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

C-coated FexCo1-x (x = 0.50, 0.45, 0.30, 0.35, 0.30, 0.25) nanoparticles we re produced using a rf plasma torch. The only C source was acetylene used a s a carrier gas. Structural determination by x-ray diffraction indicated a single disordered bcc alpha-FeCo phase along with graphitic C for all compo sitions. A Scherrer analysis of the peak widths revealed particles to have an average diameter of 50 nm. A broad log-normal size distribution was foun d from transmission electron microscopy observations. Magnetic hysteresis l oops have been measured to temperatures exceeding 1050 K and revealed relat ively high room temperature coercivities (200-400 Oe), with a strong compos itional variation similar to that observed in bulk alloys. Larger coercivit ies are consistent with particles near the monodomain size for these alloys . The temperature dependence of the magnetization revealed the effects of a tomic ordering. The variation of the saturation magnetization as a function of temperature showed a discontinuity near the bulk order-disorder (alpha- alpha') transformation temperature, as well as loss of magnetization at the alpha --> gamma structural phase transition temperature. Other features of M(T) near 500-550 degrees C are consistent with prior observations of a "5 50 degrees C structural anomaly" which has been observed in bulk alloys wit h less than perfect order. (C) 1998 American Institute of Physics. [S0021-8 979(98)28711-1].