(Pr, Ho)-Fe-B magnets for low-temperature applications

We have investigated the effect of HoH2 hydride addition on the hysteresis loop parameters of sintered Pr-Fe-Ti-Al-Cu-B magnets. The magnets were prepared by traditional powder metallurgy technology, and 3 wt.% HoH2 was added to the powder at the fine-milling stage. The magnets exhibited a monotonic increase in all hysteretic parameters with decreasing temperature down to 4.2 K. The coercive force and maximum energy product at 295 K (4.2 K) were 1344 (5402) kA/m and 221 (336) kJ/m3, respectively. The structure of the magnets was studied in detail by scanning electron microscopy and energy dispersive X-ray spectroscopy, which demonstrated the formation of the so-called “core-shell” structure, which is assumed to favor the marked improvement in the hysteretic properties of the samples analyzed. The surface domain structure was measured in the directions perpendicular and parallel to the magnet texture using magnetic force microscopy. The data obtained indicated fine labyrinth-like and strip domain patterns in the directions perpendicular and parallel to the magnet texture, with an average domain width of 1.2–1.8 µm.