Magnetic properties and ordering in C-coated FexCo1−x alloy nanocrystals

C-coated ${\mathrm{Fe}}_x{\mathrm{Co}}_{\text{1−x}}$ ($\text{x=0.50,}$ 0.45, 0.40, 0.35, 0.30, 0.25) nanoparticles were produced using a rf plasma torch. The only C source was acetylene used as a carrier gas. Structural determination by x-ray diffraction indicated a single disordered bcc α-FeCo phase along with graphitic C for all compositions. A Scherrer analysis of the peak widths revealed particles to have an average diameter of 50 nm. A broad log-normal size distribution was found from transmission electron microscopy observations. Magnetic hysteresis loops have been measured to temperatures exceeding 1050 K and revealed relatively high room temperature coercivities (200–400 Oe), with a strong compositional variation similar to that observed in bulk alloys. Larger coercivities are consistent with particles near the monodomain size for these alloys. The temperature dependence of the magnetization revealed the effects of atomic ordering. The variation of the saturation magnetization as a function of temperature showed a discontinuity near the bulk order–disorder ${\mathrm{(\alpha \to \alpha }}^{\prime })$ transformation temperature, as well as loss of magnetization at the $\mathrm{\alpha \to \gamma }$ structural phase transition temperature. Other features of $\mathrm{M(T)}$ near 500–550 $°\mathrm{C}$ are consistent with prior observations of a “550 $°\mathrm{C}$ structural anomaly” which has been observed in bulk alloys with less than perfect order.