atmosphere. They were then annealed at 1073 K for 2 hours and quenched in ice-water. The compositions of the alloys were determined by energy dispersive x-ray analysis to be Ni 50.0 Mn 34.0 In 16.0 and Ni 49.5 Mn 35.5 In 15.0 . Magnetization measurements were performed using a superconducting quantum interference device magnetometer equipped with pressure cell in fields up to 5 T in the temperature range 4 -340 K and for pressures up to 10 kbar. High-pressure differential thermal analysis (HP-DTA) was carried out in a calorimeter capable of operating in the temperature and pressure ranges 183 -473 K and 0 -3 kbar respectively. The calorimeter is similar to that described in [9]. Powder samples were mixed with an inert perfluorinated liquid (Galden, from Bioblock Scientifics) before they were hermetically sealed in order to ensure pressure transmission. Thermal curves were recorded as a function of temperature for selected pressure values. HP-DTA scans were run on heating and cooling at 1 K min −1 rates. Figure 1 shows the temperature dependence of the magnetization in a low external magnetic field of H = 50 Oe...

At certain compositions Ni-Mn-X Heusler alloys ͑X: group IIIA-VA elements͒ undergo martensitic transformations, and many of them exhibit inverse magnetocaloric effects. In alloys where X is Sn, the isothermal entropy change is largest among the Heusler alloys, particularly in Ni 50 Mn 37 Sn 13 , where it reaches a value of 20 J kg −1 K −1 for a field of 5 T. We substitute Ni with Fe and Co in this alloy, each in amounts of 1 and 3 at % to perturb the electronic concentration and examine the resulting changes in the magnetocaloric properties. Increasing both Fe and Co concentrations causes the martensitic transition temperature to decrease, whereby the substitution by Co at both compositions or substituting 1 at % Fe leads to a decrease in the magnetocaloric effect. On the other hand, the magnetocaloric effect in the alloy with 3 at % Fe leads to an increase in the value of the entropy change to about 30 J kg −1 K −1 at 5 T.

Ni50Mn34In16 undergoes a martensitic transformation around 250 K and exhibits a field induced reverse martensitic transformation and substantial magnetocaloric effects. We substitute small amounts Ga for In, which are isoelectronic, to carry these technically important properties to close to room temperature by shifting the martensitic transformation temperature.There is growing interest in searching for materials other than Ni-Mn-Ga which may have interesting properties concerning applications relevant to magnetic-fieldinduced strains. Such search on Ni-Mn based Heusler systems has led to the observation of giant magnetocaloric effects (MCE) [1,2,3,4,5,6], large strains related to field-induced transformations, and substantial contribution to the understanding of martensitic transformations in ferromagnetic Heusler materials. The valence electron concentration (e/a) dependence of M s in NiMnX is linear, but with different slope for each X-species [7]. Therefore, it should be possible to manipulate M s not only by varying e/a, but also by holding e/a constant and replacing one X species with another. In this manner one may have the possibility of shifting and adjusting favorable features occurring around the martensitic transformation of a particular alloy to higher or lower temperatures. Ni 50 Mn 34 In 16 [(e/a) ≈ 7.87] shows a field induced reverse martensitic transformation at M s ≈ 250 K and associated with it, a large field induced strain and a magnetocaloric effect [8,9]. In view of technical interest, it would be desirable to shift the transition temperature to around room temperature without altering the favorable features. On the other hand, in view of understanding the electronic properties of such systems close to the martensitic transformation, it would be interesting to understand to what extent the valence electron concentration can be employed as a meaningful parameter. To test this possibility, we substitute 2% Ga for In in Ni 50 Mn 34 In 16 . From interpolation at constant (e/a), this amount of Ga is expected to shift M s to around room temperature. We compare in this study the magnetic and magnetocaloric properties of the isoelectronic compounds Ni 50 Mn 34 In 16 [8] and Ni 50 Mn 34 In 14 Ga 2 an discuss to what extent the features around M s are preserved. The magnetocaloric properties are studied from the entropy-change as well as from direct temperaturechange measurements.