Friák Martin, Slávik Anton, Miháliková Ivana, Holec David, Všianská Monika, Šob Mojmír, Palm Martin, Neugebauer Jörg
Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Žižkova 22, CZ-616 62 Brno, Czech Republic.
Department of Condensed Matter Physics, Faculty of Science, Masaryk University, Kotlářská 2, CZ-611 37 Brno, Czech Republic.
Materials (Basel). 2018 Sep 14;11(9):1732. doi: 10.3390/ma11091732.
The intermetallic compound Fe 2 AlTi (alternatively Fe 2 TiAl) is an important phase in the ternary Fe-Al-Ti phase diagram. Previous theoretical studies showed a large discrepancy of approximately an order of magnitude between the ab initio computed magnetic moments and the experimentally measured ones. To unravel the source of this discrepancy, we analyze how various mechanisms present in realistic materials such as residual strain effects or deviations from stoichiometry affect magnetism. Since in spin-unconstrained calculations the system always evolves to the spin configuration which represents a local or global minimum in the total energy surface, finite temperature spin effects are not well described. We therefore turn the investigation around and use constrained spin calculations, fixing the global magnetic moment. This approach provides direct insight into local and global energy minima (reflecting metastable and stable spin phases) as well as the curvature of the energy surface, which correlates with the magnetic entropy and thus the magnetic configuration space accessible at finite temperatures. Based on this approach, we show that deviations from stoichiometry have a huge impact on the local magnetic moment and can explain the experimentally observed low magnetic moments.
金属间化合物Fe₂AlTi（也写作Fe₂TiAl）是三元Fe-Al-Ti相图中的一个重要相。先前的理论研究表明，从头算计算得到的磁矩与实验测量值之间存在大约一个数量级的巨大差异。为了揭示这种差异的来源，我们分析了实际材料中存在的各种机制，如残余应变效应或化学计量比偏差对磁性的影响。由于在自旋无约束计算中，系统总是会演化为代表总能量表面局部或全局最小值的自旋构型，因此有限温度下的自旋效应无法得到很好的描述。因此，我们改变研究方法，采用约束自旋计算，固定全局磁矩。这种方法能直接洞察局部和全局能量最小值（反映亚稳态和稳定自旋相）以及能量表面的曲率，而能量表面的曲率与磁熵相关，进而与有限温度下可及的磁构型空间相关。基于这种方法，我们表明化学计量比偏差对局部磁矩有巨大影响，并且可以解释实验观察到的低磁矩现象。
