Magnetic phase transitions in ${\mathrm{Pr}}_5{\mathrm{Ge}}_4$

The magnetic structure and magnetic phase transitions of the ${\mathrm{Pr}}_5{\mathrm{Ge}}_4$ compound are investigated by means of neutron powder diffraction (NPD) and small-angle neutron scattering (SANS). Both NPD and SANS indicate the existence of two magnetic phase transitions at 25 K and 42 K. Refinement of the crystal structure based on the high-resolution NPD data shows that from room temperature to 1.6 K the compound maintains the ${\mathrm{Sm}}_5{\mathrm{Ge}}_4$-type structure $(Pnma,$ $Z=4),$ in which Ge atoms occupy two $4c$ sites and one $8d$ site and Pr atoms occupy two $8d$ sites and one $4c$ site. Refinements of the magnetic structures reveal that the two magnetic transitions essentially correspond to the long-range orderings of the Pr moments on different crystallographic sites. The Pr moments on the $4c$ site order ferromagnetically with ${\mathrm{Pn}}^{\prime }{\mathrm{ma}}^{\prime }$ symmetry at 42 K and induce small ordered Pr moments on the $8d$ sites. The long-range ordering of the Pr moments on the $8d$ sites develops at 25 K with ${\mathrm{Pnm}}^{\prime }{a}^{\prime }$ symmetry. The separate ordering of the Pr moments on different crystallographic sites can be readily understood by the salient difference in the rare earth metal environments between the different crystallographic sites. The Pr moments on the $4c$ site in ${\mathrm{Pr}}_5{\mathrm{Ge}}_4$ compound align along the $b$ axis, in contrast to other reported $R_5{\mathrm{Ge}}_4$ compounds, and can be attributed to the contribution of higher-order terms of crystal-field to magnetocrystalline anisotropy of the various $R$ atoms in the $R_5{\mathrm{Ge}}_4$ compounds.