The probe of the nature of magnetic phase transition by magnetocaloric effect in FePO₄ compound

The polycrystalline FePO₄ sample was synthesized by the solid-state reaction. The structure of the FePO₄ compound was investigated by powder X-ray diffraction and shows that the compound adopts a trigonal crystal structure conforming to space group P3₁21 [152]. This comprehensive study elucidates the intricate magnetic properties and magnetocaloric effects in FePO₄ through a multi-faceted experimental analysis. Temperature-dependent magnetic susceptibility measurements reveal an antiferromagnetic (AFM) ordering transition at the Néel temperature T_N ≈ 25 K±5K, characterized by a broad maximum in χ(T) and corroborated by a linear Curie-Weiss regime in the paramagnetic phase yielding an effective magnetic moment of μ_eff = 5.77 µ_B as the theoretical one (5.92μB), consistent with localized Fe³⁺ (3d⁵) moments. Isothermal magnetization curves confirm the AFM ground state, exhibiting linear behavior at low fields and a lack of saturation up to 5 T. Analysis of the magnetic phase transition via Arrott plots and the field dependence of the critical exponent *n* confirms a second-order nature. The material exhibits a tunable magnetocaloric response, with the magnetic entropy change (ΔS_M) showing both conventional (negative) and inverse (positive) effects at different temperatures. The relative cooling power (RCP) demonstrates significant field dependency, scaling from a modest 0.26 J kg⁻¹ at 1 T to a substantial 57.35 J kg⁻¹ at 9 T, highlighting FePO₄'s potential as a versatile material for tunable magnetic cooling technologies, particularly in the cryogenic temperature regime.