Ferromagnetic shape memory alloys of the Ni-Mn-Ga family demonstrate a very spectacular illustration of magnetoelastic coupling. In these alloys the thermoelastic martensitic transition may occur in the ferromagnetic state. As a result giant magnetoinduced deformations up to several percents due to martensite domain boundaries movement [1] and magnetically controlled shape memory effect due to martensitic transition [2] can be observed in single- and polycrystals, respectively. The behavior of martensitic transition under the action of external field and stress is of extreme impotence in view of possible applications of polycrystalline alloys in sensor and actuator technology.

The aim of the present work is to study experimentally and theoretically the martensitic transition in polycrystalline Ni_2.14Mn_0.81Fe_0.05Ga under the action of external uniaxial compressive stresses up to 40 MPa and magnetic fields up to 5 T. The measurements of low-field magnetic susceptibility near martensitic transition reveal that external stress tends to shift the martensitic transition hysteretic loop to a higher temperature region and to smoother the transition, widening the transition loop. Strong magnetic field up to 5 T shifts the hysteretic loop to higher temperatures without a pronounced deformation of the loop. In situ optical observations show the evolution of martensitic and ferromagnetic domains structure caused by the external stress. The theoretical and experimental results are in good agreement.

This work is supported by RFBR (Grants 03-02-17443, 04-02-81058, 03-02-39006).

[1] R.C. O’Handley, K.Ullakko, US Patent No 5958154, Sep.28.1999.

[2] A.N.Vasilev, A.V.Glebov, I.E.Dikshtein, et.al, Russian Patent No 2221076, Jan.10.2004.

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