Magnetic evidence for structural-phase transformations in Fe-Co alloy nanocrystals produced by a carbon arc

In previously reported work ${\mathrm{Fe}}_x{\mathrm{Co}}_{\text{1−x}}[\mathrm{C}]$ ($\text{x=0.0}$ , 0.2, 0.4, 0.5, 0.6, and 0.8 nominally) nanoparticles were prepared by a Kratschmer–Huffman carbon-arc method. ${\mathrm{Fe}}_{\mathrm{0.5}}{\mathrm{Co}}_{\mathrm{0.5}}\mathrm{[C]}$ exhibited the largest magnetizations heretofore observed in similarly produced nanoparticles. Here we present a more detailed study of the magnetic properties of ${\mathrm{Fe}}_{\mathrm{0.5}}{\mathrm{Co}}_{\mathrm{0.5}}\mathrm{[C]}$ nanocrystals. Magnetic hysteresis loops have been measured to temperatures exceeding 1050 K. This is attributed to rotational processes in monodomain particles and is shown to be sensitive to ordering of the particles. Low-field thermomagnetic data clearly show features which we attribute to the α→α^′ disorder–order and α→γ phase transformations, respectively.