Structure and magnetic properties of Fe-Co alloy nanoparticles synthesized by pulsed-laser inert gas condensation

Fe-Co alloy nanoparticles of different compositions (Fe content of 76, 51, and 30 at%), along with pure Fe and Co nanoparticles, were prepared by pulsed-laser inert gas condensation, consisting in laser ablation of Fe-Co alloy targets under helium atmosphere. From the morphological point of view, the obtained nanoparticles have nearly spherical shape, follow a lognormal size distribution and exhibit little aggregation. X-ray diffraction and high-resolution electron microscopy coupled with electron energy loss spectroscopy show that the Fe-Co nanoparticles are single crystals with body-centered cubic structure. Furthermore, in the majority of nanoparticles the composition is highly uniform across the whole diameter and there is little variation in composition from one nanoparticle to another. Exposure to non-inert atmosphere leads to the formation of a core@shell metal@oxide morphology characterized by a spinel oxide shell of 2–3 nm around the metallic alloy core. All samples display a ferromagnetic behavior, characterized by a hysteretic magnetization loop. The saturation magnetization attains a maximum value of 2.43 Bohr magnetons per atom for Fe content of 76 at%, in agreement with the Slater-Pauling curve for alloys of 3d elements. Instead, the coercive field, ranging from 29 to 60 kA m−1, is much larger than the reported values for polycrystalline bulk Fe-Co compounds and monotonically increases from pure Fe to pure Co. These results demonstrate that pulsed-laser inert gas condensation allows to prepare high-quality nanoalloys with tailorable magnetic properties, overcoming the limitations of thermal evaporation methods with respect to compositional control.