Size-Induced Chemical and Magnetic Ordering in Individual Fe-Au Nanoparticles

摘要

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 L1_2-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 L1_2-ordered Fe_3Au and FeAu_3 compounds along with a previously discovered L1_0-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 firstprinciple calculations. All ordered compounds exhibit a substantial magnetization at room temperature. The Fe_3Au had a high saturation magnetization of about 143.6 emu/g with a ferromagnetic spin structure. The FeAu_3 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 FeAu_3, consistent with the experimental results.