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.

International audienceWe studied the effect of Fe and Co 3d elements selective substitution 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 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 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

We have studied the elastocaloric effect in a Co-doped Ni-Mn-Ga magnetic Heusler shape memory alloy in the vicinity of its martensitic transition. Measurements of the length change as a function of temperature have been carried out across the transition under applied compression stresses and magnetic fields. The isothermal stress-induced entropy changes have been computed from the experimental data. Results evidence a significant elastocaloric effect associated with the large entropy change of the structural phase transition. The alloy also exhibits a magnetocaloric effect at low applied magnetic fields. It is shown that application of a magnetic field below 1 T increases the estimated elastocaloric relative cooling power by about 20%. A comparison of elasto- and magnetocaloric properties indicates that a similar relative cooling power is reached under application of 10 MPa or 0.8 T.

We have studied the effect of Fe addition on the structural and magnetic transitions in the magnetic shape memory alloy Ni-Mn-Ga by substituting systematically each atomic species by Fe. Calorimetric and ac susceptibility measurements have been carried out in order to study the magnetic and structural transformation properties. We find that the addition of Fe modifies the structural and magnetic transformation temperatures. Magnetic transition temperatures are displaced to higher values when Fe is substituted into Ni-Mn-Ga, while martensitic and premartensitic transformation temperatures shift to lower values. Moreover, it has been found that the electron per 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 related ternary system Ni-Mn-Ga.