The magnetic and thermodynamic properties of a Ni2.19Mn0.81Ga alloy with coupled magnetic and structural (martensitic) phase transitions were studied experimentally and theoretically. The magnetocaloric effect was measured by a direct method in magnetic fields 0−26 kOe at temperatures close to the magnetostructural transition temperature. For theoretical description of the alloy properties near the magnetostructural transition a statistical model is suggested, that takes into account the coexistence of martensite and austenite domains in the vicinity of martensite transformation point.Keywords: Ferromagnetism; Magnetostructural phase transition; Magnetization; Magnetocaloric effectIn recent year Ni 2 MnGa-based ferromagnetic shape memory alloys have attracted considerable attention as a new class of actuator materials (see, for a recent review, Ref.[1]). Giant magnetocaloric effect (MCE) has also been observed in these alloys. Solid state materials with a large MCE are considered to be very perspective in creation of solid state refrigerators with high efficiency, manufacturability and ecological compatibility (Ref. [2] and references therein). In Ni-Mn-Ga alloys, the martensitic transition temperature is very sensitive to composition, which is very important for practical applications of these alloys. For instance, upon a partial substitution of Mn for Ni the martensitic transition temperature T m increases and Curie temperature T C decreases until they merge in one first-order magnetostructural phase transition (MSPT) [1]. The values of MCE in the vicinity of such a transition are likely to be maximal. The aim of our work has been to study, experimentally and theoretically, magnetic and thermodynamic properties of Ni 2.19 Mn 0.81 Ga alloy with MSPT and to determine directly the magnitude of MCE.Polycrystalline Ni 2.19 Mn 0.81 Ga alloy was prepared by a conventional arc-melting method in argon atmosphere. MCE was measured by a direct method in a calorimetric system at constant temperatures in the vicinity of MSPT. The sample for the measurement of MCE was fixed by glass wires of 10 mm in diameter glued to the sample. The change in the temperature of the sample, ∆T , with the change in magnetic field, ∆H, was measured by a differential thermocouple. A cooper screen was placed near the sample in order to decrease radiational losses. The magnetic field up to 26 kOe was created by an electromagnet.The temperature dependence of ∆T at turning the magnetic field off is presented in Fig. 1. The experimental results obtained evidence that the ∆T dependence has a peak at T ≈ 340 K. This temperature corresponds A statistical model is suggested for the theoretical description of the alloy properties near MSPT. The model takes into account coexistence of three types of martensitic and one type of austenitic structural domains in the vicinity of MSPT point [3,4]. Each structural domain is divided by 180• magnetic domains. It is postulated that the rate of transformation between structural domains is proportional to the net p...

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The kinetics of magnetic and structural domains and relaxation of magnetization and strain under an external stress and a magnetic field in ferromagnetic Ni-Mn-Ga alloys close to a structural phase transition on the basis of time-depending Ginzburg-Landau equation with a thermal noise were theoretically investigated. The influence of an external magnetic field on the kinetics of structural domains was established to have the same influence as the stress has but not so great. The evolution of structural domain pictures was obtained. One type of structural domains was established to prevail in an alloy under an external stress or a magnetic field with time. The time dependencies of magnetization and strain were calculated.