  The processing of promising magnetocaloric materials into spheres is one of
the important issues on developing high-performance magnetic refrigeration
systems. In the present study, we achieved in producing spherical particles of
a giant magnetocaloric compound HoB2 by a crucible-free gas atomization
process, despite its high melting point of 2350 C. The particle size
distribution ranges from 100 to 710 micrometers centered at 212-355 micrometers
with the highest yield of 14-20wt% of total melted electrode, which is suitable
for magnetic refrigeration systems. The majority of the resulting particles are
mostly spherical with no contamination during the processing, while unique
microstructures are observed on the surface and inside. These spherical
particles exhibit sharp magnetic transitions and huge magnetic entropy change
of 0.34 J cm^{-3} K^{-1} for a magnetic field change of 5 T at 15.5 K. The high
sphericality and the high magnetocaloric performance suggest that the HoB2
gas-atomized particles have good potential as magnetic refrigerants for use in
magnetic refrigerators for hydrogen liquefaction.
