Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA.
J Phys Condens Matter. 2012 May 9;24(18):185501. doi: 10.1088/0953-8984/24/18/185501. Epub 2012 Apr 5.
In this work, we perform first-principles DFT calculations to investigate the interplay between magnetic and structural properties in Ni(2)MnGa. We demonstrate that the relative stability of austenite (cubic) and non-modulated martensite (tetragonal) phases depends critically on the magnetic interactions between Mn atoms. While standard approximate DFT functionals stabilize the latter phase, a more accurate treatment of electronic localization and magnetism, obtained with DFT+U, suppresses the non-modulated tetragonal structure for the stoichiometric compound, in better agreement with experiments. We show that the Anderson impurity model, with Mn atoms treated as magnetic impurities, can explain this observation and that the fine balance between super-exchange RKKY type interactions mediated by Ni d and Ga p orbitals determines the equilibrium structure of the crystal. The Anderson model is also demonstrated to capture the effect of the number of valence electrons per unit cell on the structural properties, often used as an empirical parameter to tune the behavior of Ni(2)MnGa based alloys. Finally, we show that off-stoichiometric compositions with excess Mn promote transitions to a non-modulated tetragonal structure, in agreement with experiments.
在这项工作中，我们进行了第一性原理 DFT 计算，以研究 Ni(2)MnGa 中磁学和结构性质之间的相互作用。我们证明了奥氏体（立方）和非调制马氏体（四方）相的相对稳定性取决于 Mn 原子之间的磁相互作用。虽然标准的近似 DFT 泛函稳定了后一相，但通过 DFT+U 对电子局域化和磁性进行更精确的处理，抑制了化学计量化合物中非调制四方结构，与实验结果更吻合。我们表明，Anderson 杂质模型可以解释这种现象，Mn 原子被视为磁性杂质，Ni d 和 Ga p 轨道介导的超交换 RKKY 型相互作用之间的精细平衡决定了晶体的平衡结构。Anderson 模型还被证明可以捕捉每个单元胞中价电子数对结构性质的影响，通常被用作调整 Ni(2)MnGa 基合金行为的经验参数。最后，我们表明，具有过量 Mn 的非化学计量组成促进了向非调制四方结构的转变，这与实验结果一致。
