Asynchronous field-induced ferromagnetism of the two Fe sub-lattices in (Hf,Ta)Fe2: Emergence of an additional first-order phase transition

The field-induced change of the magnetic phase transitions in partially Ta-substituted Hf0.84Ta0.16Fe2 has been examined by combining dc-magnetization, ac-susceptibility, magnetoresistance, and analysis of the isothermal entropy change [ΔS ∞ (μ0H)n] field-exponent (n). The system exhibits a first-order magnetic transition (FOMT) from a ferromagnetic (FM) to an antiferromagnetic (AFM) state at TC (262 K) and a second-order AFM to paramagnetic (PM) transition at TN (323 K) for lower applied field μ0H = 0.1 T. The shift of TC and TN follows opposite trends with increasing μ0H as expected for this material class. However, a field-induced FOMT has been observed associated with TN above 2 T, as confirmed by the negative slope of the inverse Arrott plot, the phase transition peak in ac-susceptibility, a slope change in isothermal magnetoresistance, and an overshoot of n greater than 2. The result can be ascribed to the asynchronous field-induced FM ordering of adjacent antiferromagnetically coupled 6h Fe sub-lattices at TN, which results in the increase of the temperature range of the FOMT.