Abstract
The precise determination of the stability of different martensitic phases is an essential task in the successful design of (magnetic) shape memory alloys. We evaluate the effect of electron delocalization correction on the predictive power of density functional theory for Ni-Mn-Ga, the prototype magnetic shape memory compound. Using the corrected Hubbard-model-based generalized gradient approximation (GGA+U), we varied the Coulomb repulsion parameter U from 0 eV to 3 eV to reveal the evolution of predicted material parameters. The increasing localization on Mn sites results in the increasing stabilization of 10M modulated structure in stoichiometric Ni2MnGa in agreement with experiment whereas uncorrected GGA and meta-GGA functional provide the lowest energy for 4O modulated structure and non-modulated structure, respectively. GGA+U calculations indicate that 10M structure is more stable than other martensitic structures for U > 1.2 eV.The key features of density of states (DOS) responsible for the stabilization or destabilization of particular martensitic phases calculated with GGA+U are found also in DOS calculated with advanced quasi-particle self-consistent GW (QSGW) method. It supports the physical background of Hubbard correction. Moreover, the calculations with U = 1.8 eV provide the best agreement with experimental data for lattice parameters of stoichiometric and off-stoichiometric alloys.
Highlights
Magnetic shape memory (MSM) materials from the family of Heusler alloys offer a wide variety of multifunctional properties such as magnetic field-induced strain (MFIS), magnetic field induced martensitic transformation or magneto-caloric effect [1,2,3,4,5]
Ab initio calculations of electronic structure based on density functional theory (DFT) succeeded in prediction of several properties of NiMn-Ga, e.g. instability of austenite [21,24,25], stability of modulated structures [17,30] and even in prediction of martensitic transformation temperature [31]
In the present work we provide deeper analysis of electron localization described by Hubbard correction parameter U on electronic structure of Ni-Mn-Ga alloys and on stability of different and modulated martensitic structures including 10M, 14M, NM, 4O and 6O martensite as well as their lattice parameters
Summary
Magnetic shape memory (MSM) materials from the family of Heusler alloys offer a wide variety of multifunctional properties such as magnetic field-induced strain (MFIS), magnetic field induced martensitic transformation or magneto-caloric effect [1,2,3,4,5]. The initial soft-mode shuffling in premartensite is later stabilized within the martensitic transformation by Jahn-Teller effect [28,29], resulting in the observed five-layer or seven-layer lattice modulation in tetragonally distorted structure of martensite. Ab initio calculations of electronic structure based on density functional theory (DFT) succeeded in prediction of several properties of NiMn-Ga, e.g. instability of austenite [21,24,25], stability of modulated structures [17,30] and even in prediction of martensitic transformation temperature [31]. In case of Ni2MnGa, the increased localization affects overall shape of DOS including Ni-eg peak responsible for instability of austenite [53] and the character of Fermi surface with significant impact on martensitic transformation and the structure of resulting phase. The results of GGA+U calculations are further compared with alternative increased precision methods, i.e. meta-GGA functional SCAN and GW (QSGW) method
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