Low-Temperature Magnetism of Rare-Earth Orthophosphates

Rare-earth orthophosphates, REPO4, are highly insulating materials in which localized 4f moments interact through a phosphate framework, providing a model system for weak cooperative magnetism in the absence of itinerant electrons. Here, we report a systematic study of single and mixed rare-earth orthophosphates spanning the lanthanide series, Y, and Sc. Susceptibility and magnetization measurements reveal a progression from weakly paramagnetic behavior in the light rare-earth compositions to large low-temperature moments and antiferromagnetism in the heavier members. High-temperature Curie–Weiss analysis yields effective moments that follow the expected free-ion trends, while SmPO4 and EuPO4 deviate because of Van Vleck contributions and low-lying excited multiplets. The onset and strength of magnetic order are determined by moment and structure, with TbxGd(1–x)PO4 compositions showing enhanced ordering temperatures associated with the composition and xenotime structure. Structural analysis indicates that xenotime provides shorter, more regular RE–O coordination and a more linear RE–O–P exchange segment than monazite, supporting enhanced superexchange. Within the mixed Tb–Gd series, the highest ordering temperature, TN = 3.1 K, occurs for Tb0.5Gd0.5PO4, exceeding those of both parent compounds. These results establish REPO4 as a benchmark family for understanding how localized 4f moments, crystal-field anisotropy, and structure-dependent exchange combine to produce low-temperature magnetic order.