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

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 <i>ab initio</i> 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 <i>ab initio</i> 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