The Heusler-type alloy Ni 2ϩx Mn 1Ϫx Ga exhibits well defined shape memory properties in a ferromagnetic state, which means that the martensitic transition temperature is lower than the Curie point of this material. The change of composition makes these characteristic temperatures approach each other. To study this behavior, the measurements of specific heat, ac magnetic susceptibility, and dc resistivity were performed. The phase diagram of the cubic ferromagnet describing possible structural and magnetic transitions is obtained theoretically. This diagram is compared with experimental data on Ni 2ϩx Mn 1Ϫx Ga. An estimate is given of the magnetic-field influence on the temperature of martensitic transformation in the studied alloys.

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Magnetocaloric properties of a Ni 50 Mn 36 Co 1 Sn 13 ferromagnetic shape memory alloy have been studied experimentally in the vicinity of a first-order magnetostructural phase-transition low-temperature paramagnetic martensite↔ high-temperature ferromagnetic austenite. The magnetic entropy change ⌬S m calculated from the magnetization M͑T͒ data measured upon cooling is higher than that estimated from M͑T͒ measured upon heating. Contrary to ⌬S m , the adiabatic temperature change ⌬T ad measured upon cooling is significantly smaller than that measured upon heating. The apparent discrepancy between ⌬S m and ⌬T ad ͑larger ⌬S m , smaller ⌬T ad upon cooling, and smaller ⌬S m , larger ⌬T ad upon heating͒ is caused by the hysteretical behavior of this magnetostructural transition, a feature common for all the alloys in the family of Ni 50 Mn 25+x Z 25−x ͑Z =In,Sn,Sb͒ ferromagnetic shape memory Heusler compounds. The hysteresis causes the magnetocaloric parameters to depend strongly on the temperature and field history of the experimental processes.

The temperature dependencies of the resistivity and magnetization of a series of Ni2+xMn1−xGa (x = 0 -0.09) alloys were investigated. Along with the anomalies associated with ferromagnetic and martensitic transitions, well-defined anomalies were observed at the temperature of premartensitic transformation. The premartensitic phase existing in a temperature range 200 -260 K in the stoichiometric Ni2MnGa is progressively suppressed by the martensitic phase with increasing Ni content and vanishes in Ni2.09Mn0.91Ga composition.In Ni 2 MnGa, like in many other Heusler alloys containing manganese, the indirect exchange interaction between magnetic ions results in ferromagnetism which is usually described in terms of the local magnetic moment at the Mn site 1 . For the stoichiometric Ni 2 MnGa a structural transition of the martensitic type from the parent cubic to a complex tetragonally based structure occurs at T M = 202 K while ferromagnetic ordering sets at T C = 376 K 2 . The martensitic transition temperature T M was found to be sensitive to the composition, and values of T M between 175 and 450 K have been reported. A specific feature of the Ni-Mn-Ga system is that in alloys with a high T M (> 270 K) a number of intermartensite transformations can be induced by an external stress 3,4 , whereas in the alloys with lower T M the martensitic transition is preceded by a weakly first order premartensitic phase transition 5-8 . The inelastic neutron scattering experiments 9 performed on stoichiometric Ni 2 MnGa showed the existence of a soft [ξξ0]TA 2 phonon mode in a wide temperature interval, which is common to martensitic alloys having bcc structure. An important observation in these measurements was that the TA 2 phonon branch at a wave vector of ξ 0 ≈ 0.33 incompletely condenses at the premartensitic transition T P ≈ 260 K, well above the martensitic transition T M = 220 K. On cooling from T P to T M the frequency of the soft mode increases. This would suggest that the soft mode formation is associated with the premartensitic phase transformation rather then with the martensitic one. By transmission electron microscopy observation 10 it was established that the premartensitic phase consists in a micromodulated "tweed" structure without macroscopic tetragonal distortions so that the parent cubic symmetry is preserved. The modulation of the premartensitic phase was found to correspond to the wave vector ξ 0 ≈ 0.33.It is necessary to stress that although no premartensitic transition was found in compositions with high T M , the precursor phenomenon (softening of the [ξξ0]TA 2 phonon branch at the wave vector ξ 0 ≈ 0.33) has been clearly observed by inelastic neutron scattering 11,12 . Except for the partial condensation of the TA 2 phonon branch observed by Zheludev et al., the only essential difference between the inelastic neutron scattering results performed on three samples of different stoichiometry 9,11,12 is that the width in ξ where softening occurs becomes broader as T M increases. At the presence the reason...