Nanocrystalline magnetic PrFeCrB alloys

Ribbons of Pr5Fe77−xCrxB18 (x=0, 1, 2, 2.5, 3, 4, 5) were produced by melt spinning and then annealed to develop an enhanced-remanence nanocrystalline magnetic material. These nanocomposites with Cr present a coercive field at least 50% higher than the Cr-free ones, which makes them promising materials for bonded magnets. Four different types of annealing were used in order to develop the nanocrystalline state and to optimize the magnetic properties of these alloys. The first was a conventional annealing, where the ribbons were wrapped in a tantalum foil and annealed in an argon atmosphere, but not encapsulated. The second was a flash annealing, where the ribbons were annealed by passing a current through them. The third was a conventional annealing in an external magnetic field. Finally, the fourth was a conventional annealing, where the ribbons were wrapped in a tantalum foil and encapsulated in quartz tubes with argon gas and then annealed. The annealed samples were studied by magnetic measurements, X-ray diffraction, scanning and transmission electron microscopy and atomic force microscopy. The best magnetic properties are found for Pr5Fe74Cr3B18, annealed by the fourth method, which resulted in the lowest oxygen content in the annealed nanocrystalline material as confirmed by scanning electron microscopy. The value for the coercive field for this composition is at least 50% higher than for the material without Cr (≈560 vs. ≈320 kA/m) and 40% higher than for the Nd2Fe14B/Fe3B nanocomposite with Cr. Curie temperature measurements and X-ray diffraction data showed that the main phases present in all the samples are Pr2Fe14B, Fe3B and α-Fe, Pr2Fe14B being the majoritary phase. From Curie temperature measurements it was also found that Cr atoms preferentially dissolve in the Fe3B phase.