A first-principles study of the martensitic instabilities in magnetic shape memory alloys
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A first-principles study of the martensitic instabilities in magnetic shape memory alloys Peter Entel, Markus Ernst Gruner, and Alfred Hucht Physics Department, University of Duisburg-Essen, Lotharstr. 1, Duisburg, 47048, Germany ABSTRACT Magnetic shape memory alloys are of growing technological interest as they exhibit giant magnetic field induced strain. The prototype Heusler system Ni-Mn-Ga is characterized by a variety of martensitic phases with a large magnetocrystalline anisotropy and high mobility of twin boundaries as the key ingredients for its functionality as well as anomalous features of the electronic structure and phonon dispersions in the austenitic phase. Within this contribution, we demonstrate how insights from first-principles calculations complement the physical picture. INTRODUCTION Among the magnetic shape memory Heusler alloys, Ni-Mn-Ga near stoichiometry displays the largest shape change in the martensitic 5M or 7M structure with a strain of the order of 10% in an external magnetic field of less than one Tesla. In addition, the alloys exhibit a sequence of intermediate martensites with the modulated structures usually appearing at c/a1 (see, e.g., [1]). Typically, the martensitic phase changes are accompanied by a shift of a peak in the electronic density arising from the non-bonding Ni states, a reconstruction of the associated Fermi surface, and, in some cases, by pronounced phonon anomalies. These appear in the cubic high-temperature austenitic and premartensitic phases but also in the modulated phases. In addition, in the modulated phases twin boundaries are highly mobile and can be rearranged under the action of an external magnetic field. This is due to their considerable magneto-crystalline anisotropy, which builds up in martensite and which is at the origin of the magnetic shape memory effect. Ab initio calculations on the basis of density functional theory confirm the above scenario. RESULTS Phase changes and electronic spectrum The changes of the electronic spectrum when changing from the cubic L21 austenitic phase with c/a=1 to the tetragonal non-modulated or modulated structure with c/a1 have been investigated by several authors [2-4], whereby the crossover from the high-temperature cubic L21 structure to modulated martensite with c/a=0.94 at lower temperature has been attributed to the Jahn-Teller effect. Here, the repopulation among the Ni-d states with decreasing temperature can lower the energy of the system by a simultaneous tetragonal lattice distortion, which shifts the Fermi energy to a lowdensity of states region (pseudogap). When passing from c/a=0.94 modulated martensite, which is stable below the martensitic transformation temperature TM (see Fig. 1), to tetragonal nonmodulated martensite with c/a>1 at still lower temperatures, the reconstruction of the Fermi surface is associated with a substantial shift of Ni-dx2-y2 states from below EF to above EF. The
reconstruction of the Fermi surface has recently be confirmed by ARPES measurements [5]. The change
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