Department of Mechanical & Materials Engineering, ‡Nebraska Center for Materials and Nanoscience, and §Department of Physics & Astronomy, University of Nebraska , Lincoln, Nebraska 68588, United States.
ACS Nano. 2014 Aug 26;8(8):8113-20. doi: 10.1021/nn5022007. Epub 2014 Jul 15.
Formation of chemically ordered compounds of Fe and Au is inhibited in bulk materials due to their limited mutual solubility. However, here we report the formation of chemically ordered L12-type Fe3Au and FeAu3 compounds in Fe-Au sub-10 nm nanoparticles, suggesting that they are equilibrium structures in size-constrained systems. The stability of these L12-ordered Fe3Au and FeAu3 compounds along with a previously discovered L10-ordered FeAu has been explained by a size-dependent equilibrium thermodynamic model. Furthermore, the spin ordering of these three compounds has been computed using ab initio first-principle calculations. All ordered compounds exhibit a substantial magnetization at room temperature. The Fe3Au had a high saturation magnetization of about 143.6 emu/g with a ferromagnetic spin structure. The FeAu3 nanoparticles displayed a low saturation magnetization of about 11 emu/g. This suggests a antiferromagnetic spin structure, with the net magnetization arising from uncompensated surface spins. First-principle calculations using the Vienna ab initio simulation package (VASP) indicate that ferromagnetic ordering is energetically most stable in Fe3Au, while antiferromagnetic order is predicted in FeAu and FeAu3, consistent with the experimental results.
由于有限的互溶性，Fe 和 Au 的化学有序化合物在块状材料中难以形成。然而，我们在此报告了在 Fe-Au 亚 10nm 纳米颗粒中形成化学有序的 L12 型 Fe3Au 和 FeAu3 化合物，这表明它们在尺寸受限的系统中是平衡结构。通过一个依赖于尺寸的平衡热力学模型，解释了这些 L12 有序 Fe3Au 和 FeAu3 化合物以及之前发现的 L10 有序 FeAu 的稳定性。此外，还使用基于第一性原理的从头算计算来计算这三种化合物的自旋有序性。所有有序化合物在室温下都表现出相当大的磁化强度。Fe3Au 具有约 143.6 emu/g 的高饱和磁化强度，具有铁磁自旋结构。FeAu3 纳米颗粒显示出约 11 emu/g 的低饱和磁化强度。这表明具有反铁磁自旋结构，净磁化强度来自未补偿的表面自旋。使用维也纳从头算模拟包 (VASP) 的第一性原理计算表明，在 Fe3Au 中，铁磁有序在能量上是最稳定的，而在 FeAu 和 FeAu3 中则预测为反铁磁有序，这与实验结果一致。
