A Comprehensive Research on Thermal Expansion and Critical Magnetic Behavior in HoFe ₁₂₋ <i> _x </i> Nb <i> _x </i> Compounds

ABSTRACT Thermal and magnetic properties of HoFe 12− x Nb x series of compounds were systematically investigated using variable‐temperature high‐resolution synchrotron x‐ray diffraction, neutron powder diffraction, and magnetic measurements. Rietveld refinement of temperature‐dependent x‐ray diffraction data reveals a pronounced thermal expansion anomaly near the Curie temperature ( T c ), indicative of strong magnetoelastic coupling effect. Neutron diffraction study on HoFe 11.3 Nb 0.7 confirms that the Ho and Fe magnetic moments exhibit antiparallel coupling, forming a ferrimagnet, and below T c , its easy‐magnetization direction is parallel to the crystallographic c ‐axis. The critical magnetic behavior near T c was analyzed by the Kouvel–Fisher method and modified Arrott plots, confirming a second‐order magnetic phase transition with the extracted critical exponents being consistent with mean‐field theory. Moreover, scaling analysis shows that field‐dependent magnetization data near T c collapse into two universal curves, confirming the reliability of the derived critical exponents. The analysis of magnetic data reveals that the HoFe 11.35 Nb 0.65 compound exhibits maximum magnetic entropy changes of ‐ ΔS max = 2.03 and 3.81 J·kg −1 K −1 near T c under magnetic field changes of 2 and 5 T, respectively. The relative cooling power (RCP) values reach 76.8 and 191.6 J·kg −1 . These results demonstrate that HoFe 11.35 Nb 0.65 is a promising candidate for high‐temperature magnetic refrigeration materials.