Unconventional magnetism in ThCr₂Si₂-type phosphides, La_1-xNd_xCo₂P₂

Quaternary phases La_1-xNd_xCo₂P₂ (x = 0, 0.12, 0.25, 0.37, 0.50, 0.63, 0.75, 0.88, 1.0) have been synthesized from Sn flux to investigate the origins of drastic differences in properties between ferromagnetic LaCo₂P₂ and antiferromagnetic NdCo₂P₂. Powder and single-crystal X-ray diffraction indicate that all La_1-xNd_xCo₂P₂ samples are isostructural and crystallize in the ThCr₂Si₂ structure type. The unit cell parameters and volume change non-linearly with the Nd content (x), with the x < 0.50 samples being closer to LaCo ₂P₂ and the ones with x > 0.50 being closer to NdCo₂P₂. These structural differences are also reflected in the magnetic behavior. The samples with lower Nd content are characterized by ferromagnetic ordering in the Co sublattice with the T_C increasing from 132 K for x = 0 to 262 K for x = 0.50, while the samples with higher Nd content exhibit suppressed magnetization in the Co sublattice and canted antiferromagnetic ordering with T_C ∼ 270 K. Refinement of neutron powder diffraction patterns for x = 0.50 and 0.75 reveals a gradual ordering of the Nd 4f moments under the influence of Co 3d moments below 100 K. At low temperatures and zero field, these samples exhibit antiferromagnetic ordering of both Nd and Co magnetic moments, but under applied field they demonstrate the stabilization of a ferrimagnetic state with antiparallel alignment of the 4f and 3d moments, as indicated by isothermal magnetization measurements. The re-entrant ferrimagnetic transition is also observed in samples with x > 0.50 if the temperature is lowered below 5 K. The occurrence of this low-temperature magnetic transition was confirmed by alternating-current susceptibility measurements.