Slow relaxation of magnetization is observed in a neodymium(III) single-ion magnet based on phosphine oxide, which successfully extends our pentagonal bipyramidal family to light lanthanides. Comprehensive magnetic characterizations reveal that the magnetic dynamics follow the power law that corresponds to a Raman process, despite an energy splitting of 207 cm(-1) evidenced by the ab initio calculation. Compared with a similar complex, the magnetic dynamics and magneto-structural correlations are clarified, providing deeper insight into the pursuit of promising light lanthanide single-molecule magnets.